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

The relationship of 19 functional polymorphisms in iodothyronine deiodinase and psychological well-being in hypothyroid patients

PURPOSE

Levothyroxine supplementation is insufficient for the management of one tenth of patients with hypothyroidism. Iodothyronine deiodinases have been suggested to play a role in residual hypothyroid symptoms of these patients by controlling local thyroid hormone homeostasis. Previous research has suggested a relationship between commonly inherited variations in type 2 iodothyronine deiodinase and impaired well-being. We evaluated the prevalence of iodothyronine deiodinase genotypes and their association with psychological well-being in the Korean hypothyroid population.

METHODS

A prospective observational study. We enrolled 196 hypothyroid subjects (136 chronic autoimmune thyroiditis and 60 thyroid cancer) and assessed baseline well-being using six validated questionnaires. Genotyping was conducted for 19 single nucleotide polymorphisms in type 1, 2, and 3 iodothyronine deiodinase using Sequenom MassARRAY matrix-assisted laser desorption/ionization time-of-flight mass spectrometry in all patients.

RESULTS

Frequencies of iodothyronine deiodinase genotypes and well-being scores were not different in hypothyroid subjects according to their disease types. Minor genotypes of a few iodothyronine deiodinase 1 variants (rs11206244, rs2294512, and rs4926616) were associated with reduced psychological well-being. However, iodothyronine deiodinase 2 and 3 variants had no effect on baseline well-being.

CONCLUSION

Minor variations in iodothyronine deiodinase 1 were associated with decreased well-being in the Korean hypothyroid population, whereas iodothyronine deiodinase 2 and 3 were not. Due to controversial results among different ethnicities, further studies to clarify the effects of iodothyronine deiodinase polymorphisms on psychological well-being are warranted in hypothyroid individuals.

A Step Forward in Understanding the Relevance of Genetic Variation in Type 2 Deiodinase

This article involves the study by Castagna et al. published in this issue of the Journal of Clinical Endocrinology & Metabolism on the association and functional analyses of genetic variation in DIO2.

DIO2 Thr92Ala Reduces Deiodinase-2 Activity and Serum-T3 Levels in Thyroid-Deficient Patients

Context

A substantial proportion of athyreotic levothyroxine (LT4)-treated patients experience hypothyroid-like symptoms. During LT4 replacement, levels of the active hormone triiodothyronine (T3) strictly depend on type 2-deiodinase (D2)-mediated activation of LT4. The Thr92Ala polymorphism and the 258 G/A in the DIO2 gene have been associated with various clinical conditions.

Objectives

To investigate the effects of DIO2 polymorphisms in thyroid hormone homeostasis.

Design

We compared the presurgical hormonal status of thyroidectomized LT4-treated patients who had a similar thyroid-stimulating hormone (TSH) level with their postsurgery status and analyzed their DIO2 genotype in a subgroup of 102/140 (72.8%) of patients. We measured the enzymatic properties of Thr92Ala in living cells and in relevant generated mouse models.

Subjects and methods

A total of 140 thyroidectomized subjects were included. Serum free T3 (FT3), free thyroxine, and TSH levels were directly measured. Immunohistochemistry and immunoblotting were performed for D2 protein.

Results

The DIO2 genotyping revealed an association between low FT3 values and Thr92Ala. Specifically, the mean postsurgery FT3 levels were significantly lower in patients carrying the mutated allele(s) than in wild-type patients, in whom FT3 postsurgical levels were similar to presurgery levels. The -258 G/A variation was not associated with hormonal alteration. We found that endogenous wild-type D2 and Thr92Ala share the same subcellular localization but differ in protein stability. Importantly, Thr92Ala reduced D2-mediated thyroxine to T3 conversion.

Conclusions

Thyroidectomized patients carrying Thr92Ala are at increased risk of reduced intracellular and serum T3 concentrations that are not adequately compensated for by LT4, thus providing evidence in favor of customized treatment of hypothyroidism in athyreotic patients.

Genetics of thyroid function

Recent studies show that subtle variations in thyroid function, including subclinical thyroid dysfunction, and even variation in thyroid function within the normal range, are associated with morbidity and mortality. It is estimated that 40-65% of the inter-individual variation in serum TSH and FT4 levels is determined by genetic factors. To identify these factors, various linkage and candidate gene studies have been performed in the past, which have identified only a few genes. In the last decade, genome-wide association studies identified many new genes, while recent whole-genome sequencing efforts have also been proven to be effective. In the current review, we provide a systematic overview of these studies, including strengths and limitations. We discuss new techniques which will further clarify the genetic basis of thyroid function in the near future, as well as the potential use of these genetic markers in personalizing the management of thyroid disease patients.

New insights into thyroid hormone action

Thyroid hormones (TH) are endocrine messengers essential for normal development and function of virtually every vertebrate. The hypothalamic-pituitary-thyroid axis is exquisitely modulated to maintain nearly constant TH (T4 and T3) levels in circulation. However peripheral tissues and the CNS control the intracellular availability of TH, suggesting that circulating concentrations of TH are not fully representative of what each cell type sees. Indeed, recent work in the field has identified that TH transporters, deiodinases and thyroid hormone receptor coregulators can strongly control tissue-specific sensitivity to a set amount of TH. Furthermore, the mechanism by which the thyroid hormone receptors regulate target gene expression can vary by gene, tissue and cellular context. This review will highlight novel insights into the machinery that controls the cellular response to TH, which include unique signaling cascades. These findings shed new light into the pathophysiology of human diseases caused by abnormal TH signaling.

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

INTRODUCTION

The presence of the Thr92Ala polymorphism of deiodinase-2 (D2) has been thought to have several effects. It may influence its enzymatic function, is associated with increased expression of genes involved in oxidative stress in brain tissue, and may predict favorable response to combination levothyroxine (LT4) plus triiodothyronine (T3) therapy. It was hypothesized that homozygous carriers of the D2-92Ala allele have different thyroid hormone parameters, and reduced health-related quality of life (HRQoL) and cognitive functioning.

METHODS

In 12,625 participants from the LifeLines cohort study with genome-wide genetic data available, the effects of the Thr92Ala polymorphism (rs225014) were evaluated in the general population and in 364 people treated with thyroid hormone replacement therapy, the latter mainly because of primary hypothyroidism. In addition to evaluating anthropometric data, medication use, and existence of metabolic syndrome, HRQoL was assessed with the RAND 36-Item Health Survey, and the Ruff Figural Fluency Test was used as a sensitive test for executive functioning. Data on thyrotropin, free thyroxine (fT4), and free T3 (fT3) levels were available in a subset of 4479 participants.

RESULTS

The mean age (±standard deviation) was 53 ± 12 years and the body mass index was 27.0 ± 4.5 kg/m2 in the LT4 users compared with 48 ± 11 years and 26.2 ± 4.1 kg/m2 in participants from the general population. The Ala/Ala genotype of the D2-Thr92Ala polymorphism was present in 11.3% of LT4 users and in 10.7% of the general population. In total, 3742/4479 subjects with thyroid hormone data available had normal TSH (0.4-4.0 mIU/L), and 88% of LT4 users were females. LT4 users had higher fT4, lower fT3, and a lower fT3/fT4 ratio, and female patients had lower scores on the HRQoL domains of physical functioning, vitality, mental health, social functioning, bodily pain, and general health compared with those not using LT4 (p < 0.005). Executive functioning scores, as part of cognitive functioning, were comparable between female LT4 users and the general population. In both groups, the D2-Thr92Ala polymorphism was not associated with differences in TSH, fT4, fT3, the fT3/fT4 ratio, presence of metabolic syndrome or other comorbidities, use of medication, HRQoL, and cognitive functioning.

CONCLUSION

The Thr92Ala polymorphism of D2 was not associated with thyroid parameters, HRQoL, and cognitive functioning in the general population and in participants on thyroid hormone replacement therapy.

Relational Stability of Thyroid Hormones in Euthyroid Subjects and Patients with Autoimmune Thyroid Disease

BACKGROUND/AIM

Operating far from its equilibrium resting point, the thyroid gland requires stimulation via feedback-controlled pituitary thyrotropin (TSH) secretion to maintain adequate hormone supply. We explored and defined variations in the expression of control mechanisms and physiological responses across the euthyroid reference range.

METHODS

We analyzed the relational equilibria between thyroid parameters defining thyroid production and thyroid conversion in a group of 271 thyroid-healthy subjects and 86 untreated patients with thyroid autoimmune disease.

RESULTS

In the euthyroid controls, the FT₃-FT₄ (free triiodothyronine-free thyroxine) ratio was strongly associated with the FT₄-TSH ratio (tau = -0.22, p < 0.001, even after correcting for spurious correlation), linking T₄ to T₃ conversion with TSH-standardized T₄ production. Using a homeostatic model, we estimated both global deiodinase activity and maximum thyroid capacity. Both parameters were nonlinearly and inversely associated, trending in opposite directions across the euthyroid reference range. Within the panel of controls, the subgroup with a relatively lower thyroid capacity (<2.5 pmol/s) displayed lower FT₄ levels, but maintained FT₃ at the same concentrations as patients with higher functional and anatomical capacity. The relationships were preserved when extended to the subclinical range in the diseased sample.

CONCLUSION

The euthyroid panel does not follow a homogeneous pattern to produce random variation among thyroid hormones and TSH, but forms a heterogeneous group that progressively displays distinctly different levels of homeostatic control across the euthyroid range. This suggests a concept of relational stability with implications for definition of euthyroidism and disease classification.

Risks and safety of combination therapy for hypothyroidism

INTRODUCTION

Hypothyroidism is currently a condition that can be treated, but not cured. Although levothyroxine reverses stigmata of hypothyroidism in most individuals, some patients feel dissatisfied with 'monotherapy', and this has stimulated interest in 'combination therapy' with both levothyroxine and liothyronine.

AREAS COVERED

A search of PubMed was conducted using terms including hypothyroidism, treatment, benefits, risks, and safety. Based on the articles identified, the body of evidence regarding the efficacy of traditional levothyroxine is reviewed. Concerns with levothyroxine therapy including impaired quality of life in treated patients, thyroxine-predominant hormone ratios, and inadvertent iatrogenic thyroid disease are discussed. The trials of combination therapy performed since 1999 were reviewed. The heterogeneity of these trials, both in terms of design and results, is discussed. The potential for new trials to determine whether combination therapy can reverse the dissatisfaction associated with monotherapy, while avoiding non-physiologic hormone ratios, inadvertent thyrotoxicosis, and unacceptable side effects is discussed. Expert commentary: Research regarding which therapy fully reverses hypothyroidism at a tissue and cellular level is ongoing. The field would be advanced by the development of an extended release preparation of liothyronine. In the future regeneration of functional thyroid follicles from stem cells may offer hope for curing hypothyroidism.

Antidepressant behavior in thyroidectomized Wistar rats is induced by hippocampal hypothyroidism

Thyroidectomy is a surgical procedure indicated in cases of several maligned or benign thyroid diseases, thus, the aim of our study was to verify how the hypothyroidism induced by thyroidectomy influences behavioral parameters and its relation to thyroid hormones metabolism and neurogenesis at hippocampus. For this purpose, Adult male Wistar rats underwent to thyroidectomy to induce hypothyroidism. Behavioral tests, the thyroid profile and hippocampal gene expression were evaluated in control and in thyroidectomized animals. It was observed that thyroidectomized group had a significant increasing in serum thyroid-stimulating hormone (TSH) and a decreasing in thyroxine (T4) levels as well as in triiodothyronine (T3) serum level. It was also observed reduction of the monocarboxylate transporter 8 (Mct8), thyroid hormone receptor alfa (Trα1), deiodinase type 2 (Dio2), ectonucleotide pyrophosphatase/phosphodiesterase 2 (Enpp2) and brain-derived neurotrophic factor (Bdnf) mRNA expression in hippocampus of thyroidectomized animals. In the forced swimming test, it was verified that thyroidectomy promotes a decrease in time of immobility and climbing when compared with the control group. In summary, we demonstrated that antidepressant behavior in thyroidectomized Wistar rats is induced by hippocampal hypothyroidism. This effect could be associated to an impaired neuronal activity in acute stress response as it is observed in forced swimming paradigm.

The History and Future of Treatment of Hypothyroidism

Thyroid hormone replacement has been used for more than a century to treat hypothyroidism. Natural thyroid preparations (thyroid extract, desiccated thyroid, or thyroglobulin), which contain both thyroxine (T4) and triiodothyronine (T3), were the first pharmacologic treatments available and dominated the market for the better part of the 20th century. Dosages were adjusted to resolve symptoms and to normalize the basal metabolic rate and/or serum protein-bound iodine level, but thyrotoxic adverse effects were not uncommon. Two major developments in the 1970s led to a transition in clinical practice: 1) The development of the serum thyroid-stimulating hormone (TSH) radioimmunoassay led to the discovery that many patients were overtreated, resulting in a dramatic reduction in thyroid hormone replacement dosage, and 2) the identification of peripheral deiodinase-mediated T4-to-T3 conversion provided a physiologic means to justify l-thyroxine monotherapy, obviating concerns about inconsistencies with desiccated thyroid. Thereafter, l-thyroxine monotherapy at doses to normalize the serum TSH became the standard of care. Since then, a subgroup of thyroid hormone-treated patients with residual symptoms of hypothyroidism despite normalization of the serum TSH has been identified. This has brought into question the inability of l-thyroxine monotherapy to universally normalize serum T3 levels. New research suggests mechanisms for the inadequacies of l-thyroxine monotherapy and highlights the possible role for personalized medicine based on deiodinase polymorphisms. Understanding the historical events that affected clinical practice trends provides invaluable insight into formulation of an approach to help all patients achieve clinical and biochemical euthyroidism.

Variation in the biochemical response to l-thyroxine therapy and relationship with peripheral thyroid hormone conversion efficiency

Several influences modulate biochemical responses to a weight-adjusted levothyroxine (l-T4) replacement dose. We conducted a secondary analysis of the relationship of l-T4 dose to TSH and free T3 (FT3), using a prospective observational study examining the interacting equilibria between thyroid parameters. We studied 353 patients on steady-state l-T4 replacement for autoimmune thyroiditis or after surgery for malignant or benign thyroid disease. Peripheral deiodinase activity was calculated as a measure of T4-T3 conversion efficiency. In euthyroid subjects, the median l-T4 dose was 1.3 μg/kg per day (interquartile range (IQR) 0.94,1.60). The dose was independently associated with gender, age, aetiology and deiodinase activity (all P<0.001). Comparable FT3 levels required higher l-T4 doses in the carcinoma group (n=143), even after adjusting for different TSH levels. Euthyroid athyreotic thyroid carcinoma patients (n=50) received 1.57 μg/kg per day l-T4 (IQR 1.40, 1.69), compared to 1.19 μg/kg per day (0.85,1.47) in autoimmune thyroiditis (P<0.01, n=76) and 1.08 μg/kg per day (0.82, 1.44) in patients operated on for benign disease (P< 0.01, n=80). Stratifying patients by deiodinase activity categories of <23, 23-29 and >29 nmol/s revealed an increasing FT3-FT4 dissociation; the poorest converters showed the lowest FT3 levels in spite of the highest dose and circulating FT4 (P<0.001). An l-T4-related FT3-TSH disjoint was also apparent; some patients with fully suppressed TSH failed to raise FT3 above the median level. These findings imply that thyroid hormone conversion efficiency is an important modulator of the biochemical response to l-T4; FT3 measurement may be an additional treatment target; and l-T4 dose escalation may have limited success to raise FT3 appropriately in some cases.

Thyroid Hormone at Near Physiologic Concentrations Acutely Increases Oxygen Consumption and Extracellular Acidification in LH86 Hepatoma Cells

Thyroid hormone (T3) has been known to regulate the basal metabolic rate for more than a century, but mechanistic understanding is lacking both at the level of the intact organism and in terms of how T3 alters energy expenditure in individual tissues. The current studies investigate the question of which metabolically relevant genes respond acutely as T3 concentrations increase through the physiologic range in liver cells. Because this has been technically unfeasible historically, we developed a modified protocol for extracellular flux analysis using a 96-well Extracellular Flux Analyzer (Seahorse Bioscience). Using a modified extracellular flux protocol and LH86 human hepatoma cells, we established an experimental system where small but significant changes in O2 consumption could be reproducibly quantified as hypothyroid cells were exposed to near-physiologic final concentrations of T3 approximately 2 orders of magnitude lower than most studies (0.04 nM free T3), in only 6-7 hours. Taking advantage of the nondestructive nature of 96-well Extracellular Flux Analyzer measurements, the acute, direct, transcriptional changes that occur were measured in the exact same cells demonstrating increased O2 consumption. An unbiased, genome-wide microarray analysis identified potential candidate genes related to fatty acid oxidation, angiogenesis, nucleotide metabolism, immune signaling, mitochondrial respiration, and cell proliferation. The identified transcriptome is likely enriched in the genes most important for mediating the energetic effects of T3 in hepatoma cells.

Scope and limitations of iodothyronine deiodinases in hypothyroidism

The coordinated expression and activity of the iodothyronine deiodinases regulate thyroid hormone levels in hypothyroidism. Once heralded as the pathway underpinning adequate thyroid-hormone replacement therapy with levothyroxine, the role of these enzymes has come into question as they have been implicated in both an inability to normalize serum levels of tri-iodothyronine (T3) and the incomplete resolution of hypothyroid symptoms. These observations, some of which were validated in animal models of levothyroxine monotherapy, challenge the paradigm that tissue levels of T3 and thyroid-hormone signalling can be fully restored by administration of levothyroxine alone. The low serum levels of T3 observed among patients receiving levothyroxine monotherapy occur as a consequence of type 2 iodothyronine deiodinase (DIO2) in the hypothalamus being fairly insensitive to ubiquitination. In addition, residual symptoms of hypothyroidism have been linked to a prevalent polymorphism in the DIO2 gene that might be a risk factor for neurodegenerative disease. Here, we discuss how these novel findings underscore the clinical importance of iodothyronine deiodinases in hypothyroidism and how an improved understanding of these enzymes might translate to therapeutic advances in the care of millions of patients with this condition.

Genetic abnormalities in thyroid hormone deiodinases

PURPOSE OF REVIEW

The purpose of this study was to review several of the most recent and most important clinical studies regarding the effects of how genetic variation in the deiodinases can influence health.

RECENT FINDINGS

Common variation in DIO1 but not DIO2 or DIO3 is robustly associated with thyroid hormone levels at genome-wide levels of significance although the effect is modest. There is growing evidence that common variation in DIO2, particularly the Thr92Ala substitution, is associated with important clinical endpoints including osteo-arthritis and intelligence quotient; however, the mechanism of action appears to vary between tissues with several unexpected pathways recently being elucidated including effects on Golgi function and DNA methylation. In addition, there is emerging evidence of interaction with iodine and thyroid status.

SUMMARY

The clinical importance of genetic variation in the deiodinases has yet to be fully elucidated and their impact is likely to vary between individuals and body systems dependent on multiple factors within tissues and coexistent diseases and environmental factors. Exploration of whether there are rare functional variants in the deiodinases is now possible in population studies, which may yield greater insight in the near future. Studies of the impact of genetic variation in the deiodinases in individuals with iodine deficiency, subclinical thyroid disease, or those on levothyroxine are urgently needed.