No Effect of the Thr92Ala Polymorphism of Deiodinase-2 on Thyroid Hormone Parameters, Health-Related Quality of Life, and Cognitive Functioning in a Large Population-Based Cohort Study

The Thr92Ala polymorphism in the type 2 deiodinase gene is linked to depression in patients with COVID-19 after hospital discharge

Background

The Thr92Ala-DIO2 polymorphism has been associated with clinical outcomes in hospitalized patients with COVID-19 and neuropsychiatric diseases. This study examines the impact of the Thr92Ala-DIO2 polymorphism on neuropsychological symptoms, particularly depressive symptoms, in patients who have had moderate to severe SARS-CoV-2 infection and were later discharged.

Methods

Our prospective cohort study, conducted from June to August 2020, collected data from 273 patients hospitalized with COVID-19. This included thyroid function tests, inflammatory markers, hematologic indices, and genotyping of the Thr92Ala-DIO2 polymorphism. Post-discharge, we followed up with 68 patients over 30 to 45 days, dividing them into depressive (29 patients) and non-depressive (39 patients) groups based on their Beck Depression Inventory scores.

Results

We categorized 68 patients into three groups based on their genotypes: Thr/Thr (22 patients), Thr/Ala (41 patients), and Ala/Ala (5 patients). Depressive symptoms were less frequent in the Thr/Ala group (29.3%) compared to the Thr/Thr (59.1%) and Ala/Ala (60%) groups (p = 0.048). The Thr/Ala heterozygous genotype correlated with a lower risk of post-COVID-19 depression, as shown by univariate and multivariate logistic regression analyses. These analyses, adjusted for various factors, indicated a 70% to 81% reduction in risk.

Conclusion

Our findings appear to be the first to show that heterozygosity for Thr92Ala-DIO2 in patients with COVID-19 may protect against post-COVID-19 depression symptoms up to 2 months after the illness.

Gene polymorphisms and thyroid hormone signaling: implication for the treatment of hypothyroidism

INTRODUCTION

Mutations and single nucleotide polymorphisms (SNPs) in the genes encoding the network of proteins involved in thyroid hormone signaling (TH) may have implications for the effectiveness of the treatment of hypothyroidism with LT4. It is conceivable that loss-of-function mutations or SNPs impair the ability of LT4 to be activated to T3, reach its targets, and ultimately resolve symptoms of hypothyroidism. Some of these patients do benefit from therapy containing LT4 and LT3.

METHODS

Here, we reviewed the PubMed and examined gene mutations and SNPs in the TH cellular transporters, deiodinases, and TH receptors, along with their impact on TH signaling, and potential clinical implications.

RESULTS

In some mechanisms, such as the Thr92Ala-DIO2 SNP, there is a compelling rationale for reduced T4 to T3 activation that limits the effectiveness of LT4 to restore euthyroidism. In other mechanisms, a potential case can be made but more studies with a larger number of individuals are needed.

DISCUSSION/CONCLUSION

Understanding the clinical impact of the genetic makeup of LT4-treated patients may help in the preemptive identification of those individuals that would benefit from therapy containing LT3.

Real Practice Assessment of Persistent Symptoms After Initiation of Levothyroxine

OBJECTIVE

Levothyroxine (LT4) is the standard treatment for hypothyroidism. However, certain patients experience persistent symptoms even after achieving euthyroid status with LT4 therapy. We aimed to determine the frequency of persistent or new symptoms in patients with hypothyroidism after initiating LT4.

METHODS

This retrospective study included patients with hypothyroidism who started on LT4 between January 2017 and December 2019 at Mayo Clinic in Rochester, Minnesota, USA. Five hundred patient charts were randomly selected for review. Patients with at least 1 documented follow-up encounter after LT4 initiation were evaluated for ≤3 follow-up visits regarding their biochemical status and symptoms.

RESULTS

We included 356 patients, a majority of whom were female (66.6%), white (92.3%), and obese (71.9%), with an average age of 59.5 years. At the baseline visit, approximately one-half of the patients (177/356, 47.7%) reported hypothyroid symptoms, with fatigue being the most common symptom. During the follow-up periods, we observed that 17.8% (28/157), 17.9% (19/106), and 19.3% (11/57) of patients had normal thyroid stimulating hormone (TSH) values but persistent symptoms, while 12.3% (19/156), 19.9% (16/107), and 8.9% (5/56) had normal TSH values but new symptoms. Overall, during each respective follow-up period, 26.7% (42/157), 27.3% (29/106), and 28% (16/57) of patients experienced persistent or new symptoms alongside normal TSH values, with fatigue being the most constant symptom.

CONCLUSION

Our findings indicate that approximately 1 in every 4 patients with hypothyroidism receiving LT4 therapy and achieving normal TSH levels experience persistent or new hypothyroid symptoms. The cause of these symptoms remains unclear, emphasizing the need for a better understanding of their underlying causes and the development of effective management strategies.

Association of DIO2 and MCT10 Polymorphisms With Persistent Symptoms in LT4-Treated Patients in the UK Biobank

CONTEXT

Some evidence suggests gene-treatment interactions might cause persistent symptoms in individuals receiving levothyroxine (LT4) treatment.

OBJECTIVE

We investigated, as previously hypothesized, if single-nucleotide variations (SNVs; formerly single-nucleotide polymorphisms) in rs225014 (Thr92Ala), rs225015, or rs12885300 (ORFa-Gly3Asp) in the deiodinase 2 gene (DIO2), or rs17606253 in the monocarboxylate transporter 10 gene (MCT10) were associated with outcomes indicative of local tissue hypothyroidism in LT4-treated patients and controls.

METHODS

We included 18 761 LT4-treated patients and 360 534 controls in a population-based cross-sectional study in the UK Biobank. LT4 treatment was defined as a diagnosis of hypothyroidism and self-reported use of LT4 without use of 3,5,3'-triiodothyronine. Outcomes were psychological well-being, cognitive function, and cardiovascular risk factors. Associations were evaluated by linear, logistic, or ordinal logistic multiple regression. Adjustments included sex, age, sex-age interaction, and genetic principal components 1 to 10.

RESULTS

Compared to controls, LT4 treatment was adversely associated with almost all outcomes, most noteworthy: Increased frequency of tiredness (P < .001), decreased well-being factor score (P < .001), increased reaction-time (P < .001), and increased body mass index (P < .001). Except for a significant association between the minor rs225015 A allele and financial dissatisfaction, there was no association of rs225014, rs225015, rs12885300, or rs17606253 with any outcomes in LT4-treated patients. For all outcomes, carrying the risk allele at these 4 SNVs did not amplify symptoms associated with LT4 treatment compared to controls.

CONCLUSION

rs225014, rs225015, rs12885300, and rs17606253 could not explain changed psychological well-being, cognitive function, or cardiovascular risk factors in LT4-treated patients. Our findings do not support a gene-treatment interaction between these SNVs and LT4 treatment.

Determination of Frequency of Type 2 Deiodinase Thr92Ala Polymorphism (rs225014) in 131I-treated Differentiated Thyroid Cancer Patients Undertaking L-thyroxine (L-T4) Suppression Therapy

Introduction

Type 2 deiodinase (DIO2) enzyme plays a vital role in peripheral T4 to T3 conversion and in the negative feedback regulation of pituitary thyroid-stimulating hormone (TSH) secretion. Thr92Ala polymorphism (rs225014) is a common single-nucleotide polymorphism (SNP) that lowers DIO2 activity and is associated with diverse physiological disorders. Differentiated thyroid cancer (DTC) patients are given L-T4 therapy after total thyroidectomy and 131I treatment to suppress TSH levels.

Aim

The aim of the study was to determine the frequency of rs225014 in DTC patients and to investigate its effect on the thyroid function tests (TFTs) and L-T4 dose required to suppress TSH levels.

Materials and Methods

The study included a DTC patient group and a control group. TFTs were estimated by RIA/IRMA kits. Genomic DNA of all the subjects was screened for rs225014 SNP by polymerase chain reaction.

Results

The frequency of Thr/Thr (wild type), Thr/Ala (heterozygous mutant), and Ala/Ala (homozygous mutant) genotypes in the DTC patients' group was 0.21, 0.52, and 0.27, respectively. T3 levels and T3/T4 ratio were significantly low in the Ala/Ala genotype in the DTC group indicating impaired DIO2 activity. L-T4 dose requirement to suppress TSH levels in the DTC patients harboring rs225014 SNP was not statistically different from the wild-type genotype.

Conclusion

The SNP rs225014 was observed to be associated with T3 and T3/T4 ratio but not with the L-T4 dose in DTC harboring SNP suggesting the presence of a compensatory pathway to overcome DIO2 impairment. However, it is essential to study the genetic makeup of DTC patients showing reduced response to TSH suppression to enable quicker decision-making in the implementation of personalized L-T4 dose to prevent any adverse effects.

Designing a combined liothyronine (LT3), L- thyroxine (LT4) trial in symptomatic hypothyroid subjects on LT4 - the importance of patient selection, choice of LT3 and trial design

Approximately 10%-15% of subjects with hypothyroidism on L-thyroxine (LT4) alone have persistent symptoms affecting their quality of life (QoL). Although the cause is unclear, there is evidence that "tissue T3 lack" may be responsible. If so, combining liothyronine (LT3) with LT4 would be helpful. However, randomized controlled trials (RCT), have not established greater efficacy for the LT3 + LT4 combination in these subjects than for LT4 alone. While the trial design may have been responsible, the use of unphysiological, short-acting LT3 preparations and non-thyroid-specific patient-reported outcome measures (PROMs) may have contributed. We recommend attention to the following aspects of trial design for future RCTs of LT3 + LT4 compared to LT4 alone: (a) Subject selection-(i) measurable symptoms (disadvantages should be recognized); (ii) using a validated thyroid specific PROM such as ThyPRO39 or the Composite scale derived from it; (iii) those taking over 1.2 μg/day or 100 μg/day (for pragmatic reasons) of LT4 defining a population likely without intrinsic thyroid activity who depend on exogenous LT4; (iv) recruiting a preponderance of subjects with autoimmune thyroiditis increasing generalisability; and (v) those with a high symptom load with a greater response to combination therapy e.g. those with the deiodinase 2 polymorphism. (b) The use of physiological LT3 preparations producing pharmacokinetic similarities to T3 profiles in unaffected subjects: two long-acting LT3 preparations are currently available and must be tested in phase 2b/3 RCTs. (c) The superiority of a crossover design in limiting numbers and costs while maintaining statistical power and ensuring that all subjects experienced the investigative medication.

Critical Approach to Hypothyroid Patients With Persistent Symptoms

Hypothyroidism is a common condition, and numerous studies have been published over the last decade to assess the potential risks associated with this disorder when inappropriately treated. The standard of care for treatment of hypothyroidism remains levothyroxine (LT4) at doses to achieve biochemical and clinical euthyroidism. However, about 15% of hypothyroid patients experience residual hypothyroid symptoms. Some population-based studies and international population-based surveys have confirmed dissatisfaction with LT4 treatment in some hypothyroid patients. It is well established that hypothyroid patients treated with LT4 exhibit higher serum thyroxine:triiodothyronine ratios and can have a persistent increase in cardiovascular risk factors. Moreover, variants in deiodinases and thyroid hormone transporter genes have been associated with subnormal T3 concentrations, persistent symptoms in LT4-treated patients, and improvement in response to the addition of liothyronine to LT4 therapy. The American (ATA) and European Thyroid Association (ETA) guidelines have recently evolved in their recognition of the potential limitations of LT4. This shift is reflected in prescribing patterns: Physicians' use of combination therapy is prevalent and possibly increasing. Randomized clinical trials have recently been published and, while they have found no improvement in treating hypothyroid patients, a number of important limitations did not allow generalizability. Meta-analyses have reported a preference rate for combination therapy in 46.2% hypothyroid patients treated with LT4. To promote discussions about an optimal study design, the ATA, ETA, and British Thyroid Association have recently published a consensus document. Our study provides a useful counterpoint on the controversial benefits of treating hypothyroid patients with combination therapy.

Levothyroxine Monotherapy: What Works Better for the Individual With Hypothyroidism?

OBJECTIVE

I explore objective data not supporting the addition of liothyronine (medication) (LT3) to levothyroxine (medication) (LT4) in patients with hypothyroidism. Accurate identification of patients with symptomatic (almost exclusively overt) hypothyroidism is important in evaluating clinical outcomes of therapies. Recent studies have documented that nearly a third of individuals who are offered thyroid hormone are euthyroid at the time of initiation. Additionally, others are clinically diagnosed without biochemical confirmation, so a sizable proportion of those started on LT4 are not hypothyroid. The assumption that nonhypothyroid symptoms will resolve with LT4 is problematic. The true underlying cause of these symptoms remains unidentified and untreated.

METHODS

In a narrative fashion I will review the positive predictive value of and correlation of symptoms consistent with hypothyroidism and confirmed hypothyroidism likely to favorably respond to thyroid hormone replacement.

RESULTS

Following a review of the reliability of thyroid-stimulating hormone (TSH) in predicting a euthyroid state, the correlation of circulating triiodothyronine (serum measurement) (T3) levels with symptoms and predictive value of T3 to forecast the outcome of adding LT3 to LT4 will be reviewed. The utility of striving for high, middle, or low TSH set points within the expected range to predict changes in clinical quality of life and the ability of blinded patients to sense subtle differences along this spectrum will be documented. In addition, the clinical impact of single nucleotide polymorphisms in the type 2 deiodinase gene will be reviewed. Finally, the overall satisfaction of selected patients with their thyroid hormone treatments will be outlined and preferences for T3-containing treatments from blinded studies will be summarized.

CONCLUSION

Basing thyroid hormone treatment decisions on patient symptoms likely results in missed diagnoses We should encourage primary care physicians to assess a differential diagnosis, exclude other diagnoses, and not assume a thyroid etiology when TSH is normal. Modifying treatment to a particular TSH target or adjusting based on a low T3 level does not seem to enhance patient outcomes. Finally, pending further trials of "symptomatic" participants, using sustained release LT3 to mimic normal physiology, and including monocarboxylate 10 transporter and Type 2 deiodinase polymorphisms and objective outcomes, I will continue to depend on therapy with LT4 monotherapy and seek alternative explanations for my patients' nonspecific symptoms.

Are We Restoring Thyroid Hormone Signaling in Levothyroxine-Treated Patients With Residual Symptoms of Hypothyroidism?

INTRODUCTION

Levothyroxine (LT4) at doses that maintain the serum thyroid-stimulating hormone levels within the normal range constitutes the standard of care for the treatment of hypothyroidism. After a few months, this eliminates the signs and symptoms of overt hypothyroidism in the majority of patients, owing to the endogenous activation of thyroxine to triiodothyronine, the biologically active thyroid hormone. Still, a small percentage of the patients (10%-20%) exhibit residual symptoms, despite having normal serum thyroid-stimulating hormone levels. These symptoms include cognitive, mood, and metabolic deficits, with a significant impairment in psychological well-being and quality of life.

OBJECTIVE

To provide a summary of progress in the approach of patients with hypothyroidism that exhibit residual symptoms despite treatment.

METHODS

We reviewed the current literature and here we focused on the mechanisms leading to a deficiency of T3 in some LT4-treated patients, the role of residual thyroid tissue and the rationale for combination therapy with LT4 + liothyronine (LT3).

RESULTS

A score of clinical trials comparing therapy with LT4 versus LT4 + LT3 concluded that both are safe and equally effective (neither is superior); however, these trials failed to recruit a sufficiently large number of patients with residual symptoms. New clinical trials that considered LT4-treated symptomatic patients revealed that such patients benefit from and prefer therapy containing LT4 + LT3; desiccated thyroid extract has also been used with similar results. A practical approach to patients with residual symptoms and on initiation of combination therapy with LT4 + LT3 is provided.

CONCLUSION

A recent joint statement of the American, British, and European Thyroid Associations recommends that a trial with combination therapy be offered to patients with hypothyroidism that do not fully benefit from therapy with LT4.

The enigma of persistent symptoms in hypothyroid patients treated with levothyroxine: A narrative review

A significant minority of patients with hypothyroidism report persistent symptoms despite achieving normal thyroid biochemistry after levothyroxine (L-T4) replacement. Four principal lines of thinking, which are not mutually exclusive, may explain this enigma. The 'low tissue liothyronine hypothesis' emphasizes the potential imperfections of L-T4 replacement therapy that may lead to hypothyroidism in some tissues such as the brain, while others (eg hypothalamus) are euthyroid. The 'Somatic Symptom and Related Disorders hypothesis' draws attention to an incidental coexistence of a diagnosis of Somatic Symptom and Related Disorders in patients with treated hypothyroidism. The 'autoimmune neuroinflammation hypothesis' highlights the potential consequences of inflammatory mediators due to thyroid autoimmunity (the commonest cause of hypothyroidism) on the brain. The 'comorbidities and psychosocial hypothesis' implicates a variety of physical and psychosocial factors that have been noted to be associated with a diagnosis of hypothyroidism, which may be primarily the cause of persistent complaints. Over the past twenty years, a great deal of time and effort has been expended pursuing the 'low tissue liothyronine hypothesis', which has failed to yield results that translate to patient benefits. This has skewed the balance in clinical practice, in favour of pursuing answers relating to L-T4 and liothyronine combination treatment, while the alternative explanations have been downplayed and potentially useful interventions have been given insufficient attention.

The Physiological Functions and Polymorphisms of Type II Deiodinase

Type II deiodinase (DIO2) is thought to provide triiodothyronine (T3) to the nucleus to meet intracellular needs by deiodinating the prohormone thyroxine. DIO2 is expressed widely in many tissues and plays an important role in a variety of physiological processes, such as controlling T3 content in developing tissues (e.g., bone, muscles, and skin) and the adult brain, and regulating adaptive thermogenesis in brown adipose tissue (BAT). However, the identification and cloning of DIO2 have been challenging. In recent years, several clinical investigations have focused on the Thr92Ala polymorphism, which is closely correlated with clinical syndromes such as type 2 diabetes, obesity, hypertension, and osteoarthritis. Thr92Ala-DIO2 was also found to be related to bone and neurodegenerative diseases and tumors. However, relatively few reviews have synthesized research on individual deiodinases, especially DIO2, in the past 5 years. This review summarizes current knowledge regarding the physiological functions of DIO2 in thyroid hormone signaling and adaptive thermogenesis in BAT and the brain, as well as the associations between Thr92Ala-DIO2 and bone and neurodegenerative diseases and tumors. This discussion is expected to provide insights into the physiological functions of DIO2 and the clinical syndromes associated with Thr92Ala-DIO2.

Severity of Fatigue and Its Relationship with TSH before and after Levothyroxine Replacement Therapy in Patients with Primary Hypothyroidism

Background: Fatigue is a common symptom in hypothyroidism; however, the effect of levothyroxine on fatigue has been little studied. The aim of this study was to evaluate the effect of levothyroxine on fatigue in Latino patients with primary hypothyroidism, as well as the association of TSH and free T4 (FT4) with the severity and persistence of fatigue. Methods: A prospective study was performed in 92 patients with primary hypothyroidism. Fatigue severity scale (FSS) scores and clinical and biochemical characteristics before and at 6 months of levothyroxine were evaluated. Results: After 6 months of levothyroxine, a reduction in FSS (53 (47–57) vs. 36 (16–38); p = 0.001) and fatigue frequency (45.7% vs. 26.1%; p = 0.008) was evident. Both before and after 6 months of levothyroxine, there was a positive correlation of the FSS score with TSH and a negative correlation with FT4. Persistent fatigue was associated with a pretreatment FSS score (r = 0.75; p = 0.001) and diabetes (r = 0.40; p = 0.001). An FSS > 34 (RR 3.9 (95% CI 1.43–10.73; p = 0.008)), an FSS > 36 (RR 3.23 (95% CI 1.21–8.6; p = 0.019)), and diabetes (RR 5.7 (95% CI 1.25–9.6; p = 0.024)) before treatment were risk factors for persistent fatigue. Conclusions: Levothyroxine improved fatigue in most patients. Diabetes and an FSS score >34 or >36 before treatment were risk factors for persistent fatigue.

Association of Levothyroxine Treatment and Thyroid Peroxidase Antibodies with Antidepressant Use: A Danish Population-Based Longitudinal Study

Background: Subjects receiving levothyroxine (LT4) treatment have increased prevalence of depression, anxiety, and antidepressant use, but whether the underlying mechanism relates to thyroid autoimmunity is still unclarified. Methods: This is a population-based longitudinal study. Baseline biochemical and questionnaire data from the Danish General Suburban Population Study (GESUS) in 2010-2013 were linked with individual-level longitudinal data in national health registries. The aim was to investigate the associations between thyroid peroxidase antibodies (TPOAbs) and LT4 treatment, separately and through interaction, and at least one redeemed prescription for antidepressants. Logistic and Cox regression were used to evaluate initiation of antidepressant use before and after the baseline examination in GESUS, respectively. All exposures and covariates were fixed at the date of baseline examination. Thyroid autoimmunity was defined as serum TPOAbs >60 U/mL. Adjustments included sex, age, education, income, Charlson comorbidity index, smoking, and alcohol. Sensitivity analyses were performed for missing variables, exclusion of lithium use, exclusion of thyroid surgery, and conservative definitions for LT4 treatment and antidepressant use requiring at least two prescriptions. Results: We included 12,894 individuals, of whom 2353 (18%) had "past or current" antidepressant use at baseline, leaving 10,541 individuals at risk for incident antidepressant use after baseline. The median follow-up was 7.8 years during which 783 individuals (7.4% of 10,541 individuals) had incident antidepressant use. TPOAb positivity was not associated with "past or current" (odds ratio [OR] 0.90 [confidence interval, CI 0.78-1.03], p = 0.13) nor incident antidepressant use (hazard ratio [HR] 1.02 [CI 0.83-1.25], p = 0.88). LT4 treatment was associated with increased "past or current" antidepressant use (OR 1.33 [CI 1.10-1.62], p = 0.004) and increased incident antidepressant use (HR 1.38 [CI 1.03-1.85], p = 0.03). There were no interactions between the effects of TPOAb positivity and LT4 treatment on the use of antidepressants in logistic (p = 0.87) or Cox regression models (p = 0.82). Sensitivity analyses were robust, except that incident use of at least two redeemed antidepressant prescriptions was not statistically significant. Conclusions: LT4 treatment, but not TPOAb positivity, was associated with increased prevalent or incident antidepressant use with at least one prescription. Our findings do not support that thyroid autoimmunity is an important factor for antidepressant use in patients receiving LT4 treatment.

Brain Fog in Hypothyroidism: What Is It, How Is It Measured, and What Can Be Done About It

Background: Some levothyroxine (LT4)-treated hypothyroid patients report a constellation of persistent and distressing cognitive symptoms that has been termed brain fog. This narrative review focuses on attempts to define and measure hypothyroid-associated brain fog, summarize possible etiologies and contributing factors, present treatment options, and propose avenues for future research. Methods: Published literature was reviewed to summarize available information on patient-reported symptoms associated with brain fog in hypothyroidism, as well as objective evidence of impairment based on neurocognitive testing and functional imaging studies. Given the limited information specific for hypothyroid-associated brain fog, relevant data from other medical conditions associated with brain fog were also reviewed and incorporated into recommendations for clinical care and future research areas. Results: Hypothyroid-associated brain fog has not been well defined or quantitated, and the underlying pathophysiology is unclear. Symptoms vary among patients but commonly include fatigue, depressed mood, and cognitive difficulties in the areas of memory and executive function. Symptoms often predate the diagnosis of hypothyroidism, and the magnitude of cognitive impairment can range from mild to severe. Regardless of severity, these symptoms are associated with impaired quality of life and cause dissatisfaction with treatment, so often lead to requests for alternate therapies. Disease-specific and psychological factors impact the experience of brain fog in complex ways, including potential limitations in LT4 monotherapy, self-knowledge of a disease state, and expectations for therapeutic effects. Conclusions: Brain fog is a variable symptom complex in people with hypothyroidism, causing significant distress and diminished quality of life. In the absence of proven therapies, individualized treatment plans are recommended, which incorporate thyroid-specific, general medical, and psychosocial approaches. In particular, cognitive rehabilitation is an underutilized technique that is beneficial in other medical conditions associated with brain fog and could improve symptoms in hypothyroid people. The limitations in our current knowledge and questions presented throughout this review highlight a major need for clinical research in this understudied area. Future research should include attention to standardization of survey instruments to quantitate brain fog in hypothyroid people, as well as rigorously designed intervention studies.

Factors influencing the levothyroxine dose in the hormone replacement therapy of primary hypothyroidism in adults

Levothyroxine (LT4) is a safe, effective means of hormone replacement therapy for hypothyroidism. Here, we review the pharmaceutical, pathophysiological and behavioural factors influencing the absorption, distribution, metabolism and excretion of LT4. Any factor that alters the state of the epithelium in the stomach or small intestine will reduce and/or slow absorption of LT4; these include ulcerative colitis, coeliac disease, bariatric surgery, Helicobacter pylori infection, food intolerance, gastritis, mineral supplements, dietary fibre, resins, and various drugs. Once in the circulation, LT4 is almost fully bound to plasma proteins. Although free T4 (FT4) and liothyronine concentrations are extensively buffered, it is possible that drug- or disorder-induced changes in plasma proteins levels can modify free hormone levels. The data on the clinical significance of genetic variants in deiodinase genes are contradictory, and wide-scale genotyping of hypothyroid patients is not currently justified. We developed a decision tree for the physician faced with an abnormally high thyroid-stimulating hormone (TSH) level in a patient reporting adequate compliance with the recommended LT4 dose. The physician should review medications, the medical history and the serum FT4 level and check for acute adrenal insufficiency, heterophilic anti-TSH antibodies, antibodies against gastric and intestinal components (gastric parietal cells, endomysium, and tissue transglutaminase 2), and Helicobacter pylori infection. The next step is an LT4 pharmacodynamic absorption test; poor LT4 absorption should prompt a consultation with a gastroenterologist and (depending on the findings) an increase in the LT4 dose level. An in-depth etiological investigation can reveal visceral disorders and, especially, digestive tract disorders.

Role of Levothyroxine/Liothyronine Combinations in Treating Hypothyroidism

Diverse causes potentially underlie decreased quality of life in biochemically euthyroid patients treated for hypothyroidism with levothyroxine. Once these contributing factors are addressed, if symptoms persist, there may be benefit to personalized use of combination therapy adding liothyronine. This approach should be carefully monitored: avoiding overtreatment and ensuring that therapy is only continued if it improves patient-reported quality of life. Most randomized clinical trials have not shown benefits, perhaps because of not targeting the most symptomatic patients. Sustained-release liothyronine preparations may soon be available for optimally designed studies assessing whether combination therapy provides superior therapy for hypothyroidism in select patients.

A Re-evaluation of Candidate Gene Studies for Well-Being in Light of Genome-Wide Evidence

Ever since twin-family studies found that a substantial amount (± 40%) of the variation in well-being can be explained by genetic variation, several candidate genes have been proposed explaining this variation. However, these candidate gene and candidate gene-by-environment interaction studies have been surrounded by controversy regarding the validity and replication of their results. In the present study, we review the existing candidate gene literature for well-being. First, we perform a systematic literature search that results in the inclusion of 41 studies. After describing the results of the included studies, we evaluated the included candidate polymorphisms by (1) looking up the results for the studied candidate SNPs in a large well-being genome-wide association study, (2) performing association analyses in UK biobank (UKB) data for the candidate variable number tandem repeats (VNTR) and the APOE ε4 allele, and (3) studying possible candidate interactions with positive and negative environmental moderators using UKB data. We find no support for any of the candidate genes or candidate gene-environment interactions for well-being, with the exception of two SNPs that were chosen based on genome-wide evidence. While the generalizability of our findings is limited by our phenotype and environment definitions, we strongly advise well-being researchers to abandon the candidate gene approach in the field of well-being and move toward genome-wide approaches.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10902-022-00538-x.

Heterozygote Advantage of the Type II Deiodinase Thr92Ala Polymorphism on Intrahospital Mortality of COVID-19

CONTEXT

The type 2 deiodinase and its Thr92Ala-DIO2 polymorphism have been linked to clinical outcomes in acute lung injury and pulmonary fibrosis.

OBJECTIVE

Our objectives were to evaluate were cumulative mortality during admission according to Thr92Ala-DIO2 polymorphism.

METHODS

Here we conducted an observational, longitudinal, and prospective cohort study to investigate a possible association between the Thr92Ala-DIO2 polymorphism and intrahospital mortality from COVID-19 in adult patients admitted between June and August 2020. Blood biochemistry, thyroid function tests, length of stay, comorbidities, complications, and severity scores were also studied according to Thr92Ala-DIO2 polymorphism.

RESULTS

In total, 220 consecutive patients (median age 62; 48-74 years) were stratified into 3 subgroups: Thr/Thr (n = 79), Thr/Ala (n = 119), and Ala/Ala (n = 23). While the overall mortality was 17.3%, the lethality was lower in Ala/Thr patients (12.6%) than in Thr/Thr patients (21.7%) or Ala/Ala patients (23%). The heterozygous genotype (Thr/Ala) was associated with a 47% reduced risk of intrahospital mortality whereas univariate and multivariate logistic regression adjusted for multiple covariates revealed a reduction that ranged from 51% to 66%. The association of the Thr/Ala genotype with better clinical outcomes was confirmed in a metanalysis of 5 studies, including the present one.

CONCLUSION

Here we provide evidence for a protective role played by Thr92Ala-DIO2 heterozygosity in patients with COVID-19. This protective effect follows an inheritance model known as overdominance, in which the phenotype of the heterozygote lies outside the phenotypical range of both homozygous.

Type 2 Deiodinase Thr92Ala Polymorphism Is Not Associated with Cognitive Impairment in Older Adults: A Cross-Sectional Study

Background: Type 2 Deiodinase (DIO2) converts thyroxine (T4) into the active hormone triiodothyronine (T3). Thr92Ala DIO2 polymorphism has been associated with reduced conversion of T4 into T3 and central nervous system hypothyroidism. However, how Thr92Ala DIO2 polymorphism affects cognitive function is still unclear. Objective: To assess the association between Thr92Ala DIO2 polymorphism and cognitive performance in older adults. Design: Cross-sectional study. Setting: University-based tertiary hospital in Brazil. Patients: > 65-year-old with no limiting clinical disease. Interventions: All participants answered a standard questionnaire before undergoing thyroid function laboratory evaluation and genotyping of the Thr92Ala DIO2 polymorphism. Main Outcomes: Cognitive impairment measured by the Word List Memory task from the Consortium to Establish a Registry for Alzheimer’s Disease Neuropsychological Battery (CERAD-NB) and the Brief Cognitive Screening Battery (BCSB). Results: A hundred individuals were included. Clinical and laboratory characteristics were similar among DIO2 genotypes (all p > 0.05). No differences were found in the Word List Memory, recall, or recognition tests of the CERAD-NB assuming a recessive model for the Ala/Ala vs. Thr/Ala-Thr/Thr genotypes. Results of Clock Drawing Test, Animal Fluency Test, Mini-Mental State Exam, and Figure Memory Test of the BCSB were similar between groups. Conclusions: These findings suggest that Thr92Ala DIO2 polymorphism is not associated with relevant cognitive impairment in older adults.

Optimal Thyroid Hormone Replacement

Hypothyroidism is a common endocrinopathy, and levothyroxine is frequently prescribed. Despite the basic tenets of initiating and adjusting levothyroxine being agreed on, there are many nuances and complexities to consistently maintaining euthyroidism. Understanding the impact of patient weight and residual thyroid function on initial levothyroxine dosage and consideration of age, comorbidities, thyrotropin goal, life stage, and quality of life as levothyroxine is adjusted can be challenging and continually evolving. Because levothyroxine is a lifelong medication, it is important to avoid risks from periods of overtreatment or undertreatment. For the subset of patients not restored to baseline health with levothyroxine, causes arising from all aspects of the patient's life (coexistent medical conditions, stressors, lifestyle, psychosocial factors) should be broadly considered. If such factors do not appear to be contributing, and biochemical euthyroidism has been successfully maintained, there may be benefit to a trial of combination therapy with levothyroxine and liothyronine. This is not supported by the majority of randomized clinical trials, but may be supported by other studies providing lower-quality evidence and by animal studies. Given this discrepancy, it is important that any trial of combination therapy be continued only as long as a patient benefit is being enjoyed. Monitoring for adverse effects, particularly in older or frail individuals, is necessary and combination therapy should not be used during pregnancy. A sustained-release liothyronine preparation has completed phase 1 testing and may soon be available for better designed and powered studies assessing whether combination therapy provides superior therapy for hypothyroidism.

Treating hypothyroidism is not always easy: When to treat subclinical hypothyroidism, TSH goals in the elderly, and alternatives to levothyroxine monotherapy

The majority of patients with hypothyroidism feel better when levothyroxine treatment restores thyroid-stimulating hormone (TSH) concentrations to normal. Increasingly, a significant minority of patients remain symptomatic and are dissatisfied with their treatment. Overzealous treatment of symptomatic patients with subclinical hypothyroidism may contribute to dissatisfaction among hypothyroidism patients, as potential hypothyroid symptoms in patients with minimal hypothyroidism rarely respond to treatment. Thyroid hormone prescriptions have increased by 30% in the United States in the last decade. The diagnosis of subclinical hypothyroidism should be confirmed by repeat thyroid function tests ideally obtained at least 2 months later, as 62% of elevated TSH levels may revert to normal spontaneously. Generally, treatment is not necessary unless the TSH exceeds 7.0-10 mIU/L. In double-blinded randomized controlled trials, treatment does not improve symptoms or cognitive function if the TSH is less than 10 mIU/L. While cardiovascular events may be reduced in patients under age 65 with subclinical hypothyroidism who are treated with levothyroxine, treatment may be harmful in elderly patients with subclinical hypothyroidism. TSH goals are age dependent, with a 97.5 percentile (upper limit of normal) of 3.6 mIU/L for patients under age 40, and 7.5 mIU/L for patients over age 80. In some hypothyroid patients who are dissatisfied with treatment, especially those with a polymorphism in type 2 deiodinase, combined treatment with levothyroxine and liothyronine may be preferred.

Inherited Disorders of Thyroid Hormone Metabolism Defect Caused by the Dysregulation of Selenoprotein Expression

Consistent activation and functioning of thyroid hormones are essential to the human body as a whole, especially in controlling the metabolic rate of all organs and systems. Impaired sensitivity to thyroid hormones describes any process that interferes with the effectiveness of thyroid hormones. The genetic origin of inherited thyroid hormone defects and the investigation of genetic defects upon the processing of thyroid hormones are of utmost importance. Impaired sensitivity to thyroid hormone can be categorized into three conditions: thyroid hormone cell membrane transport defect (THCMTD), thyroid hormone metabolism defect (THMD), and thyroid hormone action defect (THAD). THMD is caused by defects in the synthesis and processing of deiodinases that convert the prohormone thyroxine (T4) to the active hormone triiodothyronine (T3). Deiodinase, a selenoprotein, requires unique translation machinery that is collectively composed of the selenocysteine (Sec) insertion sequence (SECIS) elements, Sec-insertion sequence-binding protein 2 (SECISBP2), Sec-specific eukaryotic elongation factor (EEFSEC), and Sec-specific tRNA (TRU-TCA1-1), which leads to the recognition of the UGA codon as a Sec codon for translation into the growing polypeptide. In addition, THMD could be expanded to the defects of enzymes that are involved in thyroid hormone conjugation, such as glucuronidation and sulphation. Paucity of inherited disorders in this category leaves them beyond the scope of this review. This review attempts to specifically explore the genomic causes and effects that result in a significant deficiency of T3 hormones due to inadequate function of deiodinases. Moreover, along with SECISBP2, TRU-TCA1-1, and deiodinase type-1 (DIO1) mutations, this review describes the variants in DIO2 single nucleotide polymorphism (SNP) and thyroid stimulating hormone receptor (TSHR) that result in the reduced activity of DIO2 and subsequent abnormal conversion of T3 from T4. Finally, this review provides additional insight into the general functionality of selenium supplementation and T3/T4 combination treatment in patients with hypothyroidism, suggesting the steps that need to be taken in the future.

Effect of Liothyronine Treatment on Quality of Life in Female Hypothyroid Patients With Residual Symptoms on Levothyroxine Therapy: A Randomized Crossover Study

OBJECTIVE

The effects of levothyroxine (LT4)/liothyronine (LT3) combination therapy on quality of life (QoL) in hypothyroid patients former on LT4 monotherapy have been disappointing. We therefore wanted to test the effects of LT3 monotherapy on QoL in hypothyroid patients with residual symptoms despite thyroid stimulating hormone (TSH) values within the reference range.

DESIGN

Female hypothyroid patients with residual symptoms on LT4 monotherapy or combination LT4/LT3 therapy received LT3 and LT4 monotherapy, respectively for 12 weeks in a non-blinded randomized crossover study.

METHODS

Fifty-nine patients aged 18-65 years were included. QoL was assessed using one disease-specific questionnaire (ThyPRO) and two generic questionnaires (Fatigue Questionnaire and SF-36) at baseline and at the end of the two treatment periods. Clinical indices of cardiovascular health (resting heart rate and blood pressure), as well as thyroid tests, were assessed at baseline and at the end of the two treatment periods.

RESULTS

After 12 weeks of LT3 treatment, 12 of the 13 domains of the ThyPRO questionnaire (physical, mental and social domains) showed significant improvements. The most pronounced improvements were less tiredness (mean -21 ± 26; P<0.0001) and cognitive complaints (mean -20 ± 20; P<0.0001). LT4 monotherapy exerted minor effects on two domains only (cognitive complaints and impaired daily life). All three dimensions' scores in the Fatigue Questionnaire (physical, mental and total fatigue) improved after LT3 treatment compared to baseline (P<0.001), and in the SF-36 questionnaire 7 of 8 scales showed significantly better scores after LT3 treatment compared to baseline. There were no differences in blood pressure or resting heart rate between the two treatment groups. TSH in patients on LT3 was slightly higher (median 1.33 mU/L (interquartile range (IQR) 0.47-2.26)) than in patients on LT4 (median 0.61 mU/L (IQR 0.25-1.20; P<0.018). Five patients on LT3 dropped out of the study due to subjectively reported side effects, compared to only one on LT4.

CONCLUSIONS

LT3 treatment improved QoL in women with residual hypothyroid symptoms on LT4 monotherapy or LT4/LT3 combination therapy. Short-term LT3 treatment did not induce biochemical or clinical hyperthyroidism, and no cardiovascular adverse effects were recorded. Further studies are needed to assess the long-term safety and efficacy of LT3 monotherapy.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov, identifier NCT03627611.

The Type 2 Deiodinase Thr92Ala Polymorphism Is Associated with Higher Body Mass Index and Fasting Glucose Levels: A Systematic Review and Meta-Analysis

Background

Type 2 deiodinase (Dio2) is a selenoenzyme that is mainly expressed in the endoplasmic reticulum of the central nervous system, brown adipose tissue, and placenta and is responsible for outer ring deiodination of thyroxine (T4) to form biologically active triiodothyronine (T3). The Thr92Ala polymorphism of Dio2 has been found to be a potential risk factor for various diseases beyond the hypothalamus-pituitary-thyroid (HPT) axis.

Methods

We searched the relevant studies in the PubMed, Embase, and Cochrane Library databases and Google Scholar. A systematic review and meta-analysis of studies on the Thr92Ala polymorphism and metabolic parameters beyond the HPT axis (e.g., BMI, fasting glycemic traits, plasma lipid levels, and hypertension risk) were performed.

Results

Six eligible studies that analyzed the relationship between the Thr92Ala polymorphism and metabolic parameters beyond the thyroid were identified. All selected studies excluded patients with thyroid dysfunction, and diabetic patients were also excluded when fasting glucose and fasting insulin levels were meta-analyzed. The Thr92Ala polymorphism was found to be a significant risk factor for higher BMI (Std. mean difference 0.31 (0.01, 0.60), p = 0.04) and higher fasting glucose levels (Std. mean difference 1.18 (0.05, 2.31), p = 0.04). However, fasting insulin levels, plasma lipid levels, and hypertension risk showed a nonsignificant association with the Thr92Ala polymorphism.

Conclusion

Compared with euthyroid noncarriers (Thr/Thr), euthyroid Ala92-Dio2 carriers showed increased BMI levels, and Ala92-Dio2 carriers also had higher fasting plasma glucose levels than matched euthyroid nondiabetic noncarriers.

T4+T3 Combination Therapy: An Unsolved Problem of Increasing Magnitude and Complexity

Thyroxine (T4)+triiodothyronine (T3) combination therapy can be considered in case of persistent symptoms despite normal serum thyroid stimulating hormone in levothyroxine (LT4)-treated hypothyroid patients. Combination therapy has gained popularity in the last two decades, especially in countries with a relatively high gross domestic product. The prevalence of persistent symptoms has also increased; most frequent are complaints about energy levels and fatigue (80% to 90%), weight management (70% to 75%), memory (60% to 80%), and mood (40% to 50%). Pathophysiological explanations for persistent problems are unrealistic patient expectations, comorbidities, somatic symptoms, related disorders (Diagnostic and Statistical Manual of Mental Disorders [DSM-5]), autoimmune neuroinflammation, and low tissue T3. There is fair circumstantial evidence for the latter cause (tissue and specifically brain T3 content is normalized by T4+T3, not by T4 alone), but the other causes are viewed as more relevant in current practice. This might be related to the 'hype' that has emerged surrounding T4+T3 therapy. Although more and better-designed trials are needed to validate the efficacy of T4+T3 combination, the management of persistent symptoms should also be directed towards alternative causes. Improving the doctor-patient relationship and including more and better information is crucial. For example, dissatisfaction with the outcomes of T4 treatment for subclinical hypothyroidism can be anticipated as recent trials have demonstrated that LT4 is hardly effective in improving symptoms associated with subclinical hypothyroidism.

Use of Statins Among Patients Taking Levothyroxine: an Observational Drug Utilization Study Across Sites

CONTEXT

Treatment with levothyroxine (LT4) that normalize serum thyrotropin (TSH) is expected to restore lipid metabolism.

OBJECTIVE

To assess statin utilization in LT4-treated patients through an observational drug utilization study.

METHODS

Three sites were involved: (1) 10 723 outpatients placed on LT4 during 2006-2019 identified from the Clinical Research Data Warehouse of the University of Chicago; (2) ~1.4 million LT4 prescriptions prepared by primary care physicians during January-December 2018, identified from the IQVIA™ database of medical prescriptions in Brazil; (30 ~5.4 million patient interviews during 2009-2019, including ~0.32 million patients on LT4, identified from the Fleury Group database in Brazil.

RESULTS

On site 1, initiation of therapy with LT4 increased the frequency of statin utilization (19.1% vs 24.6%), which occurred ~1.5 years later (median 76 weeks) and, among those patients that were on statins, increased intensity of treatment by 33%, despite normalization of serum TSH levels; on site 2, after matching for sex and age, the frequency of statins prescription was higher for those patients using LT4: females, 2.1 vs 3.4% (odds ratio [OR] 1.656 [1.639-1.673]); males, 3.1 vs 4.4% (OR 1.435 [1.409-1.462]); and, on site 3, after matching for sex and age, the frequency of statin utilization was higher in those patients using LT4: females, 10 vs 18% (OR 2.02 [2.00-2.04]); males, 15 vs 25% (OR 1.92 [1.88-1.96]); all P values were <.0001.

CONCLUSION

Prescription and utilization of statins were higher in patients taking LT4. The reasons for this association should be addressed in future studies.

Hypothyrosis syndrome: role of triodthyronine in diagnostics and combination therapy (review of literature)

Hypothyroidism syndrome is one of the most common pathologies of the endocrine system. Assessment of euthyroid status can not always be carried out according to the TSH indicator. In a number of patients with normal TSH levels and a clinical picture of hypothyroidism, the serum T3 concentration is determined at the lower limit of the norm or below it with a high content of free T4 in the blood. In world practice, positive experience has been accumulated in the combined use of preparations of sodium levothyroxine and liothyronine, a synthetic form of exogenous triiodothyronine. A number of studies have noted the advantages of using combination therapy for hypothyroidism over levothyroxine monotherapy in certain groups of patients. Possible reasons for the ineffectiveness of standard treatment for hypothyroidism are described.

What Is the Quality of Life in Patients Treated with Levothyroxine for Hypothyroidism and How Are We Measuring It? A Critical, Narrative Review

Thyroid hormone replacement therapy (THRT, generally using oral levothyroxine (LT4)) is a safe, effective means of treating hypothyroidism. However, a proportion of LT4-treated patients with biochemically normal thyroid function tests complain of persistent symptoms that impact their health-related quality of life (QoL). The objectives of this critical, narrative review of the literature were to identify studies of QoL in LT4-treated patients with hypothyroidism, examine the instruments used to measure QoL, determine whether normal QoL is restored by THRT, and identify factors associated with QoL. The PubMed database was searched from 1 January 2000 to 31 December 2020. A total of 809 publications were screened, 129 full-text articles were retrieved, and 58 were analyzed. The studies of overt hypothyroidism evidenced an improvement in psychological and emotional well-being after three to six months of THRT with LT4, although contrasting results were found for patients with subclinical hypothyroidism. Combination treatment with LT4 and liothyronine was not generally associated with better QoL. In hypothyroidism, QoL appears to be influenced by a number of physiological, behavioral, cognitive and/or lifestyle factors that are not strictly related to thyroid hormone levels.

Cholinesterase homozygous genotype as susceptible biomarker of hypertriglyceridaemia for pesticide-exposed agricultural workers

PURPOSE

Dyslipidemia is an emerging metabolic disorder among pesticide-exposed agricultural workers, and this study was aimed to explore biomarkers of hypertriglyceridaemia susceptibility.

METHODS

This cross-sectional study recruited 72 pesticide-exposed subjects and 78 non-exposed controls. Lipid profile, cholinesterase activity, and thyroid hormones were analysed with routine assays. Six loci, including rs11206244 and rs2235544 for deiodinase 1, rs12885300 and rs225014 for deiodinase 2, rs1803274 for butyrylcholinesterase, and rs3757869 for acetylcholinesterase were genotyped using an improved multiplex ligation detection reaction technique.

RESULTS

Pesticide-exposed subjects showed higher levels of triglyceride than controls (p = 0.009), although there were comparable cholinesterase activity and genotype frequencies of all six loci between pesticide-exposed subjects and controls. Pesticide-exposed subjects with homozygous genotype of cholinesterase had increased triglyceride levels than controls (p < 0.05). The percentage of hypertriglyceridaemia was 28.6% and 8.8% for pesticide-exposed subjects and controls with homozygous butyrylcholinesterase genotype (p = 0.007) and 20.8% and 14.3% with homozygous acetylcholinesterase genotype (p = 0.792), respectively. Multivariate logistic regression analyses found that odds ratio of hypertriglyceridaemia is 21.92 and 4.56 for pesticide-exposed subjects with homozygous genotype of butyrylcholinesterase (p = 0.001) and acetylcholinesterase (p = 0.036), respectively.

CONCLUSIONS

Cholinesterase homozygous genotype might be a potential susceptible biomarker in screening pesticide-exposed agricultural workers vulnerable to hypertriglyceridaemia.

Thyroid function, renal events and mortality in chronic kidney disease patients: the German Chronic Kidney Disease study

BACKGROUND

Hypothyroidism and low free triiodothyronine (FT3) syndrome [low FT3 levels with normal thyroid-stimulating hormone (TSH)] have been associated with reduced kidney function cross-sectionally in chronic kidney disease (CKD) patients with severely reduced estimated glomerular filtration rate (eGFR) or end-stage kidney disease (ESKD). Results on the prospective effects of impaired thyroid function on renal events and mortality for patients with severely reduced eGFR or from population-based cohorts are conflicting. Here we evaluated the association between thyroid and kidney function with eGFR (cross-sectionally) as well as renal events and mortality (prospectively) in a large, prospective cohort of CKD patients with mild to moderately reduced kidney function.

METHODS

Thyroid markers were measured among CKD patients from the German Chronic Kidney Disease study. Incident renal endpoints (combined ESKD, acute kidney injury and renal death) and all-cause mortality were abstracted from hospital records and death certificates. Time to first event analysis of complete data from baseline to the 4-year follow-up (median follow-up time 4.04 years) of 4600 patients was conducted. Multivariable linear regression and Cox proportional hazards models were fitted for single and combined continuous thyroid markers [TSH, free thyroxine (FT4), FT3] and thyroid status.

RESULTS

Cross-sectionally, the presence of low-FT3 syndrome showed a significant inverse association with eGFR and continuous FT3 levels alone showed a significant positive association with eGFR; in combination with FT4 and TSH, FT3 levels also showed a positive association and FT4 levels showed a negative association with eGFR. Prospectively, higher FT4 and lower FT3 levels were significantly associated with a higher risk of all-cause mortality (N events = 297). Per picomole per litre higher FT3 levels the risk of reaching the composite renal endpoint was 0.73-fold lower (95% confidence interval 0.65-0.82; N events = 615). Compared with euthyroid patients, patients with low-FT3 syndrome had a 2.2-fold higher risk and patients with hypothyroidism had a 1.6-fold higher risk of experiencing the composite renal endpoint.

CONCLUSIONS

Patients with mild to moderate CKD suffering from thyroid function abnormalities are at an increased risk of adverse renal events and all-cause mortality over time.

Evidence-Based Use of Levothyroxine/Liothyronine Combinations in Treating Hypothyroidism: A Consensus Document

BACKGROUND

Fourteen clinical trials have not shown a consistent benefit of combination therapy with levothyroxine (LT4) and liothyronine (LT3). Despite the publication of these trials, combination therapy is widely used and patients reporting benefit continue to generate patient and physician interest in this area. Recent scientific developments may provide insight into this inconsistency and guide future studies.

METHODS

The American Thyroid Association (ATA), British Thyroid Association (BTA), and European Thyroid Association (ETA) held a joint conference on November 3, 2019 (live-streamed between Chicago and London) to review new basic science and clinical evidence regarding combination therapy with presentations and input from 12 content experts. After the presentations, the material was synthesized and used to develop Summary Statements of the current state of knowledge. After review and revision of the material and Summary Statements, there was agreement that there was equipoise for a new clinical trial of combination therapy. Consensus Statements encapsulating the implications of the material discussed with respect to the design of future clinical trials of LT4/LT3 combination therapy were generated. Authors voted upon the Consensus Statements. Iterative changes were made in several rounds of voting and after comments from ATA/BTA/ETA members.

RESULTS

Of 34 Consensus Statements available for voting, 28 received at least 75% agreement, with 13 receiving 100% agreement. Those with 100% agreement included studies being powered to study the effect of deiodinase and thyroid hormone transporter polymorphisms on study outcomes, inclusion of patients dissatisfied with their current therapy and requiring at least 1.2 µg/kg of LT4 daily, use of twice daily LT3 or preferably a slow-release preparation if available, use of patient-reported outcomes as a primary outcome (measured by a tool with both relevant content validity and responsiveness) and patient preference as a secondary outcome, and utilization of a randomized placebo-controlled adequately powered double-blinded parallel design. The remaining statements are presented as potential additional considerations.

DISCUSSION

This article summarizes the areas discussed and presents Consensus Statements to guide development of future clinical trials of LT4/LT3 combination therapy. The results of such redesigned trials are expected to be of benefit to patients and of value to inform future thyroid hormone replacement clinical practice guidelines treatment recommendations.

Evidence-Based Use of Levothyroxine/Liothyronine Combinations in Treating Hypothyroidism: A Consensus Document

Background: Fourteen clinical trials have not shown a consistent benefit of combination therapy with levothyroxine (LT4) and liothyronine (LT3). Despite the publication of these trials, combination therapy is widely used and patients reporting benefit continue to generate patient and physician interest in this area. Recent scientific developments may provide insight into this inconsistency and guide future studies. Methods: The American Thyroid Association (ATA), British Thyroid Association (BTA), and European Thyroid Association (ETA) held a joint conference on November 3, 2019 (live-streamed between Chicago and London) to review new basic science and clinical evidence regarding combination therapy with presentations and input from 12 content experts. After the presentations, the material was synthesized and used to develop Summary Statements of the current state of knowledge. After review and revision of the material and Summary Statements, there was agreement that there was equipoise for a new clinical trial of combination therapy. Consensus Statements encapsulating the implications of the material discussed with respect to the design of future clinical trials of LT4/LT3 combination therapy were generated. Authors voted upon the Consensus Statements. Iterative changes were made in several rounds of voting and after comments from ATA/BTA/ETA members. Results: Of 34 Consensus Statements available for voting, 28 received at least 75% agreement, with 13 receiving 100% agreement. Those with 100% agreement included studies being powered to study the effect of deiodinase and thyroid hormone transporter polymorphisms on study outcomes, inclusion of patients dissatisfied with their current therapy and requiring at least 1.2 μg/kg of LT4 daily, use of twice daily LT3 or preferably a slow-release preparation if available, use of patient-reported outcomes as a primary outcome (measured by a tool with both relevant content validity and responsiveness) and patient preference as a secondary outcome, and utilization of a randomized placebo-controlled adequately powered double-blinded parallel design. The remaining statements are presented as potential additional considerations. Discussion: This article summarizes the areas discussed and presents Consensus Statements to guide development of future clinical trials of LT4/LT3 combination therapy. The results of such redesigned trials are expected to be of benefit to patients and of value to inform future thyroid hormone replacement clinical practice guidelines treatment recommendations.

Association Between Peripheral Blood Cell Count Abnormalities and Health-Related Quality of Life in the General Population

Complete blood cell counts, including differentials, are widely available and change on aging. Peripheral blood cell counts outside the normal range have previously been associated with increased mortality rates and a number of comorbid conditions. However, data about the association between blood cell count abnormalities, other than anemia, and health-related quality of life (HRQoL) are scarce. We investigated the association between abnormalities in (differential) blood cell counts and HRQoL in 143 191 community-dwelling individuals from the prospective population-based Lifelines cohort. HRQoL was measured using the RAND 36-Item Health Survey. Logistic regression analyses were used to determine the effect of blood cell count abnormalities on the odds of having a lower score than an age- and sex-specific reference value for each domain. Leukocytosis, neutrophilia, and a high neutrophil to lymphocyte ratio were associated with impaired HRQoL across multiple domains, both for younger and older (≥60 years) individuals. Using multivariable models, we confirmed that these associations were independent of the potential confounding factors obesity, smoking, alcohol use, number of medications (as a measure of comorbidity), anemia, and mean corpuscular volume. The impact on HRQoL was most pronounced for high neutrophil levels. Further, high white blood cell counts proved to be a better marker for inferior HRQoL as compared to elevated high-sensitivity C-reactive protein levels. Decreased HRQoL in several domains was also observed for individuals with monocytosis, lymphocytosis, and thrombocytosis. Taken together, the present study demonstrates an association between inflammatory and myeloid-skewed blood cell counts and inferior HRQoL in community-dwelling individuals.

Health-Related Quality of Life in Levothyroxine-Treated Hypothyroid Women and Women without Hypothyroidism: A Case-Control Study

The use of levothyroxine is not always related to the elimination of the symptoms of hypothyroidism. The aim of this study is to compare the health-related quality of life (HRQOL) of a group of hypothyroid women under levothyroxine treatment with that of a group of non-hypothyroid women. Methodology: A case–control study was performed. We used convenience sampling. The case group consisted of 152 levothyroxine-treated hypothyroid women; the control group consisted of 238 women without hypothyroidism disorders. All of the participants were euthyroid according to the clinical practice guidelines. We used as instruments the Short Form-12 questionnaire (SF-12v1) and a sociodemographic questionnaire. Results: Hypothyroid women scored significantly lower in HRQOL in SF-12v1 mental and physical components than the control group (mental component summary: 41.23 ± 12.12 vs. 46.45 ± 10.22, p < 0.001; physical component summary: 49.64 ± 10.16 vs. 54.75 ± 5.76, p < 0.001). body mass index (BMI) and age showed an influence on the physical component (p < 0.001 in both variables). Adjusted for age and BMI, hypothyroidism was still related to worse scores (p < 0.001). Conclusion: Despite being euthyroid, women with hypothyroidism showed a poorer quality of life than women without hypothyroidism. Health professionals need to assess the HRQOL of women with hypothyroidism. Further research on HRQOL and hypothyroidism is needed.

Epidemiology of thyroid disorders in the Lifelines Cohort Study (the Netherlands)

BACKGROUND

Thyroid hormone plays a pivotal role in human metabolism. In epidemiologic studies, adequate registration of thyroid disorders is warranted. We examined the prevalence of thyroid disorders, reported thyroid medication use, thyroid hormone levels, and validity of thyroid data obtained from questionnaires in the Lifelines Cohort Study.

METHODS

We evaluated baseline data of all 152180 subjects (aged 18-93 years) of the Lifelines Cohort Study. At baseline, participants were asked about previous thyroid surgery and current and previous thyroid hormone use. At follow-up (n = 136776, after median 43 months), incident thyroid disorders could be reported in an open, non-structured question. Data on baseline thyroid hormone measurements (TSH, FT4 and FT3) were available in a subset of 39935 participants.

RESULTS

Of the 152180 participants, mean (±SD) age was 44.6±13.1 years and 58.5% were female. Thyroid medication was used by 4790 participants (3.1%); the majority (98.2%) used levothyroxine, and 88% were females. 59.3% of levothyroxine users had normal TSH levels. The prevalence of abnormal TSH levels in those not using thyroid medication was 10.8%; 9.4% had a mildly elevated (4.01-10.0 mIU/L), 0.7% had suppressed (<0.40 mIU/L), while 0.7% had elevated (>10.0 mIU/L) TSH levels. Over 98% of subjects with TSH between 4 and 10 mIU/L had normal FT4. Open text questions allowing to report previous thyroid surgery and incident thyroid disorders proved not to be reliable and severely underestimated the true incidence and prevalence of thyroid disorders.

CONCLUSIONS

Undetected thyroid disorders were prevalent in the general population, whereas the prevalence of thyroid medication use was 3.1%. Less than 60% of individuals using levothyroxine had a normal TSH level. The large group of individuals with subclinical hypothyroidism (9.4%) offers an excellent possibility to prospectively follow the natural course of this disorder. Both structured questions as well as linking to G.P.'s and pharmacists' data are necessary to improve the completeness and reliability of Lifelines' data on thyroid disorders.

Single Nucleotide Polymorphisms in Thyroid Hormone Transporter Genes MCT8, MCT10 and Deiodinase DIO2 Contribute to Inter-Individual Variance of Executive Functions and Personality Traits

Thyroid hormones are modulators of cognitive functions, and changes in hormone levels affect intelligence, memory, attention and executive function. Single nucleotide polymorphisms (SNPs) of transporter proteins MCT8, MCT10 and deiodinase 2 (DIO2) influence thyroid metabolism and could therefore contribute to inter-individual variance of cognitive functions. This study investigates the influence of these SNPs using an extensive neuropsychological test battery. 656 healthy participants aged 18-39 years were genotyped for four SNPs: MCT8 (rs5937843 and rs6647476), MCT10 (rs14399) and DIO2 (rs225014) and underwent eleven different neuropsychological tests as well as four personality questionnaires. Test results were compared between homo- and heterozygous carriers and for the X-linked MCT8 additionally between men and women. Personality questionnaires revealed that Risk Seeking was reduced in homozygous T carriers and highest in homozygous C carriers of the DIO2 SNP and that both polymorphisms of MCT8 had an additive effect on Physical Aggression in men. Neuropsychological testing indicated that MCT10 affects nonverbal reasoning abilities, DIO2 influences working memory and verbal fluency and MCT8 influences attention, alertness and planning. This pilot study suggests an influence of polymorphisms in thyroid hormone transporter genes and deiodinase on cognitive domains and personality traits.

Individualized Therapy for Hypothyroidism: Is T4 Enough for Everyone?

CONTEXT

It is well recognized that some hypothyroid patients on levothyroxine (LT4) remain symptomatic, but why patients are susceptible to this condition, why symptoms persist, and what is the role of combination therapy with LT4 and liothyronine (LT3), are questions that remain unclear. Here we explore evidence of abnormal thyroid hormone (TH) metabolism in LT4-treated patients, and offer a rationale for why some patients perceive LT4 therapy as a failure.

EVIDENCE ACQUISITION

This review is based on a collection of primary and review literature gathered from a PubMed search of "hypothyroidism," "levothyroxine," "liothyronine," and "desiccated thyroid extract," among other keywords. PubMed searches were supplemented by Google Scholar and the authors' prior knowledge of the subject.

EVIDENCE SYNTHESIS

In most LT4-treated patients, normalization of serum thyrotropin levels results in decreased serum T3/T4 ratio, with relatively lower serum T3 levels; in at least 15% of the cases, serum T3 levels are below normal. These changes can lead to a reduction in TH action, which would explain the slower rate of metabolism and elevated serum cholesterol levels. A small percentage of patients might also experience persistent symptoms of hypothyroidism, with impaired cognition and tiredness. We propose that such patients carry a key clinical factor, for example, specific genetic and/or immunologic makeup, that is well compensated while the thyroid function is normal but might become apparent when compounded with relatively lower serum T3 levels.

CONCLUSIONS

After excluding other explanations, physicians should openly discuss and consider therapy with LT4 and LT3 with those hypothyroid patients who have persistent symptoms or metabolic abnormalities despite normalization of serum thyrotropin level. New clinical trials focused on symptomatic patients, genetic makeup, and comorbidities, with the statistical power to identify differences between monotherapy and combination therapy, are needed.

Importance of Thyroid Hormone level and Genetic Variations in Deiodinases for Patients after Acute Myocardial Infarction: A Longitudinal Observational Study

This study aimed to examine the influence of thyroid hormone (TH) levels and genetic polymorphisms of deiodinases on long-term outcomes after acute myocardial infarction (AMI). In total, 290 patients who have experienced AMI were evaluated for demographic, clinical characteristics, risk factors, TH and NT-pro-BNP. Polymorphisms of TH related genes were included deiodinase 1 (DIO1) (rs11206244-C/T, rs12095080-A/G, rs2235544-A/C), deiodinase 2 (DIO2) (rs225015-G/A, rs225014-T/C) and deiodinase 3 (DIO3) (rs945006-T/G). Both all-cause and cardiac mortality was considered key outcomes. Cox regression model showed that NT-pro-BNP (HR = 2.11; 95% CI = 1.18– 3.78; p = 0.012), the first quartile of fT3, and DIO1 gene rs12095080 were independent predictors of cardiac-related mortality (HR = 1.74; 95% CI = 1.04–2.91; p = 0.034). The DIO1 gene rs12095080 AG genotype (OR = 3.97; 95% CI = 1.45–10.89; p = 0.005) increased the risk for cardiac mortality. Lower fT3 levels and the DIO1 gene rs12095080 are both associated with cardiac-related mortality after AMI.

Nice guideline on thyroid disease: where does it take us with liothyronine?

The new NICE guidelines on thyroid disease and its management do not recommend the routine use of liothyronine, but do not completely rule it out either. Guidelines from the British and European Thyroid Associations are open to a “trial of liothyronine” on an individual basis.

Some patients do not feel well on L-thyroxine despite a serum TSH in the reference range. Key issues to consider in such patients include establishing whether the patient had established hypothyroidism initially, and whether the L-thyroxine has been titrated carefully enough, possibly using small increments, to achieve a careful balance between symptoms and serum TSH concentrations. Patients should also be considered for other causes of the symptoms which may be wide-ranging.

Meta-analyses of several, but small, randomised control trials show no advantage, or disadvantage of liothyronine over L-thyroxine. However, detailed sub-analysis identifies some tantalising results eg on preferential weight loss, patient preference, and possibly genetic markers. Although linked with plausible theoretical explanations, these results may be over-interpreted. The key questions are whether a short-term trial treatment is worthwhile and safe, and whether in the future sub-groups of patients can be identified who may benefit from liothyronine. These questions remain divisive but require additional focussed research.

It could be argued that inflated costs of liothyronine in some countries have either distracted from or helped focus on the science. Costs need to be addressed. However better biomarkers of tissue level thyroid action, and a better understanding of the impact of genetic polymorphisms will help to make progress when choosing if there is a place for liothyronine in the future.

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The Genetic Basis of Thyroid Function: Novel Findings and New Approaches

CONTEXT

Genetic factors are major determinants of thyroid function. Over the last two decades, multiple genetic variants have been associated with variations in normal range thyroid function tests. Most recently, a large-scale genome-wide association study (GWAS) doubled the number of known variants associated with normal range thyrotropin (TSH) and free thyroxine (FT4) levels.

EVIDENCE ACQUISITION

This review summarizes the results of genetic association studies on normal range thyroid function and explores how these genetic variants can be used in future studies to improve our understanding of thyroid hormone regulation and disease.

EVIDENCE SYNTHESIS

Serum TSH and FT4 levels are determined by multiple genetic variants on virtually all levels of the hypothalamus-pituitary-thyroid (HPT) axis. Functional follow-up studies on top of GWAS hits has the potential to discover new key players in thyroid hormone regulation, as exemplified by the identification of the thyroid hormone transporter SLC17A4 and the metabolizing enzyme AADAT. Translational studies may use these genetic variants to investigate causal associations between thyroid function and various outcomes in Mendelian Randomization (MR) studies, to identify individuals with an increased risk of thyroid dysfunction, and to predict the individual HPT axis setpoint.

CONCLUSIONS

Recent genetic studies have greatly improved our understanding of the genetic basis of thyroid function, and have revealed novel pathways involved in its regulation. In addition, these findings have paved the way for various lines of research that can improve our understanding of thyroid hormone regulation and thyroid diseases, as well as the potential use of these markers in future clinical practice.

Patient Experiences and Perceptions Associated with the Use of Desiccated Thyroid Extract

Background and objectives: It is unclear why many patients with hypothyroidism prefer the use of desiccated thyroid extract (DTE) as a thyroid hormone replacement formulation over levothyroxine (LT4) treatment, as recommended by clinical practice guidelines. We analyzed patient-reported information from patient online forums to better understand patient preferences for and attitudes toward the use of DTE to treat hypothyroidism. Materials and Methods: We conducted a mixed-methods study by evaluating the content of online posts from three popular hypothyroidism forums from patients currently taking DTE (n = 673). From these posts, we extracted descriptive information on patient demographics and clinical characteristics and qualitatively analyzed posts’ content to explore patient perceptions on DTE and other therapies further. Results: Nearly half (46%) of the patients reported that a clinician initially drove their interest in trying DTE. Patients described many reasons for switching from a previous therapeutic approach to DTE, including lack of improvement in hypothyroidism-related symptoms (58%) and the development of side effects (22%). The majority of patients described DTE as moderately to majorly effective overall (81%) and more effective than the previous therapy (77%). The most frequently described benefits associated with DTE use were an improvement in symptoms (56%) and a change in overall well-being (34%). One-fifth of patients described side effects related to the use of DTE. Qualitative analysis of posts’ content supported these findings and raised additional issues around the need for individualizing therapy approaches for hypothyroidism (e.g., a sense of each patient has different needs), as well as difficulties obtaining DTE (e.g., issues with pharmacy availability). Conclusions: Lack of individualized treatment and a feeling of not been listened to were recurrent themes among DTE users. A subset of patients may prefer DTE to LT4 for many reasons, including perceived better effectiveness and improved overall well-being, despite the risks associated with DTE.

The Year in Basic Thyroidology

Basic research in 2019 yielded exciting discoveries and advancements in thyroidology. Specifically, there have been breakthroughs in our understanding of the molecular actions of thyroid hormone and thyroid hormone receptors, thyroid hormone metabolism and transport, autoimmunity, and thyroid cancer. Next, I summarize important studies published over the past year and whose major data I presented during the 89th American Thyroid Association annual meeting at the opening plenary session The Year in Thyroidology.

Mood Disorders in Levothyroxine-Treated Hypothyroid Women

Background: Hypothyroidism has several symptoms (weight gain, arrhythmias, mood changes, etc.). The aims of this study were (1) to assess the prevalence of anxiety and depression in levothyroxine-treated hypothyroid women and in women without hypothyroidism; (2) to identify variables associated with anxiety and depression. Methods: A case-control study was performed with 393 women. Case-group: 153 levothyroxine-treated hypothyroid women. Control-group: 240 women without hypothyroidism. Convenience sampling. Instrument: The Hamilton Hospital Anxiety and Depression Scale (HADS), and a sociodemographic questionnaire. Results: The prevalence of anxiety in levothyroxine-treated hypothyroid women was higher than in women without hypothyroidism (29.4% vs. 16.7%, χ² p < 0.001). The prevalence of depression in the case group was higher than in the control group (13.1% vs. 4.6%, χ² p < 0.001). Levothyroxine-treated hypothyroid women were more likely to have anxiety (OR = 2.08, CI: 1.28-3.38) and depression (OR = 3.13, IC = 1.45-6.45). Conclusion: In spite of receiving treatment with levothyroxine, women with hypothyroidism are more likely to have depression and anxiety. Health professionals need to assess the mood of women with hypothyroidism. Although levothyroxine is a good treatment for the symptoms of hypothyroidism, it may not be enough to prevent development or persistence of depression and anxiety by itself.

T4 + T3 combination therapy: any progress?

Guidelines on T4 + T3 combination therapy were published in 2012. This review investigates whether the issue is better understood 7 years later. Dissatisfaction with the outcome of T4 monotherapy remains high. Persistent symptoms consist mostly of fatigue, weight gain, problems with memory and thinking and mood disturbances. T4 monotherapy is associated with low serum T3 levels, which often require TSH-suppressive doses of L-T4 for normalization. Peripheral tissue thyroid function tests during T4 treatment indicate mild hyperthyroidism at TSH < 0.03 mU/L and mild hypothyroidism at TSH 0.3-5.0 mU/L; tissues are closest to euthyroidism at TSH 0.03-0.3 mU/L. This is explained by the finding that whereas T4 is usually ubiquinated and targeted for proteasomal degradation, hypothalamic T4 is rather stable and less sensitive to ubiquination. A normal serum TSH consequently does not necessarily indicate a euthyroid state. Persistent symptoms in L-T4 treated patients despite a normal serum TSH remain incompletely understood. One hypothesis is that a SNP (Thr92Ala) in DIO2 (required for local production of T3 out of T4) interferes with its kinetics and/or action, resulting in a local hypothyroid state in the brain. Effective treatment of persistent symptoms has not yet realized. One may try T4 + T3 combination treatment in selected patients as an experimental n = 1 study. The 2012 ETA guidelines are still valid for this purpose. More well-designed randomized clinical trials in selected patients are key in order to make progress. In the meantime the whole issue has become rather complicated by commercial and political overtones, as evident from skyrocketing prices of T3 tablets, aggressive pressure groups and motions in the House of Lords.

Levothyroxine Dose Adjustment to Optimise Therapy Throughout a Patient's Lifetime

Levothyroxine is the standard therapy for patients with hypothyroidism, a condition that affects up to 5% of people worldwide. While levothyroxine therapy has substantially improved the lives of millions of hypothyroid patients since its introduction in 1949, the complexity of maintaining biochemical and clinical euthyroidism in patients undergoing treatment with levothyroxine cannot be underestimated. Initial dosing of levothyroxine can vary greatly and may be based on the amount of residual thyroid function retained by the patient, the body weight or lean body mass of the patient, and thyroid-stimulating hormone levels. As levothyroxine is usually administered over a patient's lifetime, physiological changes throughout life will affect the dose of levothyroxine required to maintain euthyroidism. Furthermore, dose adjustments may need to be made in patients with concomitant medical conditions, in patients taking certain medications, as well as in elderly patients. Patients who have undergone any weight or hormonal changes may require dose adjustments, and the majority of pregnant women require increased doses of levothyroxine. Optimal treatment of hypothyroidism requires a partnership between patient and physician. The physician is tasked with vigilant appraisal of the patient's status based on a thorough clinical and laboratory assessment and appropriate adjustment of their levothyroxine therapy. The patient in turn is tasked with medication adherence and reporting of symptomatology and any changes in their medical situation. The goal is consistent maintenance of euthyroidism, without the patient experiencing the adverse events and negative health consequences of under- or overtreatment.Funding Merck.Plain Language Summary Plain language summary available for this article.

Paradigms of Dynamic Control of Thyroid Hormone Signaling

Thyroid hormone (TH) molecules enter cells via membrane transporters and, depending on the cell type, can be activated (i.e., T4 to T3 conversion) or inactivated (i.e., T3 to 3,3'-diiodo-l-thyronine or T4 to reverse T3 conversion). These reactions are catalyzed by the deiodinases. The biologically active hormone, T3, eventually binds to intracellular TH receptors (TRs), TRα and TRβ, and initiate TH signaling, that is, regulation of target genes and other metabolic pathways. At least three families of transmembrane transporters, MCT, OATP, and LAT, facilitate the entry of TH into cells, which follow the gradient of free hormone between the extracellular fluid and the cytoplasm. Inactivation or marked downregulation of TH transporters can dampen TH signaling. At the same time, dynamic modifications in the expression or activity of TRs and transcriptional coregulators can affect positively or negatively the intensity of TH signaling. However, the deiodinases are the element that provides greatest amplitude in dynamic control of TH signaling. Cells that express the activating deiodinase DIO2 can rapidly enhance TH signaling due to intracellular buildup of T3. In contrast, TH signaling is dampened in cells that express the inactivating deiodinase DIO3. This explains how THs can regulate pathways in development, metabolism, and growth, despite rather stable levels in the circulation. As a consequence, TH signaling is unique for each cell (tissue or organ), depending on circulating TH levels and on the exclusive blend of transporters, deiodinases, and TRs present in each cell. In this review we explore the key mechanisms underlying customization of TH signaling during development, in health and in disease states.

Deiodinases and their intricate role in thyroid hormone homeostasis

The deiodinase family of enzymes mediates the activation and inactivation of thyroid hormone. The role of these enzymes in the regulation of the systemic concentrations of thyroid hormone is well established and underpins the treatment of common thyroid diseases. Interest in this field has increased in the past 10 years as the deiodinases became implicated in tissue development and homeostasis, as well as in the pathogenesis of a wide range of human diseases. Three deiodinases have been identified, namely, types 1, 2 and 3 iodothyronine deiodinases, which differ in their catalytic properties and tissue distribution. Notably, the expression of these enzymes changes during the lifetime of an individual in relation to the different needs of each organ and to ageing. The systemic homeostatic role of deiodinases clearly emerges during changes in serum concentrations of thyroid hormone, as seen in patients with thyroid dysfunction. By contrast, the role of deiodinases at the tissue level allows thyroid hormone signalling to be finely tuned within a given cell in a precise time-space window without perturbing serum concentrations of thyroid hormone. This Review maps the overall functional role of the deiodinases and explores challenges and novel opportunities arising from the expanding knowledge of these 'master' components of the thyroid homeostatic system.

The Swinging Pendulum in Treatment for Hypothyroidism: From (and Toward?) Combination Therapy

Thyroid hormone replacement for hypothyroidism can be achieved via several approaches utilizing different preparations of thyroid hormones, T3, and/or T4. "Combination therapy" involves administration of both T3 and T4, and was technically the first treatment for hypothyroidism. It was lauded as a cure for the morbidity and mortality associated with myxedema, the most severe presentation of overt hypothyroidism. In the late nineteenth and the early Twentieth centuries, combination therapy per se could consist of thyroid gland transplant, or more commonly, consumption of desiccated animal thyroid, thyroid extract, or thyroglobulin. Combination therapy remained the mainstay of therapy for decades despite development of synthetic formulations of T4 and T3, because it was efficacious and cost effective. However, concerns emerged about the consistency and potency of desiccated thyroid hormone after cases were reported detailing either continued hypothyroidism or iatrogenic thyrotoxicosis. Development of the TSH radioimmunoassay and discovery of conversion of T4-to-T3 in humans led to a major transition in clinical practices away from combination therapy, to adoption of levothyroxine "monotherapy" as the standard of care. Levothyroxine monotherapy has a favorable safety profile and can effectively normalize the serum TSH, the most sensitive marker of hypothyroidism. Whether levothyroxine monotherapy restores thyroid hormone signaling within all tissues remains controversial. Evidence of persistent signs and symptoms of hypothyroidism during levothyroxine monotherapy at doses that normalize serum TSH is mounting. Hence, in the last decade there has been acknowledgment by all thyroid professional societies that there may be a role for the use of combination therapy; this represents a significant shift in the clinical practice guidelines. Further bolstering this trend are the recent findings that the Thr92AlaD2 polymorphism may reduce thyroid hormone signaling, resulting in localized and systemic hypothyroidism. This strengthens the hypothesis that treatment options could be personalized, taking into consideration genotypes and comorbidities. The development of long-acting formulations of liothyronine and continued advancements in development of thyroid regenerative therapy, may propel the field closer to adoption of a physiologic thyroid hormone replacement regimen with combination therapy.

Type 2 deiodinase polymorphism causes ER stress and hypothyroidism in the brain

Levothyroxine (LT4) is a form of thyroid hormone used to treat hypothyroidism. In the brain, T4 is converted to the active form T3 by type 2 deiodinase (D2). Thus, it is intriguing that carriers of the Thr92Ala polymorphism in the D2 gene (DIO2) exhibit clinical improvement when liothyronine (LT3) is added to LT4 therapy. Here, we report that D2 is a cargo protein in ER Golgi intermediary compartment (ERGIC) vesicles, recycling between ER and Golgi. The Thr92-to-Ala substitution (Ala92-D2) caused ER stress and activated the unfolded protein response (UPR). Ala92-D2 accumulated in the trans-Golgi and generated less T3, which was restored by eliminating ER stress with the chemical chaperone 4-phenyl butyric acid (4-PBA). An Ala92-Dio2 polymorphism-carrying mouse exhibited UPR and hypothyroidism in distinct brain areas. The mouse refrained from physical activity, slept more, and required additional time to memorize objects. Enhancing T3 signaling in the brain with LT3 improved cognition, whereas restoring proteostasis with 4-PBA eliminated the Ala92-Dio2 phenotype. In contrast, primary hypothyroidism intensified the Ala92-Dio2 phenotype, with only partial response to LT4 therapy. Disruption of cellular proteostasis and reduced Ala92-D2 activity may explain the failure of LT4 therapy in carriers of Thr92Ala-DIO2.