Prevalent polymorphism in thyroid hormone-activating enzyme leaves a genetic fingerprint that underlies associated clinical syndromes

A DIO2 missense mutation and its impact on fetal response to PRRSV infection

BACKGROUND

Porcine reproductive and respiratory syndrome virus 2 (PRRSV-2) infection during late gestation substantially lowers fetal viability and survival. In a previous genome-wide association study, a single nucleotide polymorphism on chromosome 7 was significantly associated with probability of fetuses being viable in response to maternal PRRSV-2 infection at 21 days post maternal inoculation. The iodothyronine deiodinase 2 (DIO2) gene, located ~ 14 Kilobase downstream of this SNP, was selected as a priority candidate related to fetal susceptibility following maternal PRRSV-2 infection. Our objectives were to identify mutation(s) within the porcine DIO2 gene and to determine if they were associated with fetal outcomes after PRRSV-2 challenge. Sequencing of the DIO2, genotyping identified variants, and association of DIO2 genotypes with fetal phenotypes including DIO2 mRNA levels, viability, survival, viral loads, cortisol and thyroid hormone levels, and growth measurements were conducted.

RESULTS

A missense variant (p.Asn91Ser) was identified in the parental populations from two independent PRRSV-2 challenge trials. This variant was further genotyped to determine association with fetal PRRS outcomes. DIO2 mRNA levels in fetal heart and kidney differed by the genotypes of Asn91Ser substitution with significantly greater DIO2 mRNA expression in heterozygotes compared with wild-type homozygotes (P < 0.001 for heart, P = 0.002 for kidney). While Asn91Ser did not significantly alter fetal viability and growth measurements, interaction effects of the variant with fetal sex or trial were identified for fetal viability or crown rump length, respectively. However, this mutation was not related to dysregulation of the hypothalamic-pituitary-adrenal and thyroid axis, indicated by no differences in circulating cortisol, T4, and T3 levels in fetuses of the opposing genotypes following PRRSV-2 infection.

CONCLUSIONS

The present study suggests that a complex relationship among DIO2 genotype, DIO2 expression, fetal sex, and fetal viability may exist during the course of fetal PRRSV infection. Our study also proposes the increase in cortisol levels, indicative of fetal stress response, may lead to fetal complications, such as fetal compromise, fetal death, or premature farrowing, during PRRSV infection.

Mapping Thyroid Hormone Action in the Human Brain

Background: Normal brain development, mood, and cognitive functions depend on thyroid hormone (TH) action. However, little is known about how TH mediates its actions in the human brain. This is due to limited access to human brains deprived of TH during fetal and early postnatal life, as well as from adults with altered thyroid status. One way to partially bypass these limitations is by using magnetic resonance imaging and spectroscopy, two neuroimaging techniques that provide detailed, noninvasive information on human brain structure and function. Another way is using human-induced pluripotent stem cell (hiPSCs)-derived three-dimensional in vitro systems, known as brain organoids, which allow for the study of fundamental aspects of the early stages of human brain development. Summary: This narrative review focuses on neuroimaging and brain organoid studies. Neuroimaging of human brains performed in individuals with different thyroid conditions provides information on the volume, myelination, blood flow, neural activity, and connectivity of different areas. Such studies show that suboptimal thyroid status can impact human brain development and its normal function throughout life. This is true not only for patients with sporadic congenital hypothyroidism, during pregnancy or early after birth, but also for adult patients with hypo- or hyperthyroidism, patients carrying mutations that manifest as impaired sensitivity to TH, and even for normal individuals during aging. Studies using brain organoids generated from hiPSCs of healthy individuals or patients with thyroid genetic conditions provide insights into how TH can impact the early development of the human cerebral cortex. Conclusions: The developmental alterations in children born to mothers with different degrees of gestational hypothyroidism or who developed hypothyroidism early in life are remarkable, affecting multiple brain regions and pathways, including the cerebral cortex, hippocampus, cerebellum, interhemispheric and corticospinal tracts, and associative nuclei. The data connecting such changes to poor neurological outcomes in adult patients with hypothyroidism represent an objective link between thyroid-specific functional brain alterations and behavior. Growing brain organoids require TH, which is critical for human neurogenesis and oligodendrogenesis. These models have proven useful in screening drugs with potential therapeutic effects for patients with genetic thyroid diseases.

Genetic Background Strongly Influences the Impact of Carrying the Thr92Ala-DIO2 Polymorphism in the Male Mouse

About half of the world population carries at least one allele of the Ala92-DIO2, which slows down the activity of the type 2 deiodinase (D2), the enzyme that activates T4 to T3. Carrying the Ala92-DIO2 allele has been associated with increased body mass index and insulin resistance, but this has not been reproduced in all populations. To test if the genetic background affects the impact of this polymorphism, here we studied the genetically distant C57Bl/6J (B6) and FVB/N (FVB) mice carrying the Ala92-Dio2 allele as compared to control mice carrying the Thr92-Dio2 allele. Whereas B6-Ala92-Dio2 and B6-Thr92-Dio2 mice-fed chow or high-fat diet-behaved metabolically similar in studies using indirect calorimetry, glucose- and insulin tolerance tests, and measuring white adipose tissue (WAT) weight and liver steatosis, major differences were observed between FVB-Ala92-Dio2 and FVB-Thr92-Dio2 mice: carrying the Ala92-Dio2 allele (on a chow diet) resulted in hypercholesterolemia, smaller WAT pads, hepatomegaly, steatosis, and transcriptome changes in the interscapular brown adipose tissue (iBAT) typical of ER stress and apoptosis. Acclimatization at thermoneutrality (30 °C) eliminated most of the metabolic phenotype, indicating that impaired adaptive (BAT) thermogenesis can be involved. In conclusion, the metabolic impact of carrying the Ala92-Dio2 allele depends greatly on the genetic background of the mouse, varying from no phenotype in B6 mice to a major phenotype in FVB mice. These results will help the planning of future clinical trials studying the Thr92Ala-DIO2 polymorphism and may explain why some clinical studies performed in different populations across the globe have obtained inconsistent results.

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.

Brain hypothyroidism silences the immune response of microglia in Alzheimer's disease animal model

Thyroid hormone (TH) imbalance is linked to the pathophysiology of reversible dementia and Alzheimer's disease (AD). It is unclear whether tissue hypothyroidism occurs in the AD brain and how it affects on AD pathology. We find that decreased iodothyronine deiodinase 2 is correlated with hippocampal hypothyroidism in early AD model mice before TH alterations in the blood. TH deficiency leads to spontaneous activation of microglia in wild-type mice under nonstimulated conditions, resulting in lowered innate immune responses of microglia in response to inflammatory stimuli or amyloid-β. In AD model mice, TH deficiency aggravates AD pathology by reducing the disease-associated microglia population and microglial phagocytosis. We find that TH deficiency reduces microglial ecto-5'-nucleotidase (CD73) and inhibition of CD73 leads to impaired innate immune responses in microglia. Our findings reveal that TH shapes microglial responses to inflammatory stimuli including amyloid-β, and brain hypothyroidism in early AD model mice aggravates AD pathology by microglial dysfunction.

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.

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 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.

Deiodinases control local cellular and systemic thyroid hormone availability

Iodothyronine deiodinases (DIO) are a family of selenoproteins controlling systemic and local availability of the major thyroid hormone l-thyroxine (T4), a prohormone secreted by the thyroid gland. T4 is activated to the active 3,3'-5-triiodothyronine (T3) by two 5'-deiodinases, DIO1 and DIO2. DIO3, a 5-deiodinase selenoenzyme inactivates both the prohormone T4 and its active form T3. DIOs show species-specific different patterns of temporo-spatial expression, regulation and function and exhibit different mechanisms of reaction and inhibitor sensitivities. The main regulators of DIO expression and function are the thyroid hormone status, several growth factors, cytokines and altered pathophysiological conditions. Selenium (Se) status has a modest impact on DIO expression and translation. DIOs rank high in the priority of selenium supply to various selenoproteins; thus, their function is impaired only during severe selenium deficiency. DIO variants, polymorphisms, SNPs and rare mutations have been identified. Development of DIO isozyme selective drugs is ongoing. A first X-ray structure has been reported for DIO3. This review focusses on the biochemical characteristics and reaction mechanisms, the relationships between DIO selenoproteins and their importance for local and systemic provision of the active hormone T3. Nutritional, pharmacological, and environmental factors and inhibitors, such as endocrine disruptors, impact DIO functions.

A New Treatment Protocol of Combined High-Dose Levothyroxine and Repetitive Transcranial Magnetic Stimulation for the Treatment of Rapid-Cycling Bipolar Spectrum Disorders: A Cohort Evaluation of 55 Patients

Background: Bipolar spectrum disorders (BSD) are highly disabling, with rapid cycling being treatment resistant. High-dose levothyroxine (HDT) has been reported to be effective. Diagnosis is associated with mutations in thyroid-activating enzymes and cerebral transporter protein carrier. Repetitive transcranial magnetic stimulation (rTMS) has neuroplastic effects. Methods: We report data on 55 severely symptomatic patients with rapid-cycling BSD treated with a combination protocol of HDT and rTMS. Of the patients, 31 patients (56.4%) were female and 40 (72.7%) had at least one additional diagnosis. Results: Patients were evaluated at three monthly intervals after acute treatment. Remission was measured using the Sheehan Disability Scale (SDS). The average number of medications prescribed was 1.8, with 32 patients (58.2%) needing only levothyroxine. The average dose of levothyroxine was 303.7 mcg (50 mcg−1000 mcg). A total of 53 patients were in remission (96.4%), with an average duration of 2.0 years. The SDS scores decreased significantly (Cohen’s d = 2.61 (95% C.I. 1.81 to 2.83, p < 0.001). One patient had reversible side effects. A total of 52 (94.3%) patients had Deiodinase 1 and 2 (DiO1/DiO2) or SLCO1C1 protein carrier gene mutations. Conclusion: The data support the safety and acceptability of combined HDT/rTMS. Patients achieved long remissions with substantial improvements in quality of life.

Emerging roles of ER-resident selenoproteins in brain physiology and physiopathology

The brain has a very high oxygen consumption rate and is particularly sensitive to oxidative stress. It is also the last organ to suffer from a loss of selenium (Se) in case of deficiency. Se is a crucial trace element present in the form of selenocysteine, the 21st proteinogenic amino acid present in selenoproteins, an essential protein family in the brain that participates in redox signaling. Among the most abundant selenoproteins in the brain are glutathione peroxidase 4 (GPX4), which reduces lipid peroxides and prevents ferroptosis, and selenoproteins W, I, F, K, M, O and T. Remarkably, more than half of them are proteins present in the ER and recent studies have shown their involvement in the maintenance of ER homeostasis, glycoprotein folding and quality control, redox balance, ER stress response signaling pathways and Ca2+ homeostasis. However, their molecular functions remain mostly undetermined. The ER is a highly specialized organelle in neurons that maintains the physical continuity of axons over long distances through its continuous distribution from the cell body to the nerve terminals. Alteration of this continuity can lead to degeneration of distal axons and subsequent neuronal death. Elucidation of the function of ER-resident selenoproteins in neuronal pathophysiology may therefore become a new perspective for understanding the pathophysiology of neurological diseases. Here we summarize what is currently known about each of their molecular functions and their impact on the nervous system during development and stress.

Selenoproteins in brain development and function

Expression of selenoproteins is widespread in neurons of the central nervous system. There is continuous evidence presented over decades that low levels of selenium or selenoproteins are linked to seizures and epilepsy indicating a failure of the inhibitory system. Many developmental processes in the brain depend on the thyroid hormone T3. T3 levels can be locally increased by the action of iodothyronine deiodinases on the prohormone T4. Since deiodinases are selenoproteins, it is expected that selenoprotein deficiency may affect development of the central nervous system. Studies in genetically modified mice or clinical observations of patients with rare diseases point to a role of selenoproteins in brain development and degeneration. In particular selenoprotein P is central to brain function by virtue of its selenium transport function into and within the brain. We summarize which selenoproteins are essential for the brain, which processes depend on selenoproteins, and what is known about genetic deficiencies of selenoproteins in humans. This review is not intended to cover the potential influence of selenium or selenoproteins on major neurodegenerative disorders in human.

Age Worsens the Cognitive Phenotype in Mice Carrying the Thr92Ala-DIO2 Polymorphism

The Thr92Ala-Dio2 polymorphism has been associated with reduced cognition in 2-month-old male mice and increased risk for cognitive impairment and Alzheimer's disease in African Americans. This has been attributed to reduced thyroid hormone (TH) signaling and endoplasmic reticulum (ER) stress in the brain. Here we studied the Thr92Ala-Dio2 mouse model and saw that older male mice (7-8-month-old) exhibited a more severe cognition impairment, which extended to different aspects of declarative and working memories. A similar phenotype was observed in 4-5-month-old female mice. There were no structural alterations in the prefrontal cortex (PFC) and hippocampus of the Thr92Ala-Dio2 mouse. Nonetheless, in both male and female PFC, there was an enrichment in genes associated with TH-dependent processes, ER stress, and Golgi apparatus, while in the hippocampus there was additional enrichment in genes associated with inflammation and apoptosis. Reduced TH signaling remains a key mechanism of disease given that short-term treatment with L-T3 rescued the cognitive phenotype observed in males and females. We conclude that in mice, age is an additional risk factor for cognitive impairment associated with the Thr92Ala-Dio2 polymorphism. In addition to reduced TH signaling, ER-stress, and involvement of the Golgi apparatus, hippocampal inflammation and apoptosis were identified as potentially important mechanisms of a disease.

Type 2 deiodinase p.Thr92Ala polymorphism does not affect the severity of obesity and weight loss after bariatric surgery

A single nucleotide polymorphism in the Type 2 deiodinase (DIO2) gene (p.Thr92Ala) was found to be associated with hypertension, type 2 diabetes mellitus (T2DM), insulin resistance, and body mass index (BMI). We retrospectively evaluated 182 patients to assess whether the DIO2 p.Thr92Ala was associated with severe obesity and response to bariatric surgery. Genomic DNA was extracted from peripheral blood leukocytes before surgery. Glycemic control parameters, cardiometabolic risk biomarkers (waist circumference, lipid assessment and blood pressure) and hormonal parameters were assessed at baseline and after surgery. Based on genotype evaluation, 78/182 (42.9%) patients were homozygous wild-type (Thr/Thr), 83/182 (45.6%) heterozygous (Thr/Ala), and 21/182 (11.5%) rare homozygous (Ala/Ala). Age at the time of the first evaluation in our Unit was significantly lower in patients with DIO2 p.Thr92Ala. No significant association was observed between DIO2 p.Thr92Ala and BMI, excess weight, waist circumference, Homa Index. The prevalence of comorbidities was not associated with allele distribution except for hypertension that was more frequent in wild-type patients (p = 0.03). After bariatric surgery, excess weight loss (EWL) % and remission from comorbidities occurred without differences according to genotypes. DIO2 p.Thr92Ala does not affect the severity of obesity and its complications, but it seems to determine an earlier onset of morbid obesity. The presence of polymorphism seems not to impact on the response to bariatric surgery, both in terms of weight loss and remission of comorbidities.

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.

Higher thyroid function is associated with accelerated hippocampal volume loss in Alzheimer's disease

BACKGROUND

In epidemiological studies, higher thyroid hormone (TH) levels have been associated with lower brain volume and increased risk of Alzheimer's disease (AD) in elderly individuals. However, the relationships between serum THs and hippocampal atrophy rates have previously not been investigated.

METHODS

A prospective study of patients with AD (n = 55), stable mild cognitive impairment (sMCI; n = 84) and healthy controls (n = 29) recruited at a single memory clinic. We investigated whether serum THs were associated with magnetic resonance imaging (MRI)-estimated hippocampal volumes at baseline and with longitudinal alterations, defined as annualized percent changes.

RESULTS

Serum levels of free triiodothyronine (FT3) and FT3/free thyroxine (FT4) ratio were reduced in AD and sMCI patients compared with the controls (p < 0.05). Hierarchical linear regression analyses showed that higher serum FT3/FT4 ratio was associated with greater baseline hippocampal volume in all study groups. Only in AD patients, higher serum FT4 was associated with lower baseline volume of the left hippocampus. Finally, exclusively in the AD group, higher serum levels of FT3 and FT3/FT4 ratio, and lower serum TSH levels, were associated with greater annual hippocampal volume loss.

CONCLUSIONS

In all study groups, FT3/FT4 ratio was related to baseline hippocampal volume. However, only in AD patients, higher levels of THs were associated with greater annual loss of hippocampal volume, suggesting that excessive TH levels exert a deleterious effect on the hippocampus in the presence of existing AD neuropathology.

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.

A new approach for the treatment of subthreshold bipolar disorders: Targeted high dose levothyroxine and repetitive transcranial magnetic stimulation for mitochondrial treatment

Bipolar spectrum disorder includes Bipolar I, Bipolar II and subthreshold bipolar disorders (BD). The condition is highly prevalent, disabling and associated with high mortality. Failure of diagnosis is high. Subthreshold presentations present as 4 or more changes in polarity, are generally less responsive to standard treatment and as a result, drug combinations are often needed. High Dose Levothyroxine (HDT) has been reported to be safe and effective with this condition. Treatment response has been associated with mutations in thyroid activating enzymes and intra cerebral transporter protein carrier. Repetitive Transcranial Magnetic Stimulation (rTMS) has been shown to be effective in bipolar depression and has been proved to have neuroplastic effect. Present authors had reported clinical evidence of safe and effective use of a combination treatment protocol. Potential mechanisms of action of the combined treatment protocol and the role of mitochondria function are discussed.

Deiodinases and the Three Types of Thyroid Hormone Deiodination Reactions

Thyroid hormone (TH) signaling is strictly regulated by iodothyronine deiodinase activity, which both preserves the circulating levels of the biologically active triiodothyronine (T3) and regulates TH homeostasis at the local level, in a cell- and time-dependent manner. Three deiodinases have been identified-namely iodothyronine deiodinase 1 (DIO1), DIO2, and DIO3-that differ in their catalytic properties and tissue distribution. The deiodinases represent a dynamic system that changes in the different stages of life according to their functions and roles in various cell types and tissues. Deiodinase activity at the tissue level permits cell-targeted fine regulation of TH homeostasis, mediating the activation (DIO1 and DIO2) and inactivation (DIO3) of THs. Deiodinase homeostasis is the driving force that leads T3-target cells towards customized TH signaling, which takes into account both the hormonal circulating levels and the tissue-specific response. This review analyzes the complex role of deiodinases in physiological and pathological contexts, exploring new challenges and opportunities deriving from a deeper knowledge of the dynamics underlying their roles and functions.

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.

Deiodinases and the Metabolic Code for Thyroid Hormone Action

Deiodinases modify the biological activity of thyroid hormone (TH) molecules, ie, they may activate thyroxine (T4) to 3,5,3'-triiodothyronine (T3), or they may inactivate T3 to 3,3'-diiodo-L-thyronine (T2) or T4 to reverse triiodothyronine (rT3). Although evidence of deiodination of T4 to T3 has been available since the 1950s, objective evidence of TH metabolism was not established until the 1970s. The modern paradigm considers that the deiodinases not only play a role in the homeostasis of circulating T3, but they also provide dynamic control of TH signaling: cells that express the activating type 2 deiodinase (D2) have enhanced TH signaling due to intracellular build-up of T3; the opposite is seen in cells that express type 3 deiodinase (D3), the inactivating deiodinase. D2 and D3 are expressed in metabolically relevant tissues such as brown adipose tissue, skeletal muscle and liver, and their roles have been investigated using cell, animal, and human models. During development, D2 and D3 expression customize for each tissue/organ the timing and intensity of TH signaling. In adult cells, D2 is induced by cyclic adenosine monophosphate (cAMP), and its expression is invariably associated with enhanced T3 signaling, expression of PGC1 and accelerated energy expenditure. In contrast, D3 expression is induced by hypoxia-inducible factor 1α (HIF-1a), dampening T3 signaling and the metabolic rate. The coordinated expression of these enzymes adjusts TH signaling in a time- and tissue-specific fashion, affecting metabolic pathways in health and disease states.

Assessment of children in the autistic spectrum disorder that carry the Thr92Ala-DIO2 polymorphism

INTRODUCTION

A polymorphism in the type 2 deiodinase (Thr92Ala-DIO2) gene has been associated with behavioral and cognitive dysfunction as well as neurodegeneration and oxidative stress in the central nervous system.

OBJECTIVE

To test whether the minor allele (Ala92) frequency (MAF) is increased in children in the autism spectrum disorder (ASD), and whether carriers of the minor allele exhibit more severe symptoms and/or worse adaptive behavior.

STUDY DESIGN

ASD children were evaluated at baseline and yearly throughout the study by psychologists using the following tools: autism behavior checklist, Vineland Adaptative Behaviour Scales II, non-verbal intelligence test SON-R 21/2-7, SON-R 6-40, Weschler scale for intelligence, and autism treatment evaluation checklist.

SETTINGS

Academic outpatient mental health facility in Sao Paulo, Brazil.

PARTICIPANTS

ASD boys and girls younger than 18 years of age. 132 consecutive ASD children, mostly boys (~ 80%); ~ 50% was classified as verbal. Exclusion criteria were coexistence of sensory and/or physical impairment, or any associated genetic syndromes.

RESULTS

Median follow-up was for an uninterrupted period of 937 days (139-1375 days), which did not vary significantly among the genotypes. The MAF was 47% in ASD patients vs. 51% in a local reference population with similar ethnic background; the clinical severity and progression were not affected by the minor allele. Carriers of the minor allele exhibited higher adaptive behavior in the domains "daily living skills" and "communication", which correlated positively with the dose of the minor allele.

CONCLUSION

The MAF is not different in ASD children, but carriers of the Thr92Ala-DIO2 polymorphism exhibited higher adaptive behavior.

The Neurobiology of Selenium: Looking Back and to the Future

Eighteen years ago, unexpected epileptic seizures in Selenop-knockout mice pointed to a potentially novel, possibly underestimated, and previously difficult to study role of selenium (Se) in the mammalian brain. This mouse model was the key to open the field of molecular mechanisms, i.e., to delineate the roles of selenium and individual selenoproteins in the brain, and answer specific questions like: how does Se enter the brain; which processes and which cell types are dependent on selenoproteins; and, what are the individual roles of selenoproteins in the brain? Many of these questions have been answered and much progress is being made to fill remaining gaps. Mouse and human genetics have together boosted the field tremendously, in addition to traditional biochemistry and cell biology. As always, new questions have become apparent or more pressing with solving older questions. We will briefly summarize what we know about selenoproteins in the human brain, glance over to the mouse as a useful model, and then discuss new questions and directions the field might take in the next 18 years.

Selenium in Endocrinology-Selenoprotein-Related Diseases, Population Studies, and Epidemiological Evidence

Selenium (Se), apart from iodine, iron, and calcium, is one of the nutrient-derived key elements strongly affecting the endocrine system. However, no specific hormonal "feedback" regulation for Se status has yet been identified, in contrast to the fine-tuned hormone network regulating Ca2+ and phosphate balance or hepcidin-related iron status. Since its discovery as an essential trace element, the effects of Se excess or deficiency on the endocrine system or components of the hypothalamic-pituitary-periphery feedback circuits, the thyroid hormone axis, glucoregulatory and adrenal hormones, male and female gonads, the musculoskeletal apparatus, and skin have been identified. Analysis of the Se status in the blood or via validated biomarkers such as the hepatically derived selenoprotein P provides valuable diagnostic insight and a rational basis for decision making on required therapeutic or preventive supplementation of risk groups or patients. Endocrine-related epidemiological and interventional evidence linking Se status to beneficial or potentially adverse actions of selected selenoproteins mediating most of the (patho-) physiological effects are discussed in this mini-review. Autoimmune thyroid disease, diabetes and obesity, male fertility, as well as osteoporosis are examples for which observational or interventional studies have indicated Se effects. The currently prevailing concept relating Se and selenoproteins to "oxidative stress," reactive oxygen species, radical hypotheses, and related strategies of pharmacological approaches based on various selenium compounds will not be the focus. The crucial biological function of several selenoproteins in cellular redox-regulation and specific enzyme reactions in endocrine pathways will be addressed and put in clinical perspective.

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.

Temporal Pole Responds to Subtle Changes in Local Thyroid Hormone Signaling

To study thyroid hormone (TH) signaling in the human brain, we analyzed published microarray data sets of the temporal pole (Brodmann area 38) of 19 deceased donors. An index of TH signaling built on the expression of 19 well known TH-responsive genes in mouse brains (T3S+) varied from 0.92 to 1.1. After Factor analysis, T3S+ correlated independently with the expression of TH transporters (MCT8, LAT2), TH receptor (TR) beta and TR coregulators (CARM1, MED1, KAT2B, SRC2, SRC3, NCOR2a). Unexpectedly, no correlation was found between T3S+ vs DIO2, DIO3, SRC1, or TRα. An unbiased systematic analysis of the entire transcriptome identified a set of 1649 genes (set #1) with strong positive correlation with T3S+ (r > 0.75). Factor analysis of set #1 identified 2 sets of genes that correlated independently with T3S+, sets #2 (329 genes) and #3 (191 genes). When processed through the Molecular Signatures Data Base (MSigDB), both sets #2 and #3 were enriched with Gene Ontology (GO)-sets related to synaptic transmission and metabolic processes. Ranking individual human brain donors according to their T3S+ led us to identify 1262 genes (set #4) with >1.3-fold higher expression in the top half. The analysis of the overlapped genes between sets #1 and #4 resulted in 769 genes (set #5), which have a very similar MSigDB signature as sets #2 and #3. In conclusion, gene expression in the human temporal pole can be assessed through T3S+ and fluctuates with subtle variations in local TH signaling.

Altered thyroid hormone profile in patients with Alzheimer's disease

BACKGROUND

Epidemiological studies have linked higher levels of thyroid hormones (THs) to increased risk of Alzheimer's disease (AD), whereas in advanced AD, THs have been unchanged or even decreased. In early AD dementia, little is known whether THs are related to AD neuropathology or brain morphology.

METHODS

This was a cross-sectional study of 36 euthyroid AD patients and 34 healthy controls recruited at a single memory clinic. Levels of THs were measured in serum and cerebrospinal fluid (CSF). In addition, we determined AD biomarkers (amyloid-β_1-42, total tau and phosphorylated tau) in CSF and hippocampal and amygdalar volumes using magnetic resonance imaging.

RESULTS

Serum free thyroxine (FT4) levels were elevated, whereas serum free triiodothyronine (FT3)/FT4 and total T3 (TT3)/total T4 (TT4) ratios were decreased, in AD patients compared to controls. In addition, serum TT4 was marginally higher in AD (p = 0.05 vs. the controls). Other TH levels in serum as well as CSF concentrations of THs were similar in both groups, and there were no correlations between THs and CSF AD biomarkers. However, serum FT3 correlated positively with left amygdalar volume in AD patients and serum TT3 correlated positively with left and right hippocampal volume in controls.

CONCLUSIONS

Thyroid hormones were moderately altered in mild AD dementia with increased serum FT4, and in addition, the reduced T3/T4 ratios may suggest decreased peripheral conversion of T4 to T3. Furthermore, serum T3 levels were related to brain structures involved in AD development.

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.

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.

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.

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.

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.

Prevalence of a Iodothyronine Deiodinase 2 gene single nucleotide polymorphism in children with congenital hypothyroidism from Western Romania and impact on TSH levels

The aim of this study was to evaluate the prevalence of the Iodothyronine Deiodinase 2 gene Thr92Ala polymorphism in children from West of Romania with congenital hypothyroidism (CH) and association with TSH levels in response to levothyroxine monotherapy.

Genotyping in 50 children with CH and 52 healthy controls was done using real time PCR.

The results showed that there was no statistical difference between the frequencies of genotypes in patients vs. controls. Patients were treated with L-thyroxine and most had normal values for fT3 and fT4. However, high TSH values were found in 21 patients (42%) after treatment. Among patients with high TSH values, AA genotypes were significantly more prevalent (p = 0.044) than TT and AT genotypes. Our results suggest that for the D2 gene Ala92Thr polymorphism, the AA genotype may be detrimental for achieving euthyroidism in patients with CH and levothyroxine monotherapy, therefore polytherapy could be considered as a better approach in these patients.

Functional Analysis of Genetic Variation in the SECIS Element of Thyroid Hormone Activating Type 2 Deiodinase

CONTEXT

Thyroid hormone is important for normal brain development. The type 2 deiodinase (D2) controls thyroid hormone action in the brain by activating T4 to T3. The enzymatic activity of D2 depends on the incorporation of selenocysteine for which the selenocysteine-insertion sequence (SECIS) element located in the 3' untranslated region is indispensable. We hypothesized that mutations in the SECIS element could affect D2 function, resulting in a neurocognitive phenotype.

OBJECTIVE

To identify mutations in the SECIS element of DIO2 in patients with intellectual disability and to test their functional consequences.

DESIGN, SETTING, AND PATIENTS

The SECIS element of DIO2 was sequenced in 387 patients with unexplained intellectual disability using a predefined pattern of thyroid function tests. SECIS element read-through in wild-type or mutant D2 was quantified by a luciferase reporter system in transfected cells. Functional consequences were assessed by quantifying D2 activity in cell lysate or intact cell metabolism studies.

RESULTS

Sequence analysis revealed 2 heterozygous mutations: c.5703C>T and c.5730A>T, which were also present in the unaffected family members. The functional evaluation showed that both mutations did not affect D2 enzyme activity in cell lysates or intact cells, although the 5730A>T mutation decreased SECIS element read-through by 75%. In the patient harboring the c.5730A>T variant, whole genome sequencing revealed a pathogenic deletion of the STXBP1 gene.

CONCLUSIONS

We report on two families with mutations in the SECIS element of D2. Although functional analysis showed that nucleotide 5730 is important for normal SECIS element read-through, the two variants did not segregate with a distinct phenotype.

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.

Functional and Symptomatic Individuality in the Response to Levothyroxine Treatment

Background: For significant numbers of patients dissatisfied on standard levothyroxine (LT4) treatment for hypothyroidism, patient-specific responses to T4 could play a significant role. Aim: To assess response heterogeneity to LT4 treatment, identifying confounders and hidden clusters within a patient panel, we performed a secondary analysis using data from a prospective cross-sectional and retrospective longitudinal study. Methods: Multivariate and multivariable linear models adjusted for covariates (gender, age, and BMI) were stratified by disease-specific treatment indication. During follow-up, pooled observations were compared from the same patient presenting either with or without self-reported symptoms. Statistical analysis was extended to multilevel models to derive intra-class correlation coefficients and reliability measures during follow-up. Results: Equilibria between TSH, FT4, and FT3 serum concentrations in 342 patients were examined by treatment indication (benign goiter, autoimmune thyroiditis, thyroid carcinoma), consequently displaying complex interactive response patterns. Seventy-seven patients treated with LT4 and monitored for thyroid carcinoma presented, in association with changes in LT4 dose, either with hypothyroid symptoms or symptom-free. Significant biochemical differences appeared between the different presentations. Leveled trajectories by subject to relief from hypothyroid symptoms differed significantly, indicating distinct responses, and denying a single shared outcome. These were formally defined by a high coefficient of the intraclass correlation (ICC1, exceeding 0.60 in all thyroid parameters) during follow-up on multiple visits at the same LT4 dose, when lacking symptoms. The intra-personal clusters were clearly differentiated from random variability by random group resampling. Symptomatic change in these patients was strongly associated with serum FT3, but not with FT4 or TSH concentrations. In 25 patients transitioning from asymptomatic to symptomatically hyperthyroid, FT3 concentrations remained within the reference limits, whilst at the same time marked biochemical differences were apparent between the presentations. Conclusions: Considerable intra-individual clustering occurred in the biochemical and symptomatic responses to LT4 treatment, implying statistically multileveled response groups. Unmasking individual differences in the averaged treatment response hereby highlights clinically distinguishable subgroups within an indiscriminate patient panel. This, through well-designed larger clinical trials will better target the different therapeutic needs of individual patients.

Low serum concentration of free triiodothyronine (FT3) is associated with increased risk of Alzheimer's disease

BACKGROUND

In epidemiological studies, thyroid hormones (THs) have been associated with the risk of dementia. However, little is known of the relation between THs and risk of Alzheimer's disease (AD) or vascular dementia (VaD) in a memory clinic population.

METHODS

In a mono-center study, serum concentrations of thyroid-stimulating hormone (TSH), free thyroxine (FT4), and free triiodothyronine (FT3) were assessed in 302 patients. All patients had subjective or objective mild cognitive impairment and none received treatment with THs. Cox proportional hazards regression analyses was used to determine whether THs at baseline were associated with the risk of conversion to all-cause dementia, AD or VaD.

RESULTS

During the follow-up (mean 2.8 years), 82 (28%) of the patients progressed to dementia [AD, n = 55 (18%) and VaD, n = 17 (6%)]. Serum concentrations of TSH, FT4, and FT3 did not associate with all-cause dementia or VaD. Higher serum FT3 was associated with lower risk of conversion to AD [hazard ratio (HR) = 054; 95% confidence interval (CI): 0.32-0.92 per 1 pmol/L increase]. Furthermore, patients in the lowest serum FT3 quartile had a twofold increased risk of AD compared to those in the highest quartile (HR = 2.63; 95% CI: 1.06-6.47). These associations remained after adjustment for multiple covariates.

CONCLUSIONS

In a memory clinic population, there was an inverse, linear association between serum FT3 and risk of AD whereas THs did not associate with all-cause dementia or VaD. Further studies are needed to determine the underlying mechanisms as well as the clinical significance of these findings.

Pathophysiological relevance of deiodinase polymorphism

PURPOSE OF REVIEW

To assess new findings and clinical implications of deiodinase gene polymorphism. Deiodinases are enzymes that can activate or inactivate thyroid hormone molecules. Whereas the types 1 and 2 deiodinase (D1 and D2) activate thyroxine (T4) to 3,5,3'-triiodothyronine (T3) via deiodination of T4's outer ring, D1 and D3 inactivate both T4 and T3 and terminate thyroid hormone action via deiodination of T4's inner molecular ring. A number of polymorphisms have been identified in the three deiodinase genes; the most investigated and likely to have clinical relevance is the Thr92 substitution for Ala substitution in DIO2 (Thr92Ala-DIO2). There are a number of reports describing the association between the Thr92Ala-DIO2 polymorphism and clinical syndromes that include hypertension, type 2 diabetes, mental disorders, lung injury, bone turnover, and autoimmune thyroid disease; but these associations have not been reproduced in all population studies.

RECENT FINDINGS

A new report indicates that carriers of the Thr92Ala-DIO2 polymorphism exhibit lower D2 catalytic activity and localized/systemic hypothyroidism. This could explain why certain groups of levothyroxine-treated hypothyroid patients have improved quality of life when also treated with liothyronine (LT3). Furthermore, Ala92-D2 was abnormally found in the Golgi apparatus, what could constitute a disease mechanism independent of T3 signaling. Indeed, brain samples of Thr92Ala-DIO2 carriers exhibit gene profiles suggestive of brain degenerative disease. In addition, African American carriers of Thr92Ala-DIO2 exhibit an about 30% higher risk of developing Alzheimer's disease.

SUMMARY

The finding of deiodinase polymorphisms that can diminish thyroid hormone signaling and/or disrupt normal cellular function opens the door to customized treatment of hypothyroidism. Future studies should explore how the racial background modulates the clinical relevance of the Thr92Ala-DIO2 gene polymorphism.

Lessons from Randomised Clinical Trials for Triiodothyronine Treatment of Hypothyroidism: Have They Achieved Their Objectives?

Randomised controlled trials are deemed to be the strongest class of evidence in evidence-based medicine. Failure of trials to prove superiority of T3/T4 combination therapy over standard LT4 monotherapy has greatly influenced guidelines, while not resolving the ongoing debate. Novel studies have recently produced more evidence from the examination of homeostatic equilibria in humans and experimental treatment protocols in animals. This has exacerbated a serious disagreement with evidence from the clinical trials. We contrasted the weight of statistical evidence against strong physiological counterarguments. Revisiting this controversy, we identify areas of improvement for trial design related to validation and sensitivity of QoL instruments, patient selection, statistical power, collider stratification bias, and response heterogeneity to treatment. Given the high individuality expressed by thyroid hormones, their interrelationships, and shifted comfort zones, the response to LT4 treatment produces a statistical amalgamation bias (Simpson's paradox), which has a key influence on interpretation. In addition to drug efficacy, as tested by RCTs, efficiency in clinical practice and safety profiles requires reevaluation. Accordingly, results from RCTs remain ambiguous and should therefore not prevail over physiologically based counterarguments. In giving more weight to other forms of valid evidence which contradict key assumptions of historic trials, current treatment options should remain open and rely on personalised biochemical treatment targets. Optimal treatment choices should be guided by strict requirements of organizations such as the FDA, demanding treatment effects to be estimated under actual conditions of use. Various improvements in design and analysis are recommended for future randomised controlled T3/T4 combination trials.

A Common DIO2 Polymorphism and Alzheimer Disease Dementia in African and European Americans

Context

A common single nucleotide polymorphism in DIO2, Thr92AlaD2, has been associated with a transcriptome typically found in neurodegenerative diseases in postmortem human brain tissue.

Objective

To determine whether Thr92AlaD2 is associated with incident Alzheimer disease (AD).

Design

Population-based study; human brain tissue microarray.

Setting

Community-based cohorts from Chicago and northeastern Illinois and religious clergymen from across the United States constituted the primary population. A representative sample of the U.S. population was used for secondary analyses.

Participants

3054 African Americans (AAs) and 9304 European Americans (EAs).

Main Outcome Measure

Incident AD.

Results

In the primary population, AAs with Thr92AlaD2 had 1.3 times [95% confidence interval (CI), 1.02 to 1.68; P = 0.048] greater odds of developing AD. AAs from a second population with Thr92AlaD2 showed a trend toward increased odds of dementia (odds ratio, 1.33; 95% CI, 0.99 to 1.78; P = 0.06) and 1.35 times greater odds of developing cognitive impairment not demented (CIND; 95% CI, 1.09 to 1.67; P = 0.006). Meta-analysis showed that AAs with Thr92AlaD2 had 1.3 times increased odds of developing AD/dementia (95% CI, 1.07 to 1.58; P = 0.008). In EAs, no association was found between Thr92AlaD2 and AD, dementia, or CIND. Microarray of AA brain tissue identified transcriptional patterns linked to AD pathogenesis.

Conclusions

Thr92AlaD2 was associated with molecular markers known to underlie AD pathogenesis in AAs, translating to an observed phenotype of increased odds of developing AD/dementia in AAs in these populations. Thr92AlaD2 might represent one factor contributing to racial discrepancies in incident AD.

Thyroid hormone and the brain: Mechanisms of action in development and role in protection and promotion of recovery after brain injury

Thyroid hormone (TH) is essential for normal brain development and may also promote recovery and neuronal regeneration after brain injury. TH acts predominantly through the nuclear receptors, TH receptor alpha (THRA) and beta (THRB). Additional factors that impact TH action in the brain include metabolism, activation of thyroxine (T4) to triiodothyronine (T3) by the enzyme 5'-deiodinase Type 2 (Dio2), inactivation by the enzyme 5-deiodinase Type 3 (Dio3) to reverse T3 (rT3), which occurs in glial cells, and uptake by the Mct8 transporter in neurons. Traumatic brain injury (TBI) is associated with inflammation, metabolic alterations and neural death. In clinical studies, central hypothyroidism, due to hypothalamic and pituitary dysfunction, has been found in some individuals after brain injury. TH has been shown, in animal models, to be protective for the damage incurred from brain injury and may have a role to limit injury and promote recovery. Although clinical trials have not yet been reported, findings from in vitro and in vivo models inform potential treatment strategies utilizing TH for protection and promotion of recovery after brain injury.

Current evidence for the treatment of hypothyroidism with levothyroxine/levotriiodothyronine combination therapy versus levothyroxine monotherapy

OBJECTIVE

Hypothyroidism is relatively common, occurring in approximately 5% of the general US population aged ≥12 years. Levothyroxine (LT4) monotherapy is the standard of care. Approximately, 5%-10% of patients who normalise thyroid-stimulating hormone levels with LT4 monotherapy may have persistent symptoms that patients and clinicians may attribute to hypothyroidism. A long-standing debate in the literature is whether addition of levotriiodothyronine (LT3) to LT4 will ameliorate lingering symptoms. Here, we explore the evidence for and against LT4/LT3 combination therapy as the optimal approach to treat euthyroid patients with persistent complaints.

METHODS

Recent literature indexed on PubMed was searched in March 2017 using the terms "hypothyroid" or "hypothyroidism" and "triiodothyronine combination" or "T3 combination." Relevant non-review articles published in English during the past 10 years were included and supplemented with articles already known to the authors.

FINDINGS

Current clinical evidence is not sufficiently strong to support LT4/LT3 combination therapy in patients with hypothyroidism. Polymorphisms in deiodinase genes that encode the enzymes that convert T4 to T3 in the periphery may provide potential mechanisms underlying unsatisfactory treatment results with LT4 monotherapy. However, results of studies on the effect of LT4/LT3 therapy on clinical symptoms and thyroid-responsive genes have thus far not been conclusive.

CONCLUSIONS

Persistent symptoms in patients who are biochemically euthyroid with LT4 monotherapy may be caused by several other conditions unrelated to thyroid function, and their cause should be aggressively investigated by the clinician.

THERAPY OF ENDOCRINE DISEASE: T4 + T3 combination therapy: is there a true effect?

About 5%-10% of hypothyroid patients on T4 replacement therapy have persistent symptoms, despite normal TSH levels. It was hoped that T4 + T3 combination therapy might provide better outcomes, but that was not observed according to a meta-analysis of 11 randomized clinical trials comparing T4 monotherapy with T4 + T3 combination therapy. However, the issue is still subject of much research because normal thyroid function tests in serum may not necessarily indicate an euthyroid state in all peripheral tissues. This review evaluates recent developments in the field of T4 + T3 combination therapy. T4 monotherapy is associated with higher serum FT4 levels than in healthy subjects, and subnormal serum FT3 and FT3/FT4 ratios are observed in about 15% and 30% respectively. T4 + T3 combination therapy may mimic more closely thyroid function tests of healthy subjects, but it has not been demonstrated that relatively low serum FT3 or FT3/FT4 ratios are linked to persistent symptoms. One study reports polymorphism Thr92Ala in DIO2 is related to lower serum FT3 levels after thyroidectomy, and that the D2-Ala mutant reduces T4 to T3 conversion in cell cultures. Peripheral tissue function tests such as serum cholesterol reflect thyroid hormone action in target tissues. Using such biochemical markers, patients who had a normal serum TSH during postoperative T4 monotherapy, were mildly hypothyroid, whereas those with a TSH 0.03-≤0.3 mU/L were closest to euthyroidism. Peripheral tissue function tests suggest euthyroidism more often in patients randomized to T4 + T3 rather than that to T4. Preference for T4 + T3 combination over T4 monotherapy was dose-dependently related to the presence of two polymorphisms in MCT10 and DIO2 in one small study. It is not known if persistent symptoms during T4 monotherapy disappear by switching to T4 + T3 combination therapy. The number of patients on T4 + T3 therapy has multiplied in the last decade, likely induced by indiscriminate statements on the internet. Patients are sometimes not just asking but rather demanding this treatment modality. It creates tensions between patients and physicians. Only continued research will answer the question whether or not T4 + T3 combination therapy has true benefits in some patients.

Persistent hypothyroid symptoms in a patient with a normal thyroid stimulating hormone level

PURPOSE OF REVIEW

A subset of patients being treated for hypothyroidism do not feel well while taking levothyroxine (LT4) replacement therapy, despite having a normal serum thyroid stimulating hormone level. Pursuing a relative triiodothyronine deficiency as a potential explanation for patient dissatisfaction, has led to trials of combination therapy with liothyronine (LT3), with largely negative outcomes. This review attempts to reconcile these diverse findings, consider potential explanations, and identify areas for future research.

RECENT FINDINGS

Patients being treated with LT4 often have lower triiodothyronine levels than patients with endogenous thyroid function. Linking patient dissatisfaction with low triiodothyronine levels has fueled multiple combination therapy trials that have generally not shown improvement in patient quality of life, mood, or cognitive performance. Some trials, however, suggest patient preference for combination therapy. There continues, moreover, to be anecdotal evidence that patients have fewer unresolved symptoms while taking combination therapy.

SUMMARY

The 14 trials completed to date have suffered from employing doses of LT3 that do not result in steady triiodothyronine levels, and having insufficient power to analyze results based on baseline dissatisfaction with therapy and patient genotype. Future trials that are able to incorporate such features may provide insight into what thyroid hormone preparations will most improve patient satisfaction with therapy.

Hypothyroidism

Hypothyroidism is a common condition of thyroid hormone deficiency, which is readily diagnosed and managed but potentially fatal in severe cases if untreated. The definition of hypothyroidism is based on statistical reference ranges of the relevant biochemical parameters and is increasingly a matter of debate. Clinical manifestations of hypothyroidism range from life threatening to no signs or symptoms. The most common symptoms in adults are fatigue, lethargy, cold intolerance, weight gain, constipation, change in voice, and dry skin, but clinical presentation can differ with age and sex, among other factors. The standard treatment is thyroid hormone replacement therapy with levothyroxine. However, a substantial proportion of patients who reach biochemical treatment targets have persistent complaints. In this Seminar, we discuss the epidemiology, causes, and symptoms of hypothyroidism; summarise evidence on diagnosis, long-term risk, treatment, and management; and highlight future directions for research.