Resistance to thyroid hormone mediated by defective thyroid hormone receptor alpha

Nuclear receptor corepressor 1 levels differentially impact the intracellular dynamics of mutant thyroid hormone receptors associated with resistance to thyroid hormone syndrome

Thyroid hormone receptor α1 (TRα1) undergoes nucleocytoplasmic shuttling and mediates gene expression in response to thyroid hormone (T3). In Resistance to Thyroid Hormone Syndrome α (RTHα), certain TRα1 mutants have higher affinity for nuclear corepressor 1 (NCoR1) and may form stable complexes that are not released in the presence of T3. Here, we examined whether NCoR1 modulates intranuclear mobility and nuclear retention of TRα1 or RTHα-associated mutants in transfected human cells, as a way of analyzing critical structural components of TRα1 and to further explore the correlation between mutations in TRα1 and aberrant intracellular trafficking. We found no significant difference in intranuclear mobility, as measured by fluorescence recovery after photobleaching, between TRα1 and select RTHα mutants, irrespective of NCoR1 expression. Nuclear-to-cytoplasmic fluorescence ratios of RTHα mutants, however, varied from TRα1 when NCoR1 was overexpressed, with a significant increase in nuclear retention for A263V and a significant decrease for A263S and R384H. In NCoR1-knockout cells, nuclear retention of A263S, A263V, P389R, A382P, C392X, and F397fs406X was significantly decreased compared to control (wild-type) cells. Luciferase reporter gene transcription mediated by TRα1 was significantly repressed by both NCoR1 overexpression and NCoR1 knockout. Most RTHα mutants showed minimal induction regardless of NCoR1 levels, but T3-mediated transcriptional activity was decreased for R384C and F397fs406X when NCoR1 was overexpressed, and also decreased for N359Y in NCoR1-knockout cells. Our results suggest a complex interaction between NCoR1 and RTHα mutants characterized by aberrant intracellular localization patterns and transcriptional activity that potentially arise from variable repressor complex stability, and may provide insight into RTHα pathogenesis on a molecular and cellular level.

Changes in brain structure in subjects with resistance to thyroid hormone due to THRB mutations

BACKGROUND

Being critical for brain development and neurocognitive function thyroid hormones may have an effect on behaviour and brain structure. Our exploratory study aimed to delineate the influence of mutations in the thyroid hormone receptor (TR) ß gene on brain structure.

METHODS

High-resolution 3D T1-weighted images were acquired in 21 patients with a resistance to thyroid hormone ß (RTHß) in comparison to 21 healthy matched-controls. Changes in grey and white matter, as well as cortical thickness were evaluated using voxel-based morphometry (VBM) and diffusion tensor imaging (DTI).

RESULTS

RTHß patients showed elevated circulating fT4 & fT3 with normal TSH concentrations, whereas controls showed normal thyroid hormone levels. RTHß patients revealed significantly higher scores in a self-rating questionnaire for attention deficit hyperactivity disorder (ADHD). Imaging revealed alterations of the corticospinal tract, increased cortical thickness in bilateral superior parietal cortex and decreased grey matter volume in bilateral inferior temporal cortex and thalamus.

CONCLUSION

RTHb patients exhibited structural changes in multiple brain areas. Whether these structural changes are causally linked to the abnormal behavioral profile of RTHß which is similar to ADHD, remains to be determined.

Reduced pituitary size in subjects with mutations in the THRB gene and thyroid hormone resistance

BACKGROUND

Thyroid hormone action is mediated by two forms of thyroid hormone receptors (α, β) with differential tissue distribution. Thyroid hormone receptor β (TRβ) mutations lead to resistance to thyroid hormone action in tissues predominantly expressing the β form of the receptor (pituitary, liver). This study seeks to identify the effects of mutant TRβ on pituitary size.

METHODS

High-resolution 3D T1-weighted magnetic resonance images were acquired in 19 patients with RTHβ in comparison to 19 healthy matched controls. Volumetric measurements of the pituitary gland were performed independently and blinded by four different raters (two neuroradiologists, one neurologist, one neuroscientist).

RESULTS

Patients with mutant TRβ (resistance to thyroid hormone β, RTHβ) showed elevated free tri-iodothyronine/thyroxine levels with normal thyroid-stimulating hormone levels, whereas healthy controls showed normal thyroid hormone levels. Imaging revealed smaller pituitary size in RTHβ patients in comparison to healthy controls (F(1,35) = 7.05, P = 0.012, partial η2 = 0.17).

CONCLUSION

RTHβ subjects have impaired sensitivity to thyroid hormones, along with decreased size of the pituitary gland.

Endocrine Regulation on Bone by Thyroid

BACKGROUND

As an endocrine organ, the thyroid acts on the entire body by secreting a series of hormones, and bone is one of the main target organs of the thyroid.

SUMMARY

This review highlights the roles of thyroid hormones and thyroid diseases in bone homeostasis.

CONCLUSION

Thyroid hormones play significant roles in the growth and development of bone, and imbalance of thyroid hormones can impair bone homeostasis.

Paternal fenvalerate exposure transgenerationally impairs cognition and hippocampus in female offspring

The impairments of maternal fenvalerate exposure have been well documented in previous study, but little was known about the effects of paternal fenvalerate exposure. The current study aimed to assess the effects of paternal fenvalerate exposure on spatial cognition and hippocampus across generations. Adult male mice (F0) were orally administered with fenvalerate (0, 2 or 20 mg/kg) for 5 weeks. F0 males were mated with untreated-females to generate F1 generation. F1 males were mated with F1 control females to generate F2 generation. For F1 and F2 adult offspring, spatial learning and memory were detected by Morris water maze. Results showed that spatial learning and memory were impaired in F1 females but not F1 males derived from F0 males exposed to 20 mg/kg FEN. Furthermore, significant impairment of spatial learning and memory were found in F2 females but not F2 males derived from F0 males exposed to 20 mg/kg FEN. As expected, histopathology showed that neural density in hippocampal CA3 region was reduced in F1 and F2 females but not F1 and F2 males derived from F0 males exposed to 20 mg/kg FEN. Mechanistically, hippocampal thyroid hormone receptor alpha1 (TRα1) was down-regulated in F1 and F2 females derived from F0 males exposed to 20 mg/kg FEN. Correspondingly, hippocampal brain-derived neurotrophic factor, tropomyosin receptor kinase B and p75 neurotrophin receptor, three downstream genes of TR signaling, were down-regulated in F1 and F2 females. Taken together, the present study firstly found that paternal fenvalerate exposure transgenerationally impaired spatial cognition in a gender-dependent manner. Hippocampal TR signaling may, at least partially, contribute to the process of cognitive impairment induced by paternal fenvalerate exposure. Further exploration in the mode of action of fenvalerate is critically important to promote human health and environmental safety.

The Role of Thyroid Hormone in Neuronal Protection

Thyroid hormone is essential for brain development and brain function in the adult. During development, thyroid hormone acts in a spatial and temporal-specific manner to regulate the expression of genes essential for normal neural cell differentiation, migration, and myelination. In the adult brain, thyroid hormone is important for maintaining normal brain function. Thyroid hormone excess, hyperthyroidism, and thyroid hormone deficiency, hypothyroidism, are associated with disordered brain function, including depression, memory loss, impaired cognitive function, irritability, and anxiety. Adequate thyroid hormone levels are required for normal brain function. Thyroid hormone acts through a cascade of signaling components: activation and inactivation by deiodinase enzymes, thyroid hormone membrane transporters, and nuclear thyroid hormone receptors. Additionally, the hypothalamic-pituitary-thyroid axis, with negative feedback of thyroid hormone on thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) secretion, regulates serum thyroid hormone levels in a narrow range. Animal and human studies have shown both systemic and local reduction in thyroid hormone availability in neurologic disease and after brain trauma. Treatment with thyroid hormone and selective thyroid hormone analogs has resulted in a reduction in injury and improved recovery. This article will describe the thyroid hormone signal transduction pathway in the brain and the role of thyroid hormone in the aging brain, neurologic diseases, and the protective role when administered after traumatic brain injury. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

Effect of short-term and long-term traffic noise exposure on the thyroid gland in adult rats: a sexual dimorphic study

OBJECTIVES

Traffic noise, as one of the noise types, is a widespread feature of the urban environments. Traffic noise exposure can lead to hearing loss, hypertension, obesity and ischemic heart diseases. Thyroid hormones involved in the physiological and pathological conditions of the body. Therefore, this study was designed to aim the evaluation of traffic noise effects on thyroid hormones secretion and thyroid tissue structure.

METHODS

Seventy two males and females wistar rats were used in this study. After one week adaptation, they divided randomly into 12 groups; the control, short term (one day) and long term (one, two, three and four weeks) groups. Traffic sound was recorded, adjusted and played (86 dB) for animals. Female rats' cycle estrus was matched. At the end of experiment, the animals were anesthetized and cardiac blood sample was drawn. Thyroid tissue was then removed. Levels of the T3, T4, TSH, corticosterone and H&E staining were measured. p<0.05 considered to be statistically significant.

RESULTS

Findings showed that in the one-day group, T3 levels increased and T3 levels decreased in the long term groups (p<0.05). In the same way, concentration of TSH decreased in the one day, while they increased in the one, two, three and four weeks' groups (p<0.05). Histopathological evaluations showed that in the female and male animals, long-term traffic noise increased the full follicles and decreased empty follicles (p<0.05).

CONCLUSIONS

This study revealed that traffic noise exposure led to increase of T3 plasma concentration and decrement of TSH level, although in the long term, they return to basal status. It may be due to adaptation to traffic noise.

A Direct Comparison of Thyroid Hormone Receptor Protein Levels in Mice Provides Unexpected Insights into Thyroid Hormone Action

Background: Thyroid hormone (TH) action is mediated by three major thyroid hormone receptor (THR) isoforms α1, β1, and β2 (THRA1, THRB1, and THRB2). These THRs and a fourth major but non-TH binding isoform, THRA2, are encoded by two genes Thra and Thrb. Reliable antibodies against all THR isoforms are not available, and THR isoform protein levels in mammalian tissues are often inferred from messenger RNA (mRNA) levels. Methods: We generated knock-in mouse models expressing endogenously and identically 2X hemagglutenin epitope (HA)-tagged THRs (THRA1/2, THRB1, and THRB2), which could then be detected by commercially available anti-HA antibodies. Using nuclear enrichment, immunoprecipitation, and Western blotting, we determined relative THR protein expression in 16 mouse organs. Results: In all peripheral organs tested except the liver, the predominant THR isoform was THRA1. Surprisingly, in metabolically active organs such as fat and muscle, THRB1 protein levels were up to 10 times lower than that of THRA1, while their mRNA levels appeared similar. In contrast to peripheral organs, the central nervous system (CNS) had a unique pattern with relatively low levels of both THRB1 and THRA1, and high levels of THRA2 expression. As expected, THRB2 was highly expressed in the pituitary, but a previously unknown sex-specific difference in THRB2 expression was found (female mice having higher pituitary expression than male mice). Higher THRB2 expression appears to make the central axis more sensitive to TH as both serum thyrotropin and Tshb mRNA levels were lower in female mice. Conclusions: Direct comparison of THR protein abundance in different organs using endogenously tagged HA-THR mouse lines shows that expression of THR isoforms is regulated at transcriptional and posttranscriptional levels, and in organ-specific manner. The prevalence of THRA1 and low abundance of THRB1 in majority of peripheral tissues suggest that peripheral actions of these isoforms should be revisited. A unique pattern of high THRA2 in CNS warrants further exploration of this non-TH binding isoform in brain development. Finally, THRB2, in addition to cell-specific control, is also regulated in a sex-specific manner, which may change the hypothalamus-pituitary-thyroid axis set point and perhaps metabolism in males and females.

Syndromes of Resistance to Thyroid Hormone Action

Thyroid hormone (TH) action is crucial for the development of several tissues.A number of syndromes are associated with reduced responsiveness to thyroid hormones, expanding the original definition of thyroid hormone resistance, firstly described by Refetoff and collaborators in 1967, which is characterized by elevated circulating levels of T4 and T3 with measurable serum TSH concentrations, as a consequence of mutations of thyroid hormone receptor beta (TRβ), recently named as RTHβ. More recently, another form of insensitivity to TH has been identified due to mutations in the thyroid hormone receptor alpha (TRα), named RTHα. In this chapter we will focus the discussion on the phenotype of RTHβ and RTHα. These diseases share the same pathogenic mechanism caused by dominant negative mutations in TH receptor genes that reduce T3 binding or affect the recruitment of cofactors. As a consequence, thyroid hormone actions are impaired at the tissue level. The phenotypic manifestations of RTHβ and RTHα are to some extent correlated with the degree of disruption and the tissue distribution of the TRs being characterized by variable coexistence of hypothyroid or thyrotoxic manifestations in RTHβ or by a congenital hypothyroid features in RTHα despite normal TSH and borderline low free T4.

The influence of thyroid function on the coagulation system and its clinical consequences

Several studies indicate that low plasma levels of thyroid hormone shift the hemostatic system towards a hypocoagulable and hyperfibrinolytic state, whereas high levels of thyroid hormone lead to more coagulation and less fibrinolysis. Low levels of thyroid hormone thereby seem to lead to an increased bleeding risk, whereas high levels, by contrast, increase the risk of venous thromboembolism. Hypothyroidism leads to a higher incidence of acquired von Willebrand's syndrome and with increasing levels of free thyroxine, levels of fibrinogen, factor VIII and von Willebrand factor, amongst others, increase gradually, to the extent that they may lead to symptomatic venous thromboembolism in patients with hyperthyroidism. Here, we discuss the literature on the effect of thyroid hormone on the hemostatic system and the associated risk of bleeding and venous thromboembolism. Patients with hypothyroidism are at increased risk of developing bleeding complications, which could be relevant in patients undergoing invasive procedures. Furthermore, physicians should be aware of the possibility of hyperthyroidism as an underlying risk factor for venous thromboembolism, especially in unexplained cases. Clinical studies are needed to further investigate the significance for general practice of these findings. Besides the effects of hyperthyroidism on venous thromboembolism, its effects on embolism secondary to atrial fibrillation are described.

Thyroid hormone receptor localization in target tissues

The thyroid hormone receptors, TRα1, TRβ1 and other subtypes, are members of the nuclear receptor superfamily that mediate the action of thyroid hormone signaling in numerous tissues to regulate important physiological and developmental processes. Their most well-characterized role is as ligand-dependent transcription factors; TRs bind thyroid hormone response elements in the presence or absence of thyroid hormone to facilitate the expression of target genes. Although primarily residing in the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm. We have identified multiple nuclear localization signals and nuclear export signals within TRα1 and TRβ1 that interact with importins and exportins, respectively, to mediate translocation across the nuclear envelope. More recently, enigmatic cytoplasmic functions have been ascribed to other TR subtypes, expanding the diversity of the cellular response to thyroid hormone. By integrating data on localization signal motifs, this review provides an overview of the complex interplay between TR's dynamic transport pathways and thyroid hormone signaling activities. We examine the variation in TR subtype response to thyroid hormone signaling, and what is currently known about regulation of the variety of tissue-specific localization patterns, including targeting to the nucleus, the mitochondria and the inner surface of the plasma membrane.

Thyroid diseases and bone health

Thyroid hormones are essential for skeletal development and are important regulators of bone maintenance in adults. Childhood hypothyroidism causes delayed skeletal development, retarded linear growth and impaired bone mineral accrual. Epiphyseal dysgenesis is evidenced by classic features of stippled epiphyses on X-ray. In severe cases, post-natal growth arrest results in a complex skeletal dysplasia. Thyroid hormone replacement stimulates catch-up growth and bone maturation, but recovery may be incomplete dependent on the duration and severity of hypothyroidism prior to treatment. A severe phenotype characteristic of hypothyroidism occurs in children with resistance to thyroid hormone due to mutations affecting THRA encoding thyroid hormone receptor α (TRα). Discovery of this rare condition recapitulated animal studies demonstrating that TRα mediates thyroid hormone action in the skeleton. In adults, thyrotoxicosis is well known to cause severe osteoporosis and fracture, but cases are rare because of prompt diagnosis and treatment. Recent data, however, indicate that subclinical hyperthyroidism is associated with low bone mineral density (BMD) and an increased risk of fracture. Population studies have also shown that variation in thyroid status within the reference range in post-menopausal women is associated with altered BMD and fracture risk. Thus, thyroid status at the upper end of the euthyroid reference range is associated with low BMD and increased risk of osteoporotic fragility fracture. Overall, extensive data demonstrate that euthyroid status is required for normal post-natal growth and bone mineral accrual, and is fundamental for maintenance of adult bone structure and strength.

Hypothalamic effects of thyroid hormone

Thyroid hormone (TH) is a key driver of metabolism in mammals. Plasma concentrations of TH are kept within a narrow range by negative feedback regulation in the hypothalamus-pituitary-thyroid (HPT) axis. Plasma TH concentrations are an important determinant of metabolic processes in liver and brown adipose tissue (BAT). In addition to endocrine effects of TH derived from the circulation, recent studies have demonstrated additional neural routes for intrahypothalamic thyroid hormone to regulate metabolism in liver and BAT via the sympathetic and parasympathetic branch of the autonomic nervous system (ANS). This review provides an overview of studies reporting metabolic effects of selective administration of T3 within hypothalamic nuclei including the paraventricular nucleus (PVN), the ventromedial nucleus (VMH), the arcuate nucleus (Arc), and the anterior hypothalamic area (AHA). This overview of the literature suggests that intrahypothalamic T3 can have profound effects on hepatic glucose production and insulin sensitivity, energy expenditure in BAT, cardiovascular function and feeding behavior. As the experiments have been performed in experimental animals exclusively, and the timing and route of T3 administration may be an important determinant of effect size, the clinical relevance of these metabolic effects in the chronic setting remains to be established.

Thyroid hormone treatment activates protective pathways in both in vivo and in vitro models of neuronal injury

Thyroid hormone plays an important role in brain development and adult brain function, and may influence neuronal recovery after Traumatic Brain Injury (TBI). We utilized both animal and cell culture models to determine the effects of thyroid hormone treatment, post TBI or during hypoxia, on genes important for neuronal survival and neurogenesis. We show that TBI in rats is associated with a reduction in serum thyroxine (T4) and triiodothyronine (T3). A single dose of levothyroxine (T4), one hour after injury, increased serum T4 and normalized serum T3 levels. Expression of genes important for thyroid hormone action in the brain, MCT8 and Type 2 deiodinase (Dio2) mRNA, diminished after injury, but were partially restored with T4 treatment. mRNA from the Type 3 deiodinase (Dio3) gene, which inactivates T4 to reverse T3 (rT3), was induced 2.7 fold by TBI, and further stimulated 6.7-fold by T4 treatment. T4 treatment significantly increased the expression of mRNA from Bcl2, VEGFA, Sox2 and neurotrophin, genes important for neuronal survival and recovery. The cortex, compared to the hippocampus and cerebellum, sustained the greatest injury and had the most significant change in gene expression as a result of injury and the greatest response to T4 treatment. We utilized hypoxia to study the effect of neuronal injury in vitro. Neuroblastoma cells were exposed to reduced oxygen tension, 0.2%, and were compared to cells grown at control oxygen levels of 21%. T3 treatment significantly increased hypoxia inducible factor (HIF)-2α protein, but not HIF-1α. In a hypoxia time course exposure, expression of hypoxia-mediated genes (VEGF, Enolase, HIF2α, c-Jun) peaked at least 8 h earlier with T3-treatment, compared to cells grown without T3. The early induction of these genes may promote cellular growth after injury. After hypoxic injury, T3 induced mRNA expression of the genes, KLF9 and hairless, important for T3-mediated brain function. The findings from both in vitro and in vivo studies support a role of thyroid hormone in activating pathways important for neuronal protection and promotion of neuronal recovery after injury.

Anemia in Patients With Resistance to Thyroid Hormone α: A Role for Thyroid Hormone Receptor α in Human Erythropoiesis

CONTEXT

Patients with resistance to thyroid hormone (TH) α (RTHα) are characterized by growth retardation, macrocephaly, constipation, and abnormal thyroid function tests. In addition, almost all RTHα patients have mild anemia, the pathogenesis of which is unknown. Animal studies suggest an important role for TH and TH receptor (TR)α in erythropoiesis.

OBJECTIVE

To investigate whether a defect in TRα affects the maturation of red blood cells in RTHα patients.

DESIGN, SETTING, AND PATIENTS

Cultures of primary human erythroid progenitor cells (HEPs), from peripheral blood of RTHα patients (n = 11) harboring different inactivating mutations in TRα (P398R, F397fs406X, C392X, R384H, A382fs388X, A263V, A263S), were compared with healthy controls (n = 11). During differentiation, erythroid cells become smaller, accumulate hemoglobin, and express different cell surface markers. We assessed cell number and cell size, and used cell staining and fluorescence-activated cell sorter analysis to monitor maturation at different time points.

RESULTS

After ∼14 days of ex vivo expansion, both control and patient-derived progenitors differentiated spontaneously. However, RTHα-derived cells differentiated more slowly. During spontaneous differentiation, RTHα-derived HEPs were larger, more positive for c-Kit (a proliferation marker), and less positive for glycophorin A (a differentiation marker). The degree of abnormal spontaneous maturation of RTHα-derived progenitors did not correlate with severity of underlying TRα defect. Both control and RTHα-derived progenitors responded similarly when differentiation was induced. T3 exposure accelerated differentiation of both control- and RTHα patient-derived HEPs.

CONCLUSIONS

Inactivating mutations in human TRα affect the balance between proliferation and differentiation of progenitor cells during erythropoiesis, which may contribute to the mild anemia seen in most RTHα patients.

Nuclear Import and Export of the Thyroid Hormone Receptor

The thyroid hormone receptors, TRα1 and TRβ1, are members of the nuclear receptor superfamily that forms one of the most abundant classes of transcription factors in multicellular organisms. Although primarily localized to the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm. The fine balance between nuclear import and export of TRs has emerged as a critical control point for modulating thyroid hormone-responsive gene expression. Mutagenesis studies have defined two nuclear localization signal (NLS) motifs that direct nuclear import of TRα1: NLS-1 in the hinge domain and NLS-2 in the N-terminal A/B domain. Three nuclear export signal (NES) motifs reside in the ligand-binding domain. A combined approach of shRNA-mediated knockdown and coimmunoprecipitation assays revealed that nuclear entry of TRα1 is facilitated by importin 7, likely through interactions with NLS-2, and importin β1 and the adapter importin α1 interacting with both NLS-1 and NLS-2. Interestingly, TRβ1 lacks NLS-2 and nuclear import depends solely on the importin α1/β1 heterodimer. Heterokaryon and fluorescence recovery after photobleaching shuttling assays identified multiple exportins that play a role in nuclear export of TRα1, including CRM1 (exportin 1), and exportins 4, 5, and 7. Even single amino acid changes in TRs dramatically alter their intracellular distribution patterns. We conclude that mutations within NLS and NES motifs affect nuclear shuttling activity, and propose that TR mislocalization contributes to the development of some types of cancer and Resistance to Thyroid Hormone syndrome.

Resistance to Thyroid Hormone due to Heterozygous Mutations in Thyroid Hormone Receptor Alpha

BACKGROUND

Thyroid hormone (TH) acts via nuclear thyroid hormone receptors (TRs). TR isoforms (TRα1, TRα2, TRβ1, TRβ2) are encoded by distinct genes (THRA and THRB) and show differing tissue distributions. Patients with mutations in THRB, exhibiting resistance within the hypothalamic-pituitary-thyroid axis with elevated TH and nonsuppressed thyroid-stimulating hormone (TSH) levels, were first described decades ago. In 2012, the first patients with mutations in THRA were identified. Scope of this review: This review describes the clinical and biochemical characteristics of patients with resistance to thyroid hormone alpha (RTHα) due to heterozygous mutations in THRA. The genetic basis and molecular pathogenesis of the disorder together with effects of levothyroxine treatment are discussed.

CONCLUSIONS

The severity of the clinical phenotype of RTHα patients seems to be associated with the location and type of mutation in THRA. The most frequent abnormalities observed include anemia, constipation, and growth and developmental delay. In addition, serum (F)T3 levels can be high-normal to high, (F)T4 and rT3 levels normal to low, while TSH is normal or mildly raised. Despite heterogeneous consequences of mutations in THRA, RTHα should be suspected in subjects with even mild clinical features of hypothyroidism together with high/high-normal (F)T3, low/low-normal (F)T4, and normal TSH.

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.

Retinoid-Related Orphan Receptor β and Transcriptional Control of Neuronal Differentiation

The ability to generate neuronal diversity is central to the function of the nervous system. Here we discuss the key neurodevelopmental roles of retinoid-related orphan receptor β (RORβ) encoded by the Rorb (Nr1f2) gene. Recent studies have reported loss of function of the human RORB gene in cases of familial epilepsy and intellectual disability. Principal sites of expression of the Rorb gene in model species include sensory organs, the spinal cord, and brain regions that process sensory and circadian information. Genetic analyses in mice have indicated functions in circadian behavior, vision, and, at the cellular level, the differentiation of specific neuronal cell types. Studies in the retina and sensory areas of the cerebral cortex suggest that this orphan nuclear receptor acts at decisive steps in transcriptional hierarchies that determine neuronal diversity.

Unexpected Elevated Free Thyroid Hormones in Pregnancy

BACKGROUND

The use of thyrotropin and free thyroid hormone assays to evaluate thyroid function is widespread, but in some situations the results are inconsistent with the patient's thyroid status.

SUMMARY

A 35-year-old woman with a known diagnosis of chronic autoimmune thyroiditis was referred to the authors' clinic at week 26 of her second pregnancy. The patient was clinically euthyroid. Consistent with this, her serum thyrotropin (TSH) was normal (0.79 mIU/L), but she had elevated free thyroid hormones-free triiodothyronine (fT3) and free thyroxine (fT4)-as determined by a one-step chemiluminescent assay. The patient was taking levothyroxine replacement therapy (125 μg/day), and the dose was confirmed. Previous blood tests showed concordance between TSH and free thyroid hormone values. The patient was followed up throughout gestation and at 12 months postpartum. During gestation, her free thyroid hormones remained high using one-step methods, while the total thyroid hormone concentration values were within the reference range, in agreement with the TSH values. Postpartum fT4 and fT3 values returned progressively to normality, in agreement with the TSH values. The presence of circulating thyroid hormone autoantibodies (THAb) was hypothesized, which are known to interfere, although to a variable extent, with thyroid hormone one-step assays. Using stored frozen sera, this hypothesis was confirmed indirectly by measuring normal levels of fT3 and fT4 with a two-step method, and directly by demonstrating THAb against the two hormones.

CONCLUSION

Despite their relative rarity, circulating THAb may be suspected when laboratory data are not consistent and contrast with the clinical picture. To the authors' knowledge, no previous case of transient appearance of THAb in pregnancy has been described.

Positive correlation of thyroid hormones and serum copper in children with congenital hypothyroidism

Thyroid hormones are of central relevance for growth and development. However, the underlying molecular mechanisms are still not fully understood. Recent studies in humans and mice have demonstrated that serum levels of selenium (Se) and copper (Cu) are positively affected by thyroid hormones. Given the importance of these trace elements for many biochemical processes, we tested whether this interaction is found in children at risk for hypothyroidism, potentially providing a novel factor contributing to the disturbed development observed in congenital hypothyroidism (CH). We conducted a cross-sectional analysis of 84 children diagnosed with CH displaying a wide range of thyroid hormone concentrations. Serum Se and Cu concentrations were measured by total reflection X-ray fluorescence. Data for thyrotropin (TSH) were available in all, thyroxine (T4) and free thyroxine (fT4) in the majority and triiodothyronine (T3) in 29 of the children. Spearman rank analyzes were performed. Cu and thyroid hormones showed a strong positive correlation (Cu/T4, rho=0.5241, P=0.0003; Cu/T3, rho=0.6003, P=0.0006). Unlike in adults, no associations were found between Se and any of the thyroid hormones. Our data highlight that serum Cu and thyroid hormones are strongly associated already in early postnatal life. Severely hypothyroid children are thus at risk of developing a Cu deficiency if not adequately nourished or supplemented. This finding needs to be verified in larger groups of children in order not to miss an easily-avoidable risk factor for poor development.

Diverse Genotypes and Phenotypes of Three Novel Thyroid Hormone Receptor-α Mutations

CONTEXT

Recently several patients with resistance to thyroid hormone (RTH)-α due to T3 receptor-α (TRα) mutations were identified. The phenotype of these patients consists of varying degrees of growth impairment, delayed bone, mental and motor development, constipation, macrocephaly, and near-normal thyroid function tests.

OBJECTIVE

The objective of the study was to describe the clinical phenotype of three new families with RTHα and thereby gain more detailed knowledge on this novel syndrome.

DESIGN, SETTING, AND PARTICIPANTS

RTHα was suspected in three index patients from different families. Detailed clinical and biochemical assessment and imaging and genetic analyses were performed in the patients and their relatives. In addition, functional consequences of TRα mutations were investigated in vitro.

RESULTS

We studied 22 individuals from three families and identified 10 patients with heterozygous TRα mutations: C380fs387X, R384H, and A263S, respectively. The frame-shift mutation completely inactivated TRα, whereas the missense mutations produced milder defects. These mutations were associated with decreasing severity of the clinical phenotype: the patient in family 1 showed severe defects in growth, mental, and motor development, whereas the seven patients in family 3 had only mild clinical features. The most frequent abnormalities were anemia, constipation, and a delay in at least one of the developmental milestones. Serum free T3 ranged from high-normal to high and serum free T4 and rT3 from normal to low. TSH levels were normal in all patients.

CONCLUSIONS

This large case series underlines the variation in the clinical phenotype of RTHα patients. RTHα should be suspected in subjects when even mild clinical and laboratory features of hypothyroidism are present along with high/high-normal free T3, low/normal free T4, and normal TSH.

Resistance to thyroid hormone α, revelation of basic study to clinical consequences

In the past 3 years, 15 patients with resistance to thyroid hormone α (RTHα), nine THRA gene mutations have been reported, reforming classification of RTH. RTHα exhibits distinguished clinical manifestations from RTHβ, including growth retardation, skeletal dysplasia, impaired neurodevelopment, cardiovascular dysfunction, constipation and specific thyroid axis type. This review focuses on possible pathogenesis by revelatory basic science of RTHα animal models in vivo, and patients' mutant thyroid hormone receptor α (TRα) in vitro. Clinical manifestations and L-T4 effects are summarized, showing strong correlation to the severity of mutation mostly within the domain which dominated TR interaction with T3 and its corepressors/coactivators. In particular, we propose the diagnosis clues and promising treatment for clinicians.

[Impaired sensitivity to thyroid hormone]

BACKGROUND

In conditions with impaired sensitivity to thyroid hormone, reduced effect of thyroid hormone is attributable to various defects. The purpose of this article is to give an overview of these conditions, as well as to provide updated knowledge on impaired sensitivity to thyroid hormone, also known as thyroid hormone resistance, with mutations in thyroid hormone receptor β (TRβ).

MATERIAL AND METHOD

This article is based on a selection of English-language articles, and Norwegian original and review articles found in PubMed, and the authors' own experiences with this patient group.

RESULTS

Thyroid hormone resistance has long been a recognised cause of the reduced effect of thyroid hormone. Several other conditions that involve impaired sensitivity to thyroid hormone have been described in recent decades, and mutations have been identified in genes that code for thyroid hormone receptor α (TRα), a cell membrane transporter, as well as in the deiodinases that metabolise thyroxine (T4) to the bioactive form triiodothyronine (T3). The conditions vary in terms of their clinical picture and biochemical profile.

INTERPRETATION

Based on clinical and biochemical findings, thyroid hormone resistance may be erroneously interpreted as hyperthyroidism. In patients with thyroid hormone resistance, the condition may be exacerbated if it is treated as hyperthyroidism. It is therefore essential to recognise the conditions and their differential diagnoses.

MECHANISMS IN ENDOCRINOLOGY: Novel genetic causes of short stature

The fast technological development, particularly single nucleotide polymorphism array, array-comparative genomic hybridization, and whole exome sequencing, has led to the discovery of many novel genetic causes of growth failure. In this review we discuss a selection of these, according to a diagnostic classification centred on the epiphyseal growth plate. We successively discuss disorders in hormone signalling, paracrine factors, matrix molecules, intracellular pathways, and fundamental cellular processes, followed by chromosomal aberrations including copy number variants (CNVs) and imprinting disorders associated with short stature. Many novel causes of GH deficiency (GHD) as part of combined pituitary hormone deficiency have been uncovered. The most frequent genetic causes of isolated GHD are GH1 and GHRHR defects, but several novel causes have recently been found, such as GHSR, RNPC3, and IFT172 mutations. Besides well-defined causes of GH insensitivity (GHR, STAT5B, IGFALS, IGF1 defects), disorders of NFκB signalling, STAT3 and IGF2 have recently been discovered. Heterozygous IGF1R defects are a relatively frequent cause of prenatal and postnatal growth retardation. TRHA mutations cause a syndromic form of short stature with elevated T3/T4 ratio. Disorders of signalling of various paracrine factors (FGFs, BMPs, WNTs, PTHrP/IHH, and CNP/NPR2) or genetic defects affecting cartilage extracellular matrix usually cause disproportionate short stature. Heterozygous NPR2 or SHOX defects may be found in ∼3% of short children, and also rasopathies (e.g., Noonan syndrome) can be found in children without clear syndromic appearance. Numerous other syndromes associated with short stature are caused by genetic defects in fundamental cellular processes, chromosomal abnormalities, CNVs, and imprinting disorders.

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

Delayed Diagnosis of Hypothyroidism in Children: Report of 3 Cases

Introduction:

Hypothyroidism is the most common endocrine disorder in children and presented with various sign and symptoms; its diagnosis needs a high index of suspicion.

Case Presentation:

We report 3 cases with unusual presentations of hypothyroidism and with delay in diagnosis that referred to Pediatric Endocrine Outpatient Clinic in Mashhad University of Medical Sciences, Mashhad, Iran with different clinical manifestations. They had decreased Thyroxin (T4) and increased thyroid stimulating hormone (TSH) levels. One case had mental retardation and deafness, but the other two cases had normal neurodevelopment. Some additional interesting findings were as follows: short stature, delayed bone age, teeth eruption impairment, hair loss, anemia and hypercholesterolemia, persistent and long-term constipation that had led to several abdominal surgeries. After a year of hormonal replacement therapy, their growth parameters and hematological values improved.

Conclusions:

We recommend thyroid hormonal evaluation for any children with short stature, especially with delayed bone age, in order to detect and treat hypothyroidism at the right time. It seems that more attention to pediatric growth is necessary.

Genetic determination of the hypothalamic-pituitary-thyroid axis: where do we stand?

For a long time it has been known that both hypo- and hyperthyroidism are associated with an increased risk of morbidity and mortality. In recent years, it has also become clear that minor variations in thyroid function, including subclinical dysfunction and variation in thyroid function within the reference range, can have important effects on clinical endpoints, such as bone mineral density, depression, metabolic syndrome, and cardiovascular mortality. Serum thyroid parameters show substantial interindividual variability, whereas the intraindividual variability lies within a narrow range. This suggests that every individual has a unique hypothalamus-pituitary-thyroid axis setpoint that is mainly determined by genetic factors, and this heritability has been estimated to be 40-60%. Various mutations in thyroid hormone pathway genes have been identified in persons with thyroid dysfunction or altered thyroid function tests. Because these causes are rare, many candidate gene and linkage studies have been performed over the years to identify more common variants (polymorphisms) associated with thyroid (dys)function, but only a limited number of consistent associations have been found. However, in the past 5 years, advances in genetic research have led to the identification of a large number of new candidate genes. In this review, we provide an overview of the current knowledge about the polygenic basis of thyroid (dys)function. This includes new candidate genes identified by genome-wide approaches, what insights these genes provide into the genetic basis of thyroid (dys)function, and which new techniques will help to further decipher the genetic basis of thyroid (dys)function in the near future.

Aberrant Monoaminergic System in Thyroid Hormone Receptor-β Deficient Mice as a Model of Attention-Deficit/Hyperactivity Disorder

BACKGROUND

Thyroid hormone receptors are divided into 2 functional types: TRα and TRβ. Thyroid hormone receptors play pivotal roles in the developing brain, and disruption of thyroid hormone receptors can produce permanent behavioral abnormality in animal models and humans.

METHODS

Here we examined behavioralchanges, regional monoamine metabolism, and expression of epigenetic modulatory proteins, including acetylated histone H3 and histone deacetylase, in the developing brain of TRα-disrupted (TRα (0/0) ) and TRβ-deficient (TRβ (-/-) ) mice. Tissue concentrations of dopamine, serotonin (5-hydroxytryptamine) and their metabolites in the mesocorticolimbic pathway were measured.

RESULTS

TRβ (-/-) mice, a model of attention-deficit/hyperactivity disorder, showed significantly high exploratory activity and reduced habituation, whereas TRα (0/0) mice showed normal exploratory activity. The biochemical profiles of dopamine and 5-hydroxytryptamine showed significantly low dopamine metabolic rates in the caudate putamen and nucleus accumbens and overall low 5-hydroxytryptamine metabolic rates in TRβ (-/-) mice, but not in TRα (0/0) mice. Furthermore, the expression of acetylated histone H3 was low in the dorsal raphe of TRβ (-/-) mice, and histone deacetylase 2/3 proteins were widely increased in the mesolimbic system.

CONCLUSIONS

These findings suggest that TRβ deficiency causes dysfunction of the monoaminergic system, accompanied by epigenetic disruption during the brain maturation process.

A novel mutation of thyroid hormone receptor β in exon 10 in a case of thyroid hormone-resistant non-Hodgkin's lymphoma of the thyroid

Only a few previous studies have demonstrated an association between resistance to thyroid hormone (RTH) and thyroid cancer. The current study presents the case of a 67-year-old female who was referred to the Third Xiangya Hospital of Central South University with an enlargement of the neck that had grown gradually over two years and subsequently, rapidly enlarged over the two months prior to admission, alongside a slight sensation of shortness of breath. Laboratory data revealed a significantly increased level of thyroid-stimulating hormone (TSH), total triiodothyronine, total thyroxine, free triiodothyronine, free thyroxine, thyroprotein and thyroglobulin antibody; however, the levels of thyroperoxidase and TSH receptor antibody were within the normal ranges. A thyroid hormone suppression test revealed a TSH reduction of 32%, Magnetic resonance imaging of the pituitary gland was negative for abnormalities. The patient's thyroid pathology revealed a non-Hodgkin's lymphoma of the thyroid. CHOP + nimustine chemotherapy significantly reduced the clinical symptoms. The genetic analysis revealed a novel point mutation of the thyroid hormone receptor β (THRB) gene in exon 10 (g1680 G to A) in the 3'-untranslated region of the patient. To the best of our knowledge, this is the first case report of RTH with thyroid non-Hodgkin's lymphoma, which involved a mutation (g1680 G to A) in exon 10 of THRB.

Clinical phenotype of a new type of thyroid hormone resistance caused by a mutation of the TRα1 receptor: consequences of LT4 treatment

CONTEXT

Recently the first patients with inactivating mutations in T₃ receptor (TR)-α1 have been identified. These patients have low free T₄, low T₄, high T₃, low rT₃, and normal TSH serum levels, in combination with growth retardation, delayed bone development, and constipation.

OBJECTIVE

The aim of the current study was to report the effects of levothyroxine (LT4) treatment on the clinical phenotype of 2 patients (father and daughter) with a heterozygous inactivating mutation in TRα1.

SETTING AND PARTICIPANTS

Both patients were treated with LT4 for the last 5 years. To evaluate the effect of LT4 treatment, LT4 was withdrawn for 35 days and subsequently reinitiated. Data were collected from medical records, by reanalysis of serum collected over the last 6 years, and by a detailed clinical evaluation.

RESULTS

Treatment with LT4 resulted in a suppression of serum TSH and normalization of serum free T₄ and rT₃, whereas T₃ levels remained elevated in both patients. In addition, there was a normalization of the dyslipidemia as well as a response in serum IGF-I, SHBG, and creatine kinase in the index patient. All these parameters returned to pretreatment values when LT4 was briefly stopped. LT4 also resulted in an improvement of certain clinical features, such as constipation and nerve conductance. However, cognitive and fine motor skill defects remained.

CONCLUSION

This study reports the consequences of LT4 treatment over a prolonged period of time in 2 of the first patients with a heterozygous mutation in TRα1. LT4 therapy leads to an improvement of certain but not all features of the clinical phenotype.