De novo triiodothyronine formation from thyrocytes activated by thyroid-stimulating hormone

Assessing the Influence of Propylthiouracil and Phenytoin on the Metabolomes of the Thyroid, Liver, and Plasma in Rats

The thyroid hormones (THs) regulate various physiological mechanisms in mammals, such as cellular metabolism, cell structure, and membrane transport. The therapeutic drugs propylthiouracil (PTU) and phenytoin are known to induce hypothyroidism and decrease blood thyroid hormone levels. To analyze the impact of these two drugs on systemic metabolism, we focused on metabolic changes after treatment. Therefore, in a rat model, the metabolome of thyroid and liver tissue as well as from the blood plasma, after 2-week and 4-week administration of the drugs and after a following 2-week recovery phase, was investigated using targeted LC-MS/MS and GC-MS. Both drugs were tested at a low dose and a high dose. We observed decreases in THs plasma levels, and higher doses of the drugs were associated with a high decrease in TH levels. PTU administration had a more pronounced effect on TH levels than phenytoin. Both drugs had little or no influence on the metabolomes at low doses. Only PTU exhibited apparent metabolome alterations at high doses, especially concerning lipids. In plasma, acylcarnitines and triglycerides were detected at decreased levels than in the controls after 2- and 4-week exposure to the drug, while sphingomyelins and phosphatidylcholines were observed at increased levels. Interestingly, in the thyroid tissue, triglycerides were observed at increased concentrations in the 2-week exposure group to PTU, which was not observed in the 4-week exposure group and in the 4-week exposure group followed by the 2-week recovery group, suggesting an adaptation by the thyroid tissue. In the liver, no metabolites were found to have significantly changed. After the recovery phase, the thyroid, liver, and plasma metabolomic profiles showed little or no differences from the controls. In conclusion, although there were significant changes observed in several plasma metabolites in PTU/Phenytoin exposure groups, this study found that only PTU exposure led to adaptation-dependent changes in thyroid metabolites but did not affect hepatic metabolites.

Clinical parameters correlated with the psoas muscle index in Japanese individuals with type 2 diabetes mellitus

Aims

Muscle atrophy is a diabetic complication, which results in a deterioration in glycemic control in type 2 diabetes mellitus (T2DM) individuals. The psoas muscle mass index (PMI) is a reliable indicator for estimating whole-body muscle mass. We aimed to examine the relationship between clinical parameters and the PMI to clarify the mechanism underlying muscle atrophy in diabetes.

Methods

This retrospective, cross-sectional study examined 51 patients (31 men and 20 women) with T2DM and a mean HbA1c value of 9.9 ± 1.7%. These patients were admitted to Aichi Medical University Hospital and underwent abdominal computed tomography imaging from July 2020 to April 2021. Multiple clinical parameters were assessed with the PMI.

Results

In a multiple regression analysis adjusted for age and sex, the PMI was correlated with body weight, body mass index, serum concentrations of corrected calcium, aspartate aminotransferase, alanine aminotransferase, creatine kinase, thyroid-stimulating hormone (TSH), urinary C-peptide concentrations, the free triiodothyronine/free thyroxine (FT3/FT4) ratio, and the young adult mean score at the femur neck. Receiver operating characteristic curves were created using TSH concentrations and the FT3/FT4 ratio for diagnosing a low PMI. The area under the curve was 0.593 and 0.699, respectively. The cut-off value with maximum accuracy for TSH concentrations was 1.491 μIU/mL, sensitivity was 56.1%, and specificity was 80.0%. Corresponding values for the FT3/FT4 ratio were 1.723, 78.0, and 66.7%, respectively.

Conclusion

TSH concentrations and the FT3/FT4 ratio are correlated with the PMI, and their thresholds may help prevent muscle mass loss in Japanese individuals with T2DM.

The role of supporting and disruptive mechanisms of FT3 homeostasis in regulating the hypothalamic-pituitary-thyroid axis

BACKGROUND

Thyroid hormones are controlled by the hypothalamic-pituitary-thyroid (HPT) axis through a complex network of regulatory loops, involving the hormones TRH, TSH, FT4, and FT3. The relationship between TSH and FT4 is widely used for diagnosing thyroid diseases. However, mechanisms of FT3 homeostasis are not well understood.

OBJECTIVE

We used mathematical modelling to further examine mechanisms that exist in the HPT axis regulation for protecting circulating FT3 levels.

METHODS

A mathematical model consisting of a system of four coupled first-order parameterized non-linear ordinary differential equations (ODEs) was developed, accounting for the interdependencies between the hormones in the HPT axis regulation. While TRH and TSH feed forward to the pituitary and thyroid, respectively, FT4 and FT3 feed backward to both the pituitary and hypothalamus. Stable equilibrium solutions of the ODE system express homeostasis for a particular variable, such as FT3, if this variable stays in a narrow range while certain other parameter(s) and system variable(s) may vary substantially.

RESULTS

The model predicts that (1) TSH-feedforward protects FT3 levels if the FT4 production rate declines and (2) combined negative feedback by FT4 and FT3 on both TSH and TRH production rates keeps FT3 levels insensitive to moderate changes in FT4 production rates and FT4 levels. The optimum FT4 and FT3 feedback and TRH and TSH-feedforward ranges that preserve FT3 homeostasis were found by numerical continuation analysis. Model predictions were in close agreement with clinical studies and individual patient examples of hypothyroidism and hyperthyroidism.

CONCLUSIONS

These findings further extend the concept of HPT axis regulation beyond TSH and FT4 to integrate the more active sister hormone FT3 and mechanisms of FT3 homeostasis. Disruption of homeostatic mechanisms leads to disease. This provides a perspective for novel testable concepts in clinical studies to therapeutically target the disruptive mechanisms.

A glance at post-translational modifications of human thyroglobulin: potential impact on function and pathogenesis

Thyroid hormones are essential for the metabolism of vertebrates and their synthesis, storage and release in the thyroid gland are orchestrated by their large protein precursor thyroglobulin (Tg). Alterations of Tg structure and localisation often correlate with major thyroid disorders. Namely, Tg is the main antigen in autoimmune thyroid diseases, and mutations in its gene are one of the causes of congenital hypothyroidism. Post-translational modifications (PTMs) are crucial for Tg surface properties and may be affected by the disease microenvironment; yet, their role in thyroid homeostasis and pathogenesis remains elusive. The advance of electron cryo-microscopy (cryo-EM) has recently enabled the structure of Tg to be revealed in the un-iodinated and iodinated states. Moreover, ad hoc proteomic analyses have lately identified new PTMs in Tg. Here, we provide an overview of the Tg cryo-EM models obtained so far, and we build a three-dimensional map of known PTMs in Tg. Based on their location, we suggest the potential implication of each PTM in hormonogenesis, interactions with cellular partners, colloid cross-linking and hormone release. In addition, several PTMs overlap with immunogenic regions and pathogenic gene mutations. Hence, our analysis reveals a possible cross-talk between PTMs and alteration of Tg function in these disorders. In perspective, multi-omics analyses from patients, interpreted with structural and functional data, may generate more robust models to correlate phenotypes with classes of Tg functional alterations. This integrative approach will likely provide more targeted strategies to restore specific Tg functions in different thyroid pathologies.

Principles of Endocrine Regulation: Reconciling Tensions Between Robustness in Performance and Adaptation to Change

Endocrine regulation in the hypothalamic-pituitary-thyroid (HPT) axis is orchestrated by physiological circuits which integrate multiple internal and external influences. Essentially, it provides either of the two responses to overt biological challenges: to defend the homeostatic range of a target hormone or adapt it to changing environmental conditions. Under certain conditions, such flexibility may exceed the capability of a simple feedback control loop, rather requiring more intricate networks of communication between the system's components. A new minimal mathematical model, in the form of a parametrized nonlinear dynamical system, is here formulated as a proof-of-concept to elucidate the principles of the HPT axis regulation. In particular, it allows uncovering mechanisms for the homeostasis of the key biologically active hormone free triiodothyronine (FT3). One mechanism supports the preservation of FT3 homeostasis, whilst the other is responsible for the adaptation of the homeostatic state to a new level. Together these allow optimum resilience in stressful situations. Preservation of FT3 homeostasis, despite changes in FT4 and TSH levels, is found to be an achievable system goal by joining elements of top-down and bottom-up regulation in a cascade of targeted feedforward and feedback loops. Simultaneously, the model accounts for the combination of properties regarded as essential to endocrine regulation, namely sensitivity, the anticipation of an adverse event, robustness, and adaptation. The model therefore offers fundamental theoretical insights into the effective system control of the HPT axis.

Characterization of the mechanistic linkages between iodothyronine deiodinase inhibition and impaired thyroid-mediated growth and development in Xenopus laevis using iopanoic acid

Iodothyronine deiodinases (DIO) are key enzymes that influence tissue-specific thyroid hormone levels during thyroid-mediated amphibian metamorphosis. Within the larger context of evaluating chemicals for thyroid system disrupting potential, chemical activity toward DIOs is being evaluated using high-throughput in vitro screening assays as part of U.S. EPA's ToxCast program. However, existing data gaps preclude any inferences between in vitro chemical inhibition of DIOs and in vivo outcomes relevant to ecological risk assessment. This study aimed to generate targeted data in a laboratory model species (Xenopus laevis) using a model DIO inhibitor, iopanoic acid (IOP), to characterize linkages between in vitro potency, in vivo biochemical responses, and adverse organismal outcomes. In vitro potency of IOP toward DIOs was evaluated using previously developed in vitro screening assays, which showed concentration-dependent inhibition of human DIO1 (IC50: 97 µM) and DIO2 (IC50: 231 µM) but did not inhibit human or X. laevis DIO3 under the assay conditions. In vivo exposure of larval X. laevis to 0, 2.6, 5.3 and 10.5 µM IOP caused thyroid-related biochemical profiles in the thyroid gland and plasma consistent with hyperthyroxinemia but resulted in delayed metamorphosis and significantly reduced growth in the highest two exposure concentrations. Independent evaluations of dio gene expression ontogeny, together with existing literature, supported interpretation of IOP-mediated effects resulting in a proposed adverse outcome pathway for DIO2 inhibition leading to altered amphibian metamorphosis. This study highlights the types of mechanistic data needed to move toward predicting in vivo outcomes of regulatory concern from in vitro bioactivity data.

Cryo-EM structure of native human thyroglobulin

The thyroglobulin (TG) protein is essential to thyroid hormone synthesis, plays a vital role in the regulation of metabolism, development and growth and serves as intraglandular iodine storage. Its architecture is conserved among vertebrates. Synthesis of triiodothyronine (T₃) and thyroxine (T₄) hormones depends on the conformation, iodination and post-translational modification of TG. Although structural information is available on recombinant and deglycosylated endogenous human thyroglobulin (hTG) from patients with goiters, the structure of native, fully glycosylated hTG remained unknown. Here, we present the cryo-electron microscopy structure of native and fully glycosylated hTG from healthy thyroid glands to 3.2 Å resolution. The structure provides detailed information on hormonogenic and glycosylation sites. We employ liquid chromatography–mass spectrometry (LC-MS) to validate these findings as well as other post-translational modifications and proteolytic cleavage sites. Our results offer insights into thyroid hormonogenesis of native hTG and provide a fundamental understanding of clinically relevant mutations.

The iodinated thyroglobulin functions as iodine storage and carrier protein and a precursor for thyroid hormone (TH) biogenesis. Here, the authors report the structure of native, fully glycosylated human thyroglobulin, revealing the location of the hTg hormonogenic and glycosylation sites.

Intrathyroidal feedforward and feedback network regulating thyroid hormone synthesis and secretion

Thyroid hormones (THs), including T4 and T3, are produced and released by the thyroid gland under the stimulation of thyroid-stimulating hormone (TSH). The homeostasis of THs is regulated via the coordination of the hypothalamic-pituitary-thyroid axis, plasma binding proteins, and local metabolism in tissues. TH synthesis and secretion in the thyrocytes-containing thyroid follicles are exquisitely regulated by an elaborate molecular network comprising enzymes, transporters, signal transduction machineries, and transcription factors. In this article, we synthesized the relevant literature, organized and dissected the complex intrathyroidal regulatory network into structures amenable to functional interpretation and systems-level modeling. Multiple intertwined feedforward and feedback motifs were identified and described, centering around the transcriptional and posttranslational regulations involved in TH synthesis and secretion, including those underpinning the Wolff-Chaikoff and Plummer effects and thyroglobulin-mediated feedback regulation. A more thorough characterization of the intrathyroidal network from a systems biology perspective, including its topology, constituent network motifs, and nonlinear quantitative properties, can help us to better understand and predict the thyroidal dynamics in response to physiological signals, therapeutic interventions, and environmental disruptions.

Triiodothyronine alongside levothyroxine in the management of hypothyroidism?

The current guideline-based management of hypothyroidism recommends monotherapy with levothyroxine (LT4), titrated to maintain the level of thyrotropin within a euthyroid reference range. This has been successful for most people with hypothyroidism, but a substantial minority still report symptoms of hypothyroidism unexplained by a comorbid medical condition. LT4 is essentially a prodrug for triiodothyronine (T3), the thyroid hormone that acts on target tissues in the brain and the periphery. Thyroid hormone replacement with LT4 alone does not restore physiological tissue levels of thyroid hormones, particularly T3. During the last two decades, much interest has focussed on the potential of combinations of LT4 and T3 to provide a superior outcome to LT4 monotherapy for people with hypothyroidism, especially those with residual symptoms despite thyrotropin-optimized LT4. This review seeks to provide an overview of currently available evidence on combination (LT4 + T3) therapy to be used for personalized medicine in patients with hypothyroidism. A number of randomized, controlled trials (RCTs) have failed to demonstrate superiority for the combination therapy approach, largely due to non-physiological T3 doses. However, patients with hypothyroidism are highly heterogeneous in terms of their residual thyroid function, individual set points for optimal thyroid homeostasis and for the presence or absence of polymorphisms in deiodinase enzymes in tissues that activate and deactivate circulating thyroid hormones. Accordingly, these RCTs may have failed to demonstrate benefits of combination therapy in individual hypothyroid phenotypes. The pharmacokinetics of LT4 and T3 also differ, which is a barrier to their co-administration. Future clinical trials using LT4 + T3 tablets better suited for combination therapy will resolve the outstanding research questions relating to the place of LT4 + T3 combination therapy in the management of hypothyroidism.

Structure and genetic variants of thyroglobulin: Pathophysiological implications

Thyroglobulin (TG) plays a main role in the biosynthesis of thyroid hormones (TH), and, thus, it is involved in a wide range of vital functions throughout the life cycle of all vertebrates. Deficiency of TH production due to TG genetic variants causes congenital hypothyroidism (CH), with devastating consequences such as intellectual disability and impaired growth if untreated. To this day, 229 variations in the human TG gene have been identified while the 3D structure of TG has recently appeared. Although TG deficiency is thought to be of autosomal recessive inheritance, the introduction of massive sequencing platforms led to the identification of a variety of monoallelic TG variants (combined with mutations in other thyroid gene products) opening new questions regarding the possibility of oligogenic inheritance of the disease. In this review we discuss remarkable advances in the understanding of the TG architecture and the pathophysiology of CH associated with TG defects, providing new insights for the management of congenital disorders as well as counseling benefits for families with a history of TG abnormalities. Moreover, we summarize relevant aspects of TH synthesis within TG and offer an updated analysis of animal and cellular models of TG deficiency for pathophysiological studies of thyroid dyshormonogenesis while highlighting perspectives for new investigations. All in all, even though there has been sustained progress in understanding the role of TG in thyroid pathophysiology during the past 50 years, functional characterization of TG variants remains an important area of study for future advancement in the field.

Identification of Protein Changes in the Urine of Hypothyroid Patients Treated with Thyroxine Using Proteomics Approach

The thyroid gland and thyroid hormones control a multitude of homeostatic functions including maintenance of fluid and electrolyte balance and normal functioning of the kidneys. Thyroid dysfunction alters the sytemic hemodynamic and metabolic balance, thereby affecting the kidney. In this study, we aimed to identify and characterize the urinary proteome of the patients with hypothyroidism. An untargeted proteomic approach with network analysis was used to identify changes in total urinary proteome in patients with newly diagnosed overt hypothyroidism. Urine samples were collected from nine age-matched patients' before and after l-thyroxine treatment. Differences in the abundance of urinary proteins between hypothyroid and euthyroid states were determined using a two-dimensional difference in gel electrophoresis (2D-DIGE) coupled to matrix-assisted laser desorption and ionization time-of-flight (MALDI TOF) mass spectrometry. Alterations in the abundance of urinary proteins, analyzed by Progenesis software, revealed statistically significant differential abundance in a total of 49 spots corresponding to 42 proteins, 28 up and 14 down (≥1.5-fold change, analysis of variance (ANOVA), p ≤ 0.05). The proteins identified in the study are known to regulate processes related to transport, acute phase response, oxidative stress, generation of reactive oxygen species, cellular proliferation, and endocytosis. Bioinformatic analysis using Ingenuity Pathway Analysis (IPA) identified dysregulation of pathways related to amino acid metabolism, molecular transport, and small-molecule biochemistry and involved the MAPK kinase, vascular endothelial growth factor (VEGF), PI3 kinase/Akt, protein kinase C (PKC), signaling pathways. The identified proteins were involved in the regulation of thyroglobulin (Tg) and thyrotropin (TSH) metabolism. Alterations in their levels indicate the presence of a compensatory mechanism aimed at increasing the regulation of Tg in the hypothyroid state.

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.

GWAS of thyroid stimulating hormone highlights pleiotropic effects and inverse association with thyroid cancer

Thyroid stimulating hormone (TSH) is critical for normal development and metabolism. To better understand the genetic contribution to TSH levels, we conduct a GWAS meta-analysis at 22.4 million genetic markers in up to 119,715 individuals and identify 74 genome-wide significant loci for TSH, of which 28 are previously unreported. Functional experiments show that the thyroglobulin protein-altering variants P118L and G67S impact thyroglobulin secretion. Phenome-wide association analysis in the UK Biobank demonstrates the pleiotropic effects of TSH-associated variants and a polygenic score for higher TSH levels is associated with a reduced risk of thyroid cancer in the UK Biobank and three other independent studies. Two-sample Mendelian randomization using TSH index variants as instrumental variables suggests a protective effect of higher TSH levels (indicating lower thyroid function) on risk of thyroid cancer and goiter. Our findings highlight the pleiotropic effects of TSH-associated variants on thyroid function and growth of malignant and benign thyroid tumors.

p.L571P in the linker domain of rat thyroglobulin causes intracellular retention

Thyroglobulin (TG), a large glycosylated protein secreted by thyrocytes into the thyroid follicular lumen, plays an essential role in thyroid hormone biosynthesis. Rattus norvegicus TG (rTG) is encoded by a large single copy gene, 186-kb long, located on chromosome 7 composed of 48 exons encoding a 8461-kb mRNA. Although the TG gene displays sequence variability, many missense mutations do not impose any adverse effect on the TG protein, whereas other nucleotide substitutions may affect its TG stability and/or TG intracellular trafficking. In order to gain a further understanding of the protein domains regulating its intracellular fate, we cloned a full-length cDNA from rTG into the pcDNA6/V5-His B expression vector. However, transient expression of the cDNA in HEK293T cells showed that the encoded protein was not a wild-type molecule, as it was unable to be secreted in the culture supernatant. Sequencing analyses revealed three random mutations, which accidentally emerged during the course of cloning: c.1712T>C [p.L571P] in the linker domain (amino acid positions 360 to 604), c.2027A>G [p.Q676R] in TG type 1-6 repeat and c.2720A>G [p.Q907R] in the TG type 1-7 repeat. Expression of cDNAs encoding a combination of two mutations [p.Q676R-p.Q907R], [p.L571P-p.Q907R] or [p.L571P-p.Q676R] indicated that any TG bearing the p.L571P substitution was trapped intracellularly. Indeed, we expressed the single point mutant p.L571P and confirmed that this point mutation was sufficient to cause intracellular retention of mutant TG in HEK293T cells. Endo H analysis showed that the p.L571P mutant is completely sensitive to the enzyme, whereas the will-type TG acquires full N-glycan modifications in Golgi apparatus. This data suggest that the p.L571P mutant contains the mannose-type N-glycan, that was added at the first stage of glycosylation. Complex-type N-glycan formation in the Golgi apparatus does not occur, consistent with defective endoplasmic reticulum exit of the mutant TG. Moreover, predictive analysis of the 3D linker domain showed that the p.L571P mutation would result in a significant protein conformational change. In conclusion, our studies identified a novel amino acid residue within the linker domain of TG associated with its conformational maturation and intracellular trafficking.

Heterogenous biochemical expression of hormone activity in subclinical/overt hyperthyroidism and exogenous thyrotoxicosis

Background

Subclinical hyperthyroidism/thyrotoxicosis originates from different causes and clinical conditions, sharing the laboratory constellation of a suppressed TSH in the presence of thyroid hormone concentrations within the reference range.

Aim

Presentation of hyperthyroidism can manifest itself in several ways. We questioned whether there is either a consistent biochemical equivalence of thyroid hormone response to these diagnostic categories, or a high degree of heterogeneity may exist both within and between the different clinical manifestations.

Methods

This secondary analysis of a former prospective cross-sectional trial involved 461 patients with untreated thyroid autonomy, Graves’ disease or on levothyroxine (LT4) after thyroidectomy for thyroid carcinoma. TSH response and biochemical equilibria between TSH and thyroid hormones were contrasted between endogenous hyperthyroidism and thyrotoxicosis (LT4 overdose).

Results

Concentrations of FT4, FT3, TSH, deiodinase activity and BMI differed by diagnostic category. Over various TSH strata, FT4 concentrations were significantly higher in LT4-treated thyroid carcinoma patients, compared to the untreated diseases, though FT3 levels remained comparable. They were concentrated in the upper FT4- but low deiodinase range, distinguishing them from patients with thyroid autonomy and Graves’ disease. In exogenous thyrotoxicosis, TSH and FT3 were less responsive to FT4 concentrations approaching its upper normal/hyperthyroid range.

Conclusions

The presence or lack of TSH feedforward activity determines the system response in the thyroid-active (hyperthyroidism) and no-thyroid response to treatment (thyrotoxicosis). This rules out a consistent thread of thyroid hormone response running through the different diagnostic categories. TSH measurements should therefore be interpreted conditionally and differently in subclinical hyperthyroidism and thyrotoxicosis.

Defects in protein folding in congenital hypothyroidism

Primary congenital hypothyroidism (CH) is the most common endocrine disease in children and one of the most common preventable causes of both cognitive and motor deficits. CH is a heterogeneous group of thyroid disorders in which inadequate production of thyroid hormone occurs due to defects in proteins involved in the gland organogenesis (dysembryogenesis) or in multiple steps of thyroid hormone biosynthesis (dyshormonogenesis). Dysembryogenesis is associated with genes responsible for the development or growth of thyroid cells: such as NKX2-1, FOXE1, PAX8, NKX2-5, TSHR, TBX1, CDCA8, HOXD3 and HOXB3 resulting in agenesis, hypoplasia or ectopia of thyroid gland. Nevertheless, the etiology of the dysembryogenesis remains unknown for most cases. In contrast, the majority of patients with dyshormonogenesis has been linked to mutations in the SLC5A5, SLC26A4, SLC26A7, TPO, DUOX1, DUOX2, DUOXA1, DUOXA2, IYD or TG genes, which usually originate goiter. About 800 genetic mutations have been reported to cause CH in patients so far, including missense, nonsense, in-frame deletion and splice-site variations. Many of these mutations are implicated in specific domains, cysteine residues or glycosylation sites, affecting the maturation of nascent proteins that go through the secretory pathway. Consequently, misfolded proteins are permanently entrapped in the endoplasmic reticulum (ER) and are translocated to the cytosol for proteasomal degradation by the ER-associated degradation (ERAD) machinery. Despite of all these remarkable advances in the field of the CH pathogenesis, several points on the development of this disease remain to be elucidated. The continuous study of thyroid gene mutations with the application of new technologies will be useful for the understanding of the intrinsic mechanisms related to CH. In this review we summarize the present status of knowledge on the disorders in the protein folding caused by thyroid genes mutations.

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.

The role of thyroglobulin in thyroid hormonogenesis

In humans, the thyroid hormones T₃ and T₄ are synthesized in the thyroid gland in a process that crucially involves the iodoglycoprotein thyroglobulin. The overall structure of thyroglobulin is conserved in all vertebrates. Upon thyroglobulin delivery from thyrocytes to the follicular lumen of the thyroid gland via the secretory pathway, multiple tyrosine residues can become iodinated to form mono-iodotyrosine (MIT) and/or di-iodotyrosine (DIT); however, selective tyrosine residues lead to preferential formation of T₄ and T₃ at distinct sites. T₄ formation involves oxidative coupling between two DIT side chains, and de novo T₃ formation involves coupling between an MIT donor and a DIT acceptor. Thyroid hormone synthesis is stimulated by TSH activating its receptor (TSHR), which upregulates the activity of many thyroid gene products involved in hormonogenesis. Additionally, TSH regulates post-translational changes in thyroglobulin that selectively enhance its capacity for T₃ formation - this process is important in iodide deficiency and in Graves disease. 167 different mutations, many of which are newly discovered, are now known to exist in TG (encoding human thyroglobulin) that can lead to defective thyroid hormone synthesis, resulting in congenital hypothyroidism.

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.

Sustained Release T3 Therapy: Animal Models and Translational Applications

The standard of care to treat hypothyroidism is daily administration of levo-thyroxine (LT4). This is based on the understanding that deiodinases can restore production of T3 and compensate for the small amounts of T3 that are normally produced by the thyroid gland. However, pre-clinical and clinical evidence indicating that deiodinases fall short of restoring T3 production is accumulating, opening the possibility that liothyronine (LT3) might have a role in the treatment of some hypothyroid patients. LT3 tablets taken orally result in a substantial peak of circulating T3 that is dissipated during the next several hours, which is markedly distinct from the relative stability of T3 levels in normal individuals. Thus, the effort to developing new delivery strategies for LT3, including slow release tablets, liquid formulations, use of T3-related/hybrid molecules such as T3 sulfate, poly-zinc-T3 and glucagon-T3, nanoparticles containing T3, subcutaneous implant of T3-containing matrices, and stem cells for de novo development of the thyroid gland. This article reviews these strategies, their applicability in animal models and translatability to humans.

The role of functional thyroid capacity in pituitary thyroid feedback regulation

BACKGROUND

Thyroid feedback regulation and equilibria between thyroid hormones differ in the presence or absence of a functioning thyroid remnant.

MATERIALS AND METHODS

This study examines the relationship between the sensitivity of TSH feedback and thyroid capacity in untreated patients with thyroid autoimmune disease (n = 86) and healthy controls (n = 271). Functional capacity was estimated at maximum TSH stimulation, and pituitary TSH response was FT4-standardised with two established indices, the TSH index and the thyrotroph thyroid hormone resistance index.

RESULTS

The two indices correlated inversely with thyroid volume and functional thyroid capacity. Relationships were shifted upwards in patients with thyroid autoimmune disease. This positioned patients with thyroid autoimmune disease predominantly at the lower capacity range and upper part of TSH index. The relationship was modulated by serum FT3 concentrations, shifting 0.19 [95%CI: 0.12, 0.26] mIU/L per pmol FT3 increase. FT3 correlated with TSH index in total group ( τ  = 0.09, P = 0.009) and both subgroups. FT3 levels were maintained despite a substantial capacity loss by progressively increasing conversion rates of T3 from T4, only collapsing at capacities below <1.5 pmol/s.

CONCLUSION

Functional thyroid capacity and preferential T3 generation are essential elements in adjusting the sensitivity of hypothalamic-pituitary-thyroid feedback control and balancing system equilibria. This suggests that the indirect regulatory role of glandular T3 co-secretion exceeds its quantitative contribution to the thyroid hormone pool. Implications for clinical practice extend to the diagnostic use of TSH in patients with impaired thyroid reserve.

Molecular analysis of thyroglobulin mutations found in patients with goiter and hypothyroidism

Thyroid dyshormonogenesis due to thyroglobulin (TG) gene mutations have an estimated incidence of approximately 1 in 100,000 newborns. The clinical spectrum ranges from euthyroid to mild or severe hypothyroidism. Up to now, one hundred seventeen deleterious mutations in the TG gene have been identified and characterized. The purpose of the present study was to identify and characterize new mutations in the TG gene. We report eight patients from seven unrelated families with goiter, hypothyroidism and low levels of serum TG. All patients underwent clinical, biochemical and image evaluation. Sequencing of DNA, genotyping, as well as bioinformatics analysis were performed. Molecular analyses revealed three novel inactivating TG mutations: c.5560G>T [p.E1835*], c.7084G>C [p.A2343P] and c.7093T>C [p.W2346R], and four previously reported mutations: c.378C>A [p.Y107*], c.886C>T [p.R277*], c.1351C>T [p.R432*] and c.7007G>A [p.R2317Q]. Two patients carried homozygous mutations (p.R277*/p.R277*, p.W2346R/p.W2346R), four were compound heterozygous mutations (p.Y107*/p.R277* (two unrelated patients), p.R432*/p.A2343P, p.Y107*/p.R2317Q) and two siblings from another family had a single p.E1835* mutated allele. Additionally, we include the analysis of 48 patients from 31 unrelated families with TG mutations identified in our present and previous studies. Our observation shows that mutations in both TG alleles were found in 27 families (9 as homozygote and 18 as heterozygote compound), whereas in the remaining four families only one mutated allele was detected. The majority of the detected mutations occur in exons 4, 7, 38 and 40. 28 different mutations were identified, 33 of the 96 TG alleles encoded the change p.R277*. In conclusion, our results confirm the genetic heterogeneity of TG defects and the pathophysiological importance of the predicted TG misfolding and therefore thyroid hormone formation as a consequence of truncated TG proteins and/or missense mutations located within its ACHE-like domain.

Relationship between the dimerization of thyroglobulin and its ability to form triiodothyronine

Thyroglobulin (TG) is the most abundant thyroid gland protein, a dimeric iodoglycoprotein (660 kDa). TG serves as the protein precursor in the synthesis of thyroid hormones tetraiodothyronine (T₄) and triiodothyronine (T₃). The primary site for T₃ synthesis in TG involves an iodotyrosine acceptor at the antepenultimate Tyr residue (at the extreme carboxyl terminus of the protein). The carboxyl-terminal region of TG comprises a cholinesterase-like (ChEL) domain followed by a short unique tail sequence. Despite many studies, the monoiodotyrosine donor residue needed for the coupling reaction to create T₃ at this evolutionarily conserved site remains unidentified. In this report, we have utilized a novel, convenient immunoblotting assay to detect T₃ formation after protein iodination in vitro, enabling the study of T₃ formation in recombinant TG secreted from thyrocytes or heterologous cells. With this assay, we confirm the antepenultimate residue of TG as a major T₃-forming site, but also demonstrate that the side chain of this residue intimately interacts with the same residue in the apposed monomer of the TG dimer. T₃ formation in TG, or the isolated carboxyl-terminal region, is inhibited by mutation of this antepenultimate residue, but we describe the first substitution mutation that actually increases T₃ hormonogenesis by engineering a novel cysteine, 10 residues upstream of the antepenultimate residue, allowing for covalent association of the unique tail sequences, and that helps to bring residues Tyr²⁷⁴⁴ from apposed monomers into closer proximity.

Recent Advances in Thyroid Hormone Regulation: Toward a New Paradigm for Optimal Diagnosis and Treatment

In thyroid health, the pituitary hormone thyroid-stimulating hormone (TSH) raises glandular thyroid hormone production to a physiological level and enhances formation and conversion of T4 to the biologically more active T3. Overstimulation is limited by negative feedback control. In equilibrium defining the euthyroid state, the relationship between TSH and FT4 expresses clusters of genetically determined, interlocked TSH-FT4 pairs, which invalidates their statistical correlation within the euthyroid range. Appropriate reactions to internal or external challenges are defined by unique solutions and homeostatic equilibria. Permissible variations in an individual are much more closely constrained than over a population. Current diagnostic definitions of subclinical thyroid dysfunction are laboratory based, and do not concur with treatment recommendations. An appropriate TSH level is a homeostatic concept that cannot be reduced to a fixed range consideration. The control mode may shift from feedback to tracking where TSH becomes positively, rather than inversely related with FT4. This is obvious in pituitary disease and severe non-thyroid illness, but extends to other prevalent conditions including aging, obesity, and levothyroxine (LT4) treatment. Treatment targets must both be individualized and respect altered equilibria on LT4. To avoid amalgamation bias, clinically meaningful stratification is required in epidemiological studies. In conclusion, pituitary TSH cannot be readily interpreted as a sensitive mirror image of thyroid function because the negative TSH-FT4 correlation is frequently broken, even inverted, by common conditions. The interrelationships between TSH and thyroid hormones and the interlocking elements of the control system are individual, dynamic, and adaptive. This demands a paradigm shift of its diagnostic use.