Iodothyronine deiodinase structure and function: from ascidians to humans

Cytokine Storm-Induced Thyroid Dysfunction in COVID-19: Insights into Pathogenesis and Therapeutic Approaches

Angiotensin-converting enzyme 2 receptors (ACE2R) are requisite to enter the host cells for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). ACE2R is constitutive and functions as a type I transmembrane metallo-carboxypeptidase in the renin-angiotensin system (RAS). On thyroid follicular cells, ACE2R allows SARS-CoV-2 to invade the thyroid gland, impose cytopathic effects and produce endocrine abnormalities, including stiff back, neck pain, muscle ache, lethargy, and enlarged, inflamed thyroid gland in COVID-19 patients. Further damage is perpetuated by the sudden bursts of pro-inflammatory cytokines, which is suggestive of a life-threatening syndrome known as a "cytokine storm". IL-1β, IL-6, IFN-γ, and TNF-α are identified as the key orchestrators of the cytokine storm. These inflammatory mediators upregulate transcriptional turnover of nuclear factor-kappa B (NF-κB), Janus kinase/signal transducer and activator of transcription (JAK/STAT), and mitogen-activated protein kinase (MAPK), paving the pathway for cytokine storm-induced thyroid dysfunctions including euthyroid sick syndrome, autoimmune thyroid diseases, and thyrotoxicosis in COVID-19 patients. Targeted therapies with corticosteroids (dexamethasone), JAK inhibitor (baricitinib), nucleotide analogue (remdesivir) and N-acetyl-cysteine have demonstrated effectiveness in terms of attenuating the severity and frequency of cytokine storm-induced thyroid dysfunctions, morbidity and mortality in severe COVID-19 patients. Here, we review the pathogenesis of cytokine storms and the mechanisms and pathways that establish the connection between thyroid disorder and COVID-19. Moreover, cross-talk interactions of signalling pathways and therapeutic strategies to address COVID-19-associated thyroid diseases are also discussed herein.

The transcriptional landscape underlying larval development and metamorphosis in the Malabar grouper (Epinephelus malabaricus)

Most teleost fishes exhibit a biphasic life history with a larval oceanic phase that is transformed into morphologically and physiologically different demersal, benthic, or pelagic juveniles. This process of transformation is characterized by a myriad of hormone-induced changes, during the often abrupt transition between larval and juvenile phases called metamorphosis. Thyroid hormones (TH) are known to be instrumental in triggering and coordinating this transformation but other hormonal systems such as corticoids, might be also involved as it is the case in amphibians. In order to investigate the potential involvement of these two hormonal pathways in marine fish post-embryonic development, we used the Malabar grouper (Epinephelus malabaricus) as a model system. We assembled a chromosome-scale genome sequence and conducted a transcriptomic analysis of nine larval developmental stages. We studied the expression patterns of genes involved in TH and corticoid pathways, as well as four biological processes known to be regulated by TH in other teleost species: ossification, pigmentation, visual perception, and metabolism. Surprisingly, we observed an activation of many of the same pathways involved in metamorphosis also at an early stage of the larval development, suggesting an additional implication of these pathways in the formation of early larval features. Overall, our data brings new evidence to the controversial interplay between corticoids and thyroid hormones during metamorphosis as well as, surprisingly, during the early larval development. Further experiments will be needed to investigate the precise role of both pathways during these two distinct periods and whether an early activation of both corticoid and TH pathways occurs in other teleost species.

Mitochondrial function is enhanced by thyroid hormones during zebra finch development

An organism's response to its environment is largely determined by changes in the energy supplied by aerobic mitochondrial metabolism via adenosine triphosphate (ATP) production. ATP is especially important under energy-demanding conditions, such as during rapid growth. It is currently poorly understood how environmental factors influence energy metabolism and mitochondrial functioning, but recent studies suggest the role of thyroid hormones (TH). TH are key regulators of growth and metabolism and can be flexibly adjusted to environmental conditions, such as environmental temperature or food availability. To test whether TH enhancement is causally linked to mitochondrial function and growth, we provided TH orally at physiological concentrations during the main growth phase in zebra finch (Taeniopygia guttata) nestlings reared in a challenging environment. TH treatment accelerated maximal mitochondrial working capacity-a trait that reflects mitochondrial ATP production, without affecting growth. To our knowledge, this is the first study to characterize the regulation of mitochondria by TH during development in a semi-naturalistic context and to address implications for fitness-related traits, such as growth.

Selenium Status Associates with Thyroid Hormone and Thyroid Dysfunction in Older Chinese Adults

Selenium (Se) is physiologically essential for thyroid function. However, epidemiological studies on the association between Se status and thyroid function are limited and the results are inconsistent. Therefore, we explored this association in an elderly Chinese population sample. Participants in the cross-sectional study were people aged 65 years or older who provided fingernail and whole blood samples. Hyperthyroidism and hypothyroidism were defined by serum thyroid hormones concentrations, including thyroid stimulating hormone (TSH), total triiodothyronine (TT3), total thyroxine (TT4), free thyroxine (FT3), and free thyrotropin (FT4). Significant positive association was observed between whole blood and fingernail Se concentrations (r = 0.672, P < 0.001). Compared with the lowest Se quartile (Q1), the other fingernail Se quartile groups had lower TSH, higher FT3 and FT4 levels, and Q2 had higher TT3 levels after adjusting for covariates; the other whole blood Se quartile groups had lower TSH levels, Q2 had higher FT3, FT4 and TT3 levels, Q3 had higher FT3 levels, and Q4 had higher FT4 levels after adjusting for covariates. Compared with Q1, the adjusted odds ratios (OR) and 95% confidence intervals (95%CIs) of hypothyroidism for Q4 of whole blood Se was 0.141 (0.029,0.675), and the adjusted OR (95%CIs) of hyperthyroidism for Q2 and Q3 of fingernail Se were 4.121 (1.233,13.733) and 3.614 (1.095,11.926). Higher Se levels were significantly associated with lower TSH levels and higher levels of TT3, FT3 and FT4. Meanwhile, higher Se levels were associated with lower risk of hypothyroidism and higher risk of hyperthyroidism.

The association of gluten-free diet with thyroid autoimmunity in patients with celiac disease: a meta-analysis

Background: Thyroid autoimmunity is an immune response to thyroid antigens that causes varying degrees of thyroid dysfunction. The sole effective treatment for Celiac Disease (CD) is a gluten-free diet (GFD). However, the association between GFD and thyroid autoimmunity in patients with CD has not been confirmed. Methods: A comprehensive search of several databases, involving PubMed, Embase, Web of Science, Medline, and Cochrane databases, was conducted to identify studies that primarily addressed the effects of GFD on thyroid autoimmunity in CD subjects. The meta-analysis involved studies that compared the risk of ATPO and ATG antibody positivity in CD patients with GFD, the risk of developing AITD, and the risk of developing thyroid dysfunction. Fixed-effects models or random-effects models were used to calculate the odds ratios (ORs) and their 95% confidence intervals (95% CIs). Results: A total of 10 observational studies met the inclusion criteria and included 6423 subjects. The results indicated that GFD is positively associated with thyroid autoimmunity in the children subgroup of CD patients (OR = 1.61, 95%CI 1.06-2.43, P = 0.02). However, there was no significant difference in thyroid autoimmunity between the group adhering to GFD and the control group in the total CD population. Conclusion: The results seem to indicate that subjects with a more pronounced autoimmunity (such as to have an early onset of CD) appear to have a greater risk of thyroid autoimmunity.

Influence of silver nanoparticles on mRNA expression of thyroid hormone-related genes in the thyroid gland and liver of laying hens

The widespread use of silver nanoparticles (AgNPs) in consumer products and animal husbandry raises the need to study their impact on living organisms. This study was conducted on Hy-Line Brown hens at the age of 25 weeks with an average weight of 1.58 kg. Hens for 2 weeks received a solution of 50 nm AgNPs at a concentration of 100 pm (experimental group; n = 6) or a solution in which the nanoparticles were suspended (control group; n = 6). Thyroid hormones (thyroxine - T4, triiodothyronine - T3) were evaluated in the blood plasma and expression profiles of genes involved in thyroid hormone (TH) synthesis (TSHR, NIS, TPO, TG), metabolism (DIO1, DIO2, DIO3) and transport (MCT8, MCT10, LAT1) were determined in the chicken thyroid gland. Furthermore, iodothyronine deiodinase, TH transporter and TH receptor (THRA, THRB) mRNA expressions were evaluated in the livers isolated from the same chickens. AgNPs did not affect serum T4 levels but elevated serum T3 concentration. The results showed that AgNPs increased DIO3 mRNA in the thyroid gland. In turn, in the liver AgNPs administration significantly upregulated DIO2 and downregulated MCT10 mRNA levels. These results indicate that exposure to AgNPs leads to a tissue-specific alternative expression of genes engaged in TH metabolism. Moreover, the mRNA expression of DIO2 in the liver showed a positive correlation with plasma T3 levels. In conclusion, AgNPs may have an impact on TH metabolism by affecting deiodinases and TH transporter MCT10 mRNA expression.

Biosynthesis, Engineering, and Delivery of Selenoproteins

Selenocysteine (Sec) was discovered as the 21st genetically encoded amino acid. In nature, site-directed incorporation of Sec into proteins requires specialized biosynthesis and recoding machinery that evolved distinctly in bacteria compared to archaea and eukaryotes. Many organisms, including higher plants and most fungi, lack the Sec-decoding trait. We review the discovery of Sec and its role in redox enzymes that are essential to human health and important targets in disease. We highlight recent genetic code expansion efforts to engineer site-directed incorporation of Sec in bacteria and yeast. We also review methods to produce selenoproteins with 21 or more amino acids and approaches to delivering recombinant selenoproteins to mammalian cells as new applications for selenoproteins in synthetic biology.

Highly Electrophilic Intermediates in the Bypass Mechanism of Glutathione Peroxidase: Synthesis, Reactivity, and Structures of Selenocysteine-Derived Cyclic Selenenyl Amides

Selenocysteine (Sec)-derived cyclic selenenyl amides, formed by the intramolecular cyclization of Sec selenenic acids (Sec-SeOHs), have been postulated to function as protective forms in the bypass mechanism of glutathione peroxidase (GPx). However, their chemical properties have not been experimentally elucidated in proteins or small-molecule systems. Recently, we reported the first nuclear magnetic resonance observation of Sec-SeOHs and their cyclization to the corresponding cyclic selenenyl amides by using selenopeptide model systems incorporated in a molecular cradle. Herein, we elucidate the structures and reactivities of Sec-derived cyclic selenenyl amides. The crystal structures and reactions toward a cysteine thiol or a 1,3-diketone-type chemical probe indicated the highly electrophilic character of cyclic selenenyl amides. This suggests that they can serve not only as protective forms to suppress the inactivation of Sec-SeOHs in GPx but also as highly electrophilic intermediates in the reactions of selenoproteins.

Thyroid dysfunction in children and adolescents affected by undernourished and overnourished eating disorders

Eating disorders (ED) are one of the most prevalent chronic disorders in adolescents and young adults, with a significantly increasing prevalence in younger children, particularly in girls. Even if obesity in essence is not framed as an eating disorder and has always been considered a separate pathology, ED and obesity could be considered part of a continuum. It has become evident that one condition can lead to another, such as binge eating disorder (BED) and bulimia nervosa, and that they share the same repercussions in terms of psychosocial, metabolic, and nutritional health. This narrative review aims to investigate the hypothalamic-pituitary-thyroid axis in undernourished and overnourished patients with ED, including obesity, in order to highlight the relationship between weight control and thyroid function and its effects and to consider therapeutic and preventive strategies in children and adolescents. Literature data report that thyroid alterations occur in patients with ED, both underweight and overweight, and represent a continuum of changes depending on the severity and time course of the disease involving the endocrine system. Considering the relevant role thyroid hormones (TH) play not only in energy expenditure (EE) but also in metabolic control and cardiovascular risks related to dysmetabolism and mood regulation, continuous monitoring of thyroid homeostasis in patients with ED is mandatory to prevent severe complications and to start early treatment when necessary.

Cross-species applicability of an adverse outcome pathway network for thyroid hormone system disruption

Thyroid hormone system disrupting compounds are considered potential threats for human and environmental health. Multiple adverse outcome pathways (AOPs) for thyroid hormone system disruption (THSD) are being developed in different taxa. Combining these AOPs results in a cross-species AOP network for THSD which may provide an evidence-based foundation for extrapolating THSD data across vertebrate species and bridging the gap between human and environmental health. This review aimed to advance the description of the taxonomic domain of applicability (tDOA) in the network to improve its utility for cross-species extrapolation. We focused on the molecular initiating events (MIEs) and adverse outcomes (AOs) and evaluated both their plausible domain of applicability (taxa they are likely applicable to) and empirical domain of applicability (where evidence for applicability to various taxa exists) in a THSD context. The evaluation showed that all MIEs in the AOP network are applicable to mammals. With some exceptions, there was evidence of structural conservation across vertebrate taxa and especially for fish and amphibians, and to a lesser extent for birds, empirical evidence was found. Current evidence supports the applicability of impaired neurodevelopment, neurosensory development (eg, vision) and reproduction across vertebrate taxa. The results of this tDOA evaluation are summarized in a conceptual AOP network that helps prioritize (parts of) AOPs for a more detailed evaluation. In conclusion, this review advances the tDOA description of an existing THSD AOP network and serves as a catalog summarizing plausible and empirical evidence on which future cross-species AOP development and tDOA assessment could build.

In silico insights into the dimer structure and deiodinase activity of type III iodothyronine deiodinase from bioinformatics, molecular dynamics simulations, and QM/MM calculations

The homodimeric family of iodothyronine deiodinases (Dios) regioselectively remove iodine from thyroid hormones. Currently, structural data has only been reported for the monomer of the mus type III thioredoxin (Trx) fold catalytic domain (Dio3Trx), but the mode of dimerization has not yet been determined. Various groups have proposed dimer structures that are similar to the A-type and B-type dimerization modes of peroxiredoxins. Computational methods are used to compare the sequence of Dio3Trx to related proteins known to form A-type and B-type dimers. Sequence analysis and in silico protein-protein docking methods suggest that Dio3Trx is more consistent with proteins that adopt B-type dimerization. Molecular dynamics (MD) simulations of the refined Dio3Trx dimer constructed using the SymmDock and GalaxyRefineComplex databases indicate stable dimer formation along the β4α3 interface consistent with other Trx fold B-type dimers. Free energy calculations show that the dimer is stabilized by interdimer interactions between the β-sheets and α-helices. A comparison of MD simulations of the apo and thyroxine-bound dimers suggests that the active site binding pocket is not affected by dimerization. Determination of the transition state for deiodination of thyroxine from the monomer structure using QM/MM methods provides an activation barrier consistent with previous small model DFT studies.Communicated by Ramaswamy H. Sarma.

Relationship between selenium status, selenoproteins and COVID-19 and other inflammatory diseases: A critical review

The antioxidant effects of selenium as a component of selenoproteins has been thought to modulate host immunity and viral pathogenesis. Accordingly, the association of low dietary selenium status with inflammatory and immunodeficiency has been reported in the literature; however, the causal role of selenium deficiency in chronic inflammatory diseases and viral infection is still undefined. The COVID-19, characterized by acute respiratory syndrome and caused by the novel coronavirus 2, SARS-CoV-2, has infected millions of individuals worldwide since late 2019. The severity and mortality from COVID-19 have been associated with several factor, including age, sex and selenium deficiency. However, available data on selenium status and COVID-19 are limited, and a possible causative role for selenium deficiency in COVID-19 severity has yet to be fully addressed. In this context, we review the relationship between selenium, selenoproteins, COVID-19, immune and inflammatory responses, viral infection, and aging. Regardless of the role of selenium in immune and inflammatory responses, we emphasize that selenium supplementation should be indicated after a selenium deficiency be detected, particularly, in view of the critical role played by selenoproteins in human health. In addition, the levels of selenium should be monitored after the start of supplementation and discontinued as soon as normal levels are reached. Periodic assessment of selenium levels after supplementation is a critical issue to avoid over production of toxic metabolites of selenide because under normal conditions, selenoproteins attain saturated expression levels that limits their potential deleterious metabolic effects.

Effects of the synthetic progestin levonorgestrel on some aspects of thyroid physiology in common carp (Cyprinus carpio)

The objective of the present study was to assess the effects of levonorgestrel (LNG), a synthetic progestin, on early development and the thyroid system of carp using morphological, histological, immunohistochemical, and gene expression analysis. Fish were exposed to LNG at three levels (3, 31, and 310 ng L^-1) from eggs to the onset of juvenile stage (47 days). LNG had no significant effect on early development in common carp or on the occurrence of morphological anomalies. No pathological alterations of the thyroid follicles were found. Immunohistochemical examination of the thyroid follicles using antibodies against thyroxin did not show any differences in fish exposed to 310 ng L^-1 LNG compared to the controls. mRNA expression of iodothyronine deiodinases (dio1, 2, 3) was differentially affected by LNG treatment during carp development. Most importantly, dio3 was markedly downregulated in fish exposed to all three LNG levels compared to the controls at the conclusion of the experiment (47 days post-fertilization). A decrease in dio1 or dio3 or an increase in dio2 transcription observed at different time points of the study may be a sign of hypothyroidism. mRNA expression of genes npr, esr1, and esr2b in the body and npr and esr2b in the head of fish exposed to 310 ng L^-1 LNG was significantly upregulated compared to the solvent control group at the end of the test. Together, these results show that levonorgestrel caused parallel changes in the hypothalamus-pituitary-thyroid and hypothalamus-pituitary-gonad axes.

Selenium and thyroid diseases

Selenium, a non-metallic element, is a micronutrient essential for the biosynthesis of selenoproteins containing selenocysteine. In adults, the thyroid contains the highest amount of selenium per gram of tissue. Most known selenoproteins, such as glutathione peroxidase, are expressed in the thyroid and are involved in thyroid hormone metabolism, redox state regulation, and maintenance of cellular homeostasis. Some clinical studies have shown that lack of selenium will increase the prevalence of several kinds of thyroid diseases. Selenium treatment in patients with Graves' orbitopathy has been shown to delay disease progression and improve the quality of life. Selenium supplementation in Hashimoto's thyroiditis was associated with the decreased levels of anti-thyroid peroxidase antibody and improved thyroid ultrasound structure. In thyroid cancer, various selenium supplements have shown variable anticancer activity. However, published results remain the conflicting and more clinical evidence is still needed to determine the clinical significance of selenium. This article reviews the strong association between selenium and thyroid disease and provides new ideas for the clinical management of selenium in thyroid disease.

Emerging roles of selenium on metabolism and type 2 diabetes

Selenium is recognized as an essential element for human health and enters human body mainly via diet. Selenium is a key constituent in selenoproteins, which exert essential biological functions, including antioxidant and anti-inflammatory effects. Several selenoproteins including glutathione peroxidases, selenoprotein P and selenoprotein S are known to play roles in the regulation of type 2 diabetes. Although there is a close association between certain selenoproteins with glucose metabolism or insulin resistance, the relationship between selenium and type 2 diabetes is complex and remains uncertain. Here we review recent advances in the field with an emphasis on roles of selenium on metabolism and type 2 diabetes. Understanding the association between selenium and type 2 diabetes is important for developing clinical practice guidelines, establishing and implementing effective public health policies, and ultimately combating relative health issues.

Effects of porcine reproductive and respiratory syndrome virus (PRRSV) on thyroid hormone metabolism in the late gestation fetus

Porcine reproductive and respiratory syndrome virus (PRRSV) in late gestation causes a profound suppression of circulating maternal and fetal thyroid hormone during a critical window of development. To understand this relationship, we evaluated thyroid hormone metabolism at the maternal-fetal interface and within fetal tissues, along with hormone metabolite levels in serum. Fetuses were classified using an established model based on viral load in serum and thymus, and preservation status, including uninfected (UNIF), high-viral load viable (HV-VIA), and high-viral load meconium-stained (HV-MEC), with additional controls from sham-inoculated gilts (CON). Expression of three iodothyronine deiodinases, five sulfotransferases, sulfatase, and two solute carriers known to transport thyroid hormone were evaluated in maternal endometrium and fetal placenta, liver, and kidney. Serum thyroxin (T4), reverse triiodothyronine (rT3), and diiodothyronine (T2) were evaluated via liquid chromatography tandem mass spectrometry. Significant changes in gene expression were observed in all four tissues, with the liver being the most severely impacted. We observed local and fetal specific regulation of maternal tissues through significant upregulation of DIO2 and DIO3 expression in the endometrium corresponding to infected but viable fetuses relative to uninfected and control fetuses. Expression levels of DIO2 and DIO3 were significantly higher in the resilient (HV-VIA) fetuses relative to the susceptible (HV-MEC) fetuses. A substantial decrease in serum T4 was confirmed, with no corresponding increase in rT3 or T2. Collectively, these results show that thyroid hormone metabolism is altered at the maternal-fetal interface and within the PRRSV infected fetus and is associated with fetal viability.

Voyage of selenium from environment to life: Beneficial or toxic?

Selenium (Se), a naturally occurring metalloid, is an essential micronutrient for life as it is incorporated as selenocysteine in proteins. Although beneficial at low doses, Se is hazardous at high concentrations and poses a serious threat to various ecosystems. Due to this contrasting 'dual' nature, Se has garnered the attention of researchers wishing to unravel its puzzling properties. In this review, we describe the impact of selenium's journey from environment to diverse biological systems, with an emphasis on its chemical advantage. We describe the uneven distribution of Se and how this affects the bioavailability of this element, which, in turn, profoundly affects the habitat of a region. Once taken up, the subsequent incorporation of Se into proteins as selenocysteine and its antioxidant functions are detailed here. The causes of improved protein function due to the incorporation of redox-active Se atom (instead of S) are examined. Subsequently, the reasons for the deleterious effects of Se, which depend on its chemical form (organo-selenium or the inorganic forms) in different organisms are elaborated. Although Se is vital for the function of many antioxidant enzymes, how the pro-oxidant nature of Se can be potentially exploited in different therapies is highlighted. Furthermore, we succinctly explain how the presence of Se in biological systems offsets the toxic effects of heavy metal mercury. Finally, the different avenues of research that are fundamental to expand our understanding of selenium biology are suggested.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny-part III: how is substance-mediated thyroid hormone imbalance in pregnant/lactating rats or their progeny related to neurodevelopmental effects?

This review investigated which patterns of thyroid- and brain-related effects are seen in rats upon gestational/lactational exposure to 14 substances causing thyroid hormone imbalance by four different modes-of-action (inhibition of thyroid peroxidase, sodium-iodide symporter and deiodinase activities, enhancement of thyroid hormone clearance) or to dietary iodine deficiency. Brain-related parameters included motor activity, cognitive function, acoustic startle response, hearing function, periventricular heterotopia, electrophysiology and brain gene expression. Specific modes-of-action were not related to specific patterns of brain-related effects. Based upon the rat data reviewed, maternal serum thyroid hormone levels do not show a causal relationship with statistically significant neurodevelopmental effects. Offspring serum thyroxine together with offspring serum triiodothyronine and thyroid stimulating hormone appear relevant to predict the likelihood for neurodevelopmental effects. Based upon the collated database, thresholds of ≥60%/≥50% offspring serum thyroxine reduction and ≥20% and statistically significant offspring serum triiodothyronine reduction indicate an increased likelihood for statistically significant neurodevelopmental effects; accuracies: 83% and 67% when excluding electrophysiology (and gene expression). Measurements of brain thyroid hormone levels are likely relevant, too. The extent of substance-mediated thyroid hormone imbalance appears more important than substance mode-of-action to predict neurodevelopmental impairment in rats. Pertinent research needs were identified, e.g. to determine whether the phenomenological offspring thyroid hormone thresholds are relevant for regulatory toxicity testing. The insight from this review shall be used to suggest a tiered testing strategy to determine whether gestational/lactational substance exposure may elicit thyroid hormone imbalance and potentially also neurodevelopmental effects.

Thyroid metabolism and supplementation. A review framed in sports environment

OBJECTIVES

This paper aimed to consider those features that may suggest a link between thyroid hormones pharmacology and athletes' health based on current consumption trends in a population of athletes.

METHODS

Methods used were observation, description, and synthesis, mainly. Among the documents reviewed were: books, scientific articles, and review articles peer-reviewed. The review covered sources published in the period 1961 to 2021. Only references with a traceable origin were accepted (DOI numbering, ISSN and ISBN, as well as peer-reviewed journals). The data on the consumption of thyroid hormones derivatives were extracted from the Doping Control Forms of athlete samples received at Laboratorio Antidoping FMSI of Rome from 2017 to 2021.

RESULTS

An overview of the biosynthesis, pharmacology, and metabolism of thyroid hormones, including thyronamines and thyronacetic acids, was presented. Likewise, a summary is presented on the relationship between thyroid hormones and ethnic and gender differences, their physiology in sport, and the reasons why their use could be considered attractive for athletes.

CONCLUSION

Today, thyroid hormones are not listed as a prohibited substance by the World Anti-Doping Agency. However, several requests to include levothyroxine on the prohibited list are documented. The observation that the number of athletes taking thyroid hormones is growing, particularly in sports such as cycling, triathlons, and skating, should prompt an update on this topic.

In vitro effects of polychlorinated biphenyls and their hydroxylated metabolites on the synthesis and metabolism of iodothyronines in the chicken (Gallus domesticus) thyroid gland

To assess the effect of polychlorinated biphenyls (PCBs) and their hydroxylated metabolites (OH-PCBs) on thyroid hormone [TH: thyroxine (T₄) and triiodothyronine (T₃)] secretion, the concentrations of iodothyronine deiodinases (DIO1, DIO2, DIO3), and mRNA expression of genes involved in TH synthesis (TSHR, NIS, TPO, TG), metabolism (DIO1, DIO2, DIO3), and transport (OATP1C1, MCT8, MCT10, LAT1), chicken thyroid explants were incubated in medium supplemented with TSH (250 mU/ml), PCB118, PCB153, 4-OH-PCB107, and 3-OH-PCB153 (0.5 × 10^-8 M), and TSH together with each PCB and OH-PCB. The results of the in vitro experiment revealed that, except for 4-OH-PCB107, all applied PCBs and OH-PCBs inhibited basal and TSH-stimulated T₄ secretion. Moreover, they increased basal and reduced TSH-stimulated T₃ secretion. PCBs and OH-PCBs decreased the TSH-stimulated TSHR expression. Following PCB and OH-PCB exposure, significant changes in mRNA expression of NIS, TPO, and TG were observed. PCBs and OH-PCBs affected DIO1 and DIO3 transcript levels and protein abundances of each DIO. Furthermore, PCB-dependent effects on OATP1C1, MCT8, and MCT10 mRNA expression were found. In conclusion, both PCB118 and PCB153 and their OH-PCBs affect TH synthesis and deiodination processes in the chicken thyroid gland and influence TH transport across the thyrocyte membrane. In addition, the effects of PCBs and OH-PCBs depended mainly on the type of PCB congener and the exposure time. These results indicate that not only parental PCBs but also OH-PCBs are hazardous for the thyroid gland and may disrupt its endocrine function. Further studies are necessary to explain a mechanism of PCB and OH-PCB action in the avian thyroid gland.

Assay of selenol species in biological samples by the fluorescent probe Sel-green

Selenium (Se) is an essential trace element for diverse cellular functions. The biological significance of Se is predominantly dependent on its incorporation into the selenocysteine (Sec) for synthesis of selenoproteins (SePs), such as thioredoxin reductase family enzymes and glutathione peroxidase family enzymes. In general, the hyperactivity of the selenol group in Sec confers the Sec residue critical for functions of SePs. The Sec is much less abundant than its sulfur analog cysteine (Cys), and it remains a high challenge to detect Sec, especially in complex biological samples. We recently reported a selective fluorescent probe Sel-green for selenols and summarized the principles for design of selenol (and thiophenol) probes. Sel-green discriminates selenols from other biological species, especially thiols, under physiological conditions, and has been applied to detect both endogenous and exogenous selenol species in live cells. In this chapter, we describe a protocol and guideline for the selective detection of Sec by applying the Sel-green. This protocol is also suitable for detection of other selenol species. This practical and convenient assay would assist scientists to better understand the pivotal roles of Sec as well as SePs.

Gaps in the knowledge of thyroid hormones and placental biology

Thyroid hormones (THs) are required for the growth and development of the foetus, stimulating anabolism and oxygen consumption from the early stages of pregnancy to the period of foetal differentiation close to delivery. Maternal changes in the hypothalamic–pituitary thyroid axis are also well known. In contrast, several open questions remain regarding the relationships between the placenta and the maternal and foetal TH systems. The exact mechanism by which the placenta participates in regulating the TH concentration in the foetus and mother and the role of TH in the placenta are still poorly studied. In this review, we aim to summarize the available data in the area and highlight significant gaps in our understanding of the ontogeny and cell-specific localization of TH transporters, TH receptors and TH metabolic enzymes in the placenta in both human and rodent models. Significant deficiencies also exist in knowledge of the contribution of genomic and nongenomic effects of TH on the placenta and finally how the placenta reacts during pregnancy when the mother has thyroid disease. By addressing these key knowledge gaps, improved pregnancy outcomes and management of women with thyroid alterations may be possible.

Involvement of integrin αvβ3 in thyroid hormone-induced dendritogenesis

Activation and/or modulation of the membrane-associated receptors plays a critical role in brain development. Thyroid hormone (TH) acts on both nuclear receptors (thyroid hormone receptor, TR) and membrane-associated receptors, particularly integrin αvβ3 in neurons and glia. Integrin αvβ3-mediated signal transduction mediates various cellular events during development including morphogenesis, migration, synaptogenesis, and intracellular metabolism. However, the involvement of integrin αvβ3-mediated TH action during brain development remains poorly understood. Thus, we examined the integrin αvβ3-mediated effects of TH (T₃, T₄, and rT₃) in the neurons and astrocytes using primary cerebellar culture, astrocyte-enriched culture, Neuro-2A clonal cells, and co-culture of neurons and astrocytes. We found that TH augments dendrite arborization of cerebellar Purkinje cells. This augmentation was suppressed by knockdown of integrin αvβ3, as well as TRα and TRβ. A selective integrin αvβ3 antagonist, LM609, was also found to suppress TH-induced arborization. However, whether this effect was a direct action of TH on Purkinje cells or due to indirect actions of other cells subset such as astrocytes was not clarified. To further study neuron-specific molecular mechanisms, we used Neuro-2A clonal cells and found TH also induces neurite growth. TH-induced neurite growth was reduced by co-exposure with LM609 or knockdown of TRα, but not TRβ. Moreover, co-culture of Neuro-2A and astrocytes also increased TH-induced neurite growth, indicating astrocytes may be involved in neuritogenesis. TH increased the localization of synapsin-1 and F-actin in filopodia tips. TH exposure also increased phosphorylation of FAK, Akt, and ERK1/2. Phosphorylation was suppressed by co-exposure with LM609 and TRα knockdown. These results indicate that TRs and integrin αvβ3 play essential roles in TH-induced dendritogenesis and neuritogenesis. Furthermore, astrocytes-neuron communication via TR-dependent and TR-independent signaling through membrane receptors and F-actin are required for TH-induced neuritogenesis.

Interdependence of Thyroid and Corticosteroid Signaling in Vertebrate Developmental Transitions

Post-embryonic acute developmental processes mainly allow the transition from one life stage in a specific ecological niche to the next life stage in a different ecological niche. Metamorphosis, an emblematic type of these post-embryonic developmental processes, has occurred repeatedly and independently in various phylogenetic groups throughout metazoan evolution, such as in cnidarian, insects, molluscs, tunicates, or vertebrates. This review will focus on metamorphoses and developmental transitions in vertebrates, including typical larval metamorphosis in anuran amphibians, larval and secondary metamorphoses in teleost fishes, egg hatching in sauropsids and birth in mammals. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in the regulation of these life transitions. The review will address the molecular and functional evolution of these axes and their interactions. Mechanisms of integration of internal and environmental cues, and activation of these neuroendocrine axes represent key questions in an “eco-evo-devo” perspective of metamorphosis. The roles played by developmental transitions in the innovation, adaptation, and plasticity of life cycles throughout vertebrates will be discussed. In the current context of global climate change and habitat destruction, the review will also address the impact of environmental factors, such as global warming and endocrine disruptors on hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal/interrenal axes, and regulation of developmental transitions.

Effect of supplementing hydroxy selenomethionine on meat quality of yellow feather broiler

This study was conducted to evaluate the effect of supplementing hydroxy selenomethionine (OH-SeMet) on performance, selenium (Se) deposition in the breast muscle, quality and oxidative stability, and expression of selenoprotein encoding genes of breast meat of the native slow-growing yellow-feathered broiler birds. A total of 375 one-day-old local yellow male birds were randomly assigned into 5 dietary treatments, supplemented with Se 0.0, 0.2, 0.4, 0.6, and 0.8 mg/kg in the form of OH-SeMet. Each treatment consisted of 5 replicates and each replicate had 15 birds, the birds were fed on basal diet containing corn and soybean meal, and the experiment lasted for 63 d. The results showed that dietary Se supplementation linearly increased (P < 0.001) Se contents in both serum and muscle, no significant changes (P > 0.05) were observed on growth performance, yield of breast, meat color, and intramuscular fat deposition of the breast muscle. Dietary Se addition improved water-holding capacity, the pH_24h value, and tenderness of breast muscle, evidenced by a linear decreases of shear force (P < 0.05), accompanied by lower thiobarbituric acid reactive substances and higher glutathione reductase activity. The mRNA abundance of selenoprotein encoding genes also responded to dietary Se levels. It is concluded that, dietary supplementation with OH-SeMet improved muscular Se deposition and meat quality of the native yellow birds, with enhanced antioxidant capability and regulation in selenogenome.

Disturbance of the Dlk1-Dio3 imprinted domain may underlie placental Dio3 suppression and extracellular thyroid hormone disturbance in placenta-derived JEG-3 cells following decabromodiphenyl ether (BDE209) exposure

Decabromodiphenyl ether (BDE209) has been widely used as a flame retardant in the past four decades, leading to human health consequences, especially neurological impairments. Our previous in vivo studies have suggested that developmental neurotoxicity in offspring may be the result of BDE209-induced placental type III iodothyronine deiodinase (Dio3) disturbance and consequent thyroid hormone (TH) instability. Dio3 is paternally imprinted gene, and its balanced expression is crucial in directing normal development and growth. In this study, we used placenta-derived cells to investigate how BDE209 affected Dio3 expression through interfering imprinting mechanisms in the delta-like homolog 1 (Dlk1)-Dio3 imprinted region. Gene chip analysis and RT-qPCR identified miR409-3p, miR410-5p, miR494-3p, miR668-3p and miR889-5p as potential candidates involved in Dio3 deregulation. The sodium bisulfite-clonal sequencing revealed the BDE209 affect methylation status of two differentially methylated regions (DMRs), intergenic-DMR (IG-DMR) and maternally expressed gene 3-DMR (MEG3-DMR). Our data indicate that placental Dio3 may be a potential molecular target for future study of BDE209 developmental toxicity. In particular, miRNAs, IG-DMR and MEG3-DMR in the Dlk1-Dio3 imprinted locus may be informative in directing studies in TH disturbance and developmental toxicity induced by in utero exposure to environmental persistent organic pollutants (POPs), and those candidate miRNAs may prove to be convenient and noninvasive biomarkers for future large-scale population studies.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny - part II: how can key events of relevant adverse outcome pathways be addressed in toxicological assessments?

The current understanding of thyroid-related adverse outcome pathways (AOPs) with adverse neurodevelopmental outcomes in mammals has been reviewed. This served to establish if standard rodent toxicity test methods and in vitro assays allow identifying thyroid-related modes-of-action potentially leading to adverse neurodevelopmental outcomes, and the human relevance of effects - in line with the European Commission's Endocrine Disruptor Criteria. The underlying hypothesis is that an understanding of the key events of relevant AOPs provides insight into differences in incidence, magnitude, or species sensitivity of adverse outcomes. The rodent studies include measurements of serum thyroid hormones, thyroid gland pathology and neurodevelopmental assessments, but do not directly inform on specific modes-of-action. Opportunities to address additional non-routine parameters reflecting critical events of AOPs in toxicological assessments are presented. These parameters appear relevant to support the identification of specific thyroid-related modes-of-action, provided that prevailing technical limitations are overcome. Current understanding of quantitative key event relationships is often weak, but would be needed to determine if the triggering of a molecular initiating event will ultimately result in an adverse outcome. Also, significant species differences in all processes related to thyroid hormone signalling are evident, but the biological implications thereof (including human relevance) are often unknown. In conclusion, careful consideration of the measurement (e.g. timing, method) and interpretation of additional non-routine parameters is warranted. These findings will be used in a subsequent paper to propose a testing strategy to identify if a substance may elicit maternal thyroid hormone imbalance and potentially also neurodevelopmental effects in the progeny.

Deiodinases: How Nonmammalian Research Helped Shape Our Present View

Iodothyronine deiodinases are enzymes capable of activating and inactivating thyroid hormones (THs) and have an important role in regulating TH action in tissues throughout the body. Three types of deiodinases (D1, D2, and D3) were originally defined based on their biochemical characteristics. Cloning of the first complementary DNAs in the 1990s (Dio1 in rat and dio2 and dio3 in frog) allowed to confirm the existence of 3 distinct enzymes. Over the years, increasing genomic information revealed that deiodinases are present in all chordates, vertebrates, and nonvertebrates and that they can even be found in some mollusks and annelids, pointing to an ancient origin. Research in nonmammalian models has substantially broadened our understanding of deiodinases. In relation to their structure, we discovered for instance that biochemical properties such as inhibition by 6-propyl-2-thiouracil, stimulation by dithiothreitol, and temperature optimum are subject to variation. Data from fish, amphibians, and birds were key in shifting our view on the relative importance of activating and inactivating deiodination pathways and in showing the impact of D2 and D3 not only in local but also whole body T3 availability. They also led to the discovery of new local functions such as the acute reciprocal changes in D2 and D3 in hypothalamic tanycytes upon photostimulation, involved in seasonal rhythmicity. With the present possibilities for rapid and precise gene silencing in any species of interest, comparative research will certainly further contribute to a better understanding of the importance of deiodinases for adequate TH action, also in humans.

Role of thyroid hormones in normal and abnormal central nervous system myelination in humans and rodents

Thyroid hormones (THs) are instrumental in promoting the molecular mechanisms which underlie the complex nature of neural development and function within the central nervous system (CNS) in vertebrates. The key neurodevelopmental process of myelination is conserved between humans and rodents, of which both experience peak fetal TH concentrations concomitant with onset of myelination. The importance of supplying adequate levels of THs to the myelin producing cells, the oligodendrocytes, for promoting their maturation is crucial for proper neural function. In this review we examine the key TH distributor and transport proteins, including transthyretin (TTR) and monocarboxylate transporter 8 (MCT8), essential for supporting proper oligodendrocyte and myelin health; and discuss disorders with impaired TH signalling in relation to abnormal CNS myelination in humans and rodents. Furthermore, we explore the importance of using novel TH analogues in the treatment of myelination disorders associated with abnormal TH signalling.

Thyroid hormone: sex-dependent role in nervous system regulation and disease

Thyroid hormone (TH) regulates many functions including metabolism, cell differentiation, and nervous system development. Alteration of thyroid hormone level in the body can lead to nervous system-related problems linked to cognition, visual attention, visual processing, motor skills, language, and memory skills. TH has also been associated with neuropsychiatric disorders including schizophrenia, bipolar disorder, anxiety, and depression. Males and females display sex-specific differences in neuronal signaling. Steroid hormones including testosterone and estrogen are considered to be the prime regulators for programing the neuronal signaling in a male- and female-specific manner. However, other than steroid hormones, TH could also be one of the key signaling molecules to regulate different brain signaling in a male- and female-specific manner. Thyroid-related diseases and neurological diseases show sex-specific incidence; however, the molecular mechanisms behind this are not clear. Hence, it will be very beneficial to understand how TH acts in male and female brains and what are the critical genes and signaling networks. In this review, we have highlighted the role of TH in nervous system regulation and disease outcome and given special emphasis on its sex-specific role in male and female brains. A network model is also presented that provides critical information on TH-regulated genes, signaling, and disease.

Early life nicotine exposure alters mRNA and microRNA expressions related to thyroid function and lipid metabolism in liver and BAT of adult wistar rats

In rats, maternal nicotine exposure during lactation induces obesity, thyroid dysfunction, brown adipose tissue (BAT) hypofunction and liver alterations in adult offspring. Both thyroid function and lipid metabolism are influenced by gene silencing mediated by microRNAs (miRNAs). Here we investigated long-term effects of early nicotine exposure on molecular and epigenetic mechanisms closely related to thyroid and lipid metabolism, through the expression of mRNAs and miRNAs in BAT and liver of adult male and female offspring. At postnatal day 2 (PND2), lactating control (CON) or nicotine (NIC) dams were subcutaneously implanted with osmotic minipumps containing, respectively, saline or 6 mg/kg nicotine. Litters were adjusted to 3 males and 3 females. Offspring's euthanasia occurred at PND180. In the BAT, NIC females showed higher Dio2 mRNA expression, while miR-382* expression was not altered in both sexes. In the liver, NIC offspring of both sexes showed lower Dio1 mRNA expression and higher miR-224 expression, while only NIC females had higher miR-383 and miR-21 expressions. NIC offspring of both sexes showed higher mRNA expression of SCD1 in the liver; NIC males had decreased CPT1 expression, whereas NIC females had increased FASN, miR-370 and miR-122 expressions. Regardless of sex, alterations in liver Dio1, miR-224 and SCD1 expressions are involved in the disturbances caused by maternal nicotine exposure during breastfeeding. Interestingly, females had more altered miRs in the liver. Early nicotine exposure induces a sex dimorphism, particularly regarding hepatic lipid metabolism, through miRs expression.

High levels of maternal total tri-iodothyronine, and low levels of fetal free L-thyroxine and total tri-iodothyronine, are associated with altered deiodinase expression and activity in placenta with gestational diabetes mellitus

Gestational Diabetes Mellitus (GDM) is characterized by abnormal maternal D-glucose metabolism and altered insulin signaling. Dysregulation of thyroid hormones (TH) tri-iodethyronine (T3) and L-thyroxine (T4) Hormones had been associated with GDM, but the physiopathological meaning of these alterations is still unclear. Maternal TH cross the placenta through TH Transporters and their Deiodinases metabolize them to regulate fetal TH levels. Currently, the metabolism of TH in placentas with GDM is unknown, and there are no other studies that evaluate the fetal TH from pregnancies with GDM. Therefore, we evaluated the levels of maternal TH during pregnancy, and fetal TH at delivery, and the expression and activity of placental deiodinases from GDM pregnancies. Pregnant women were followed through pregnancy until delivery. We collected blood samples during 10-14, 24-28, and 36-40 weeks of gestation for measure Thyroid-stimulating hormone (TSH), Free T4 (FT4), Total T4 (TT4), and Total T3 (TT3) concentrations from Normal Glucose Tolerance (NGT) and GDM mothers. Moreover, we measure fetal TSH, FT4, TT4, and TT3 in total blood cord at the delivery. Also, we measured the placental expression of Deiodinases by RT-PCR, western-blotting, and immunohistochemistry. The activity of Deiodinases was estimated quantified rT3 and T3 using T4 as a substrate. Mothers with GDM showed higher levels of TT3 during all pregnancy, and an increased in TSH during second and third trimester, while lower concentrations of neonatal TT4, FT4, and TT3; and an increased TSH level in umbilical cord blood from GDM. Placentae from GDM mothers have a higher expression and activity of Deiodinase 3, but lower Deiodinase 2, than NGT mothers. In conclusion, GDM favors high levels of TT3 during all gestation in the mother, low levels in TT4, FT4 and TT3 at the delivery in neonates, and increases deiodinase 3, but reduce deiodinase 2 expression and activity in the placenta.

Thyroxine binding to type III iodothyronine deiodinase

Iodothyronine deiodinases (Dios) are important selenoproteins that control the concentration of the active thyroid hormone (TH) triiodothyronine through regioselective deiodination. The X-ray structure of a truncated monomer of Type III Dio (Dio3), which deiodinates TH inner rings through a selenocysteine (Sec) residue, revealed a thioredoxin-fold catalytic domain supplemented with an unstructured Ω-loop. Loop dynamics are driven by interactions of the conserved Trp207 with solvent in multi-microsecond molecular dynamics simulations of the Dio3 thioredoxin(Trx)-fold domain. Hydrogen bonding interactions of Glu200 with residues conserved across the Dio family anchor the loop’s N-terminus to the active site Ser-Cys-Thr-Sec sequence. A key long-lived loop conformation coincides with the opening of a cryptic pocket that accommodates thyroxine (T₄) through an I⋯Se halogen bond to Sec170 and the amino acid group with a polar cleft. The Dio3-T₄ complex is stabilized by an I⋯O halogen bond between an outer ring iodine and Asp211, consistent with Dio3 selectivity for inner ring deiodination. Non-conservation of residues, such as Asp211, in other Dio types in the flexible portion of the loop sequence suggests a mechanism for regioselectivity through Dio type-specific loop conformations. Cys168 is proposed to attack the selenenyl iodide intermediate to regenerate Dio3 based upon structural comparison with related Trx-fold proteins.

Zebrafish as a model for studying ovarian development: Recent advances from targeted gene knockout studies

Ovarian development is a complex process controlled by precise coordination of multiple factors. The targeted gene knockout technique is a powerful tool to study the functions of these factors. The successful application of this technique in mice in the past three decades has significantly enhanced our understanding on the molecular mechanism of ovarian development. Recently, with the advent of genome editing techniques, targeted gene knockout research can be carried out in many species. Zebrafish has emerged as an excellent model system to study the control of ovarian development. Dozens of genes related to ovarian development have been knocked out in zebrafish in recent years. Much new information and perspectives on the molecular mechanism of ovarian development have been obtained from these mutant zebrafish. Some findings have challenged conventional views. Several genes have been identified for the first time in vertebrates to control ovarian development. Focusing on ovarian development, the purpose of this review is to briefly summarize recent findings using these gene knockout zebrafish models, and compare these findings with mammalian models. These established mutants and rapid development of gene knockout techniques have prompted zebrafish as an ideal animal model for studying ovarian development.

Chronic thiamethoxam exposure impairs the HPG and HPT axes in adult Chinese rare minnow (Gobiocypris rarus): Docking study, hormone levels, histology, and transcriptional responses

Thiamethoxam has emerged as an environmental contaminant detected in aqueous environments, and its endocrine-disrupting effect at chronic exposure in teleosts remains unknown. In the present study, a docking experiment and an in vivo test were integrated to systematically explore the toxic mechanisms of thiamethoxam in fish. Histological analysis, plasma VTG and hormone level (E2, 11-KT, T3 and T4) determinations, and HPG and HPT gene expression quantification were performed after Chinese rare minnow (Gobiocypris rarus) was exposed to thiamethoxam (0, 0.5, 5, and 50 μg/L) for 90 days. According to the docking study, thiamethoxam had different interactions with ERα, AR and TRα via hydrogen bonding. A decrease in body length and plasma T4 was observed in both genders. The histological damage in liver and delayed gonadal development were observed in both genders at 50 μg/L thiamethoxam treatment. In males, the following HPG axis genes were upregulated: gnrh and cyp19b in the brain; vtg and cyp19a in the liver; and cyp17 and cyp19a in the gonad. In females, erɑ in the liver was significantly upregulated with 0.5 μg/L thiamethoxam treatment, and cyp17 in the gonad was upregulated with all treatment. The suppression of cyp19a, gnrh, cyp11a, and ttr was observed at the concentration of 5 μg/L in the female liver. Taken together, the endocrine system of Chinese rare minnow might be disrupted after chronic exposure to thiamethoxam.

In ovo exposure to brominated flame retardants Part II: Assessment of effects of TBBPA-BDBPE and BTBPE on hatching success, morphometric and physiological endpoints in American kestrels

Tetrabromobisphenol A bis(2,3-dibromopropyl ether) (TBBPA-BDBPE) and 1,2-bis(2,4,6-tribromophenoxy)ethane (BTPBE) are both brominated flame retardants (BFRs) that have been detected in birds; however, their potential biological effects are largely unknown. We assessed the effects of embryonic exposure to TBBPA-BDBPE and BTBPE in a model avian predator, the American kestrel (Falco sparverius). Fertile eggs from a captive population of kestrels were injected on embryonic day 5 (ED5) with a vehicle control or one of three doses within the range of concentrations that have been detected in biota (nominal concentrations of 0, 10, 50 or 100 ng/g egg; measured concentrations 0, 3.0, 13.7 or 33.5 ng TBBPA-BDBPE/g egg and 0, 5.3, 26.8 or 58.1 ng BTBPE/g egg). Eggs were artificially incubated until hatching (ED28), at which point blood and tissues were collected to measure morphological and physiological endpoints, including organ somatic indices, circulating and glandular thyroid hormone concentrations, thyroid gland histology, hepatic deiodinase activity, and markers of oxidative stress. Neither compound had any effects on embryo survival through 90% of the incubation period or on hatching success, body mass, organ size, or oxidative stress of hatchlings. There was evidence of sex-specific effects in the thyroid system responses to the BTBPE exposures, with type 2 deiodinase (D2) activity decreasing at higher doses in female, but not in male hatchlings, suggesting that females may be more sensitive to BTBPE. However, there were no effects of TBBPA-BDBPE on the thyroid system in kestrels. For the BTPBE study, a subset of high-dose eggs was collected throughout the incubation period to measure changes in BTBPE concentrations. There was no decrease in BTBPE over the incubation period, suggesting that BTBPE is slowly metabolized by kestrel embryos throughout their ∼28-d development. These two compounds, therefore, do not appear to be particularly toxic to embryos of the American kestrel.

The relationship between ingested thyroid hormones, thyroid homeostasis and iodine metabolism in humans and teleost fish

Oral l-thyroxine (T4) therapy is used to treat human hypothyroidism but T4 fed to teleost fish does not raise plasma thyroid hormone (TH) levels nor induce growth, even though oral 3,5,3'-triiodo-l-thyronine (T3) is effective. This suggests a major difference in TH metabolism between teleosts and humans, often used as a starting thyroid model for lower vertebrates. To gain further insight on the proximate (mechanistic) and ultimate (survival value) factors underlying this difference, the several steps in TH homeostasis from intestinal TH uptake to hypothalamic-hypophyseal regulation were compared between humans and teleosts, and following dietary TH challenges. A major proximate factor limiting trout T4 uptake is a potent constitutive thiol-inhibited intestinal complete T4 deiodination that is ineffective for T3. At the hepatic level, T4 deiodination, conjugation and extensive biliary excretion with negligible T4 enterohepatic recycling can further block teleost T4 uptake to plasma. Such protection of plasma T4 from dietary T4 may be particularly critical for piscivorous fish consuming thyroid tissue, rich in T4 but not T3. It would prevent disruption by unregulated ingested T4 of the characteristic acute and transient changes in teleost plasma T4 due to diel rhythms, food intake and stress-related factors. These marked natural short-term fluctuations in teleost plasma T4 levels are enabled by the relatively small and rapidly-cleared plasma T4 pool, stemming largely from properties of the plasma T4-binding proteins. Humans, however, due mainly to plasma T4-binding globulin, have a relatively massive circulating pool of T4 and an extremely well-buffered free T4 level, consistent with the major TH role in regulating basal metabolic rate. Furthermore, this large well-buffered and slowly-cleared plasma T4 pool, in conjuction with enterohepatic recycling and relaxation of hypothalamic-hypophyseal negative feedback, allows humans to temporarily 'store' ingested T4 in plasma, thereby sparing endogenous TH secretion and conserving thyroidal iodine reserves. Indeed, iodine conservation is likely the key ultimate factor determining the divergent evolution of the human and teleost systems. For humans, ingested iodine in the form of I^-, or TH and their derivatives, is the sole iodine source and may be limiting in many environments. However, most freshwater teleosts, in addition to their ability to assimilate dietary I^-, can derive sufficient I^- from their copious gill irrigation, with no selective advantage in absorbing dietary T4 which would disrupt their natural acute and transient changes in plasma T4. Thus T4 may act also as a vitamin (vitamone) in humans but not in teleosts; in contrast, T3, naturally ingested at much lower levels, may act as a vitamone in both humans and teleosts.

Insights from zebrafish deficiency models to understand the impact of local thyroid hormone regulator action on early development

Thyroid hormones (THs) stimulate and coordinate a wide range of processes to ensure normal development, mainly by binding of the most active TH 3,5,3'-triiodothyronine (T₃) to nuclear receptors resulting in changes in gene transcription. Local TH action is monitored at three distinct levels by different types of regulators: transmembrane transporters (TH influx and efflux), deiodinases (TH activation and inactivation) and nuclear receptors (TH signalling). Since TH regulators are strongly conserved among vertebrate species, the externally and rapidly developing zebrafish (Danio rerio) has become one of the favourite models to study their role in TH-dependent development. Most regulators are expressed in zebrafish from early stages in development in a dynamic and tissue-specific pattern. Transient or permanent disruption of a given regulator severely perturbs development of multiple organs. These zebrafish deficiency models help to explain why, next to overall hypo-/hyperthyroidism, inactivating mutations in the genes encoding TH regulators such as MCT8 and THRA/B have irreversible adverse effects on human development. Zebrafish are also increasingly used as a high-throughput model to assess the toxicity of various xenobiotics and their impact on development. While adverse effects on TH metabolism and gene expression have been shown, information on direct interaction with TH regulators is scarce, albeit essential to fully understand their mechanism of action. For the future, the combination of novel gene silencing tools, fluorescent reporter lines and (single-cell) transcriptomics holds promise for new zebrafish models to further elucidate the role of each TH regulator in vertebrate development.

Serum Iodine Is Correlated with Iodine Intake and Thyroid Function in School-Age Children from a Sufficient-to-Excessive Iodine Intake Area

BACKGROUND

An alternative feasible and convenient method of assessing iodine intake is needed.

OBJECTIVE

The aim of this study was to examine the utility of serum iodine for assessing iodine intake in children.

METHODS

One blood sample and 2 repeated 24-h urine samples (1-mo interval) were collected from school-age children in Shandong, China. Serum free triiodothyronine (FT3), free thyroxine (FT4), thyroid-stimulating hormone (TSH), thyroglobulin (Tg), total iodine (StI), and non-protein-bound iodine (SnbI) concentrations and urine iodine (UIC) and creatinine (UCr) concentrations were measured. Iodine intake was estimated based on two 24-h urine iodine excretions (24-h UIE). Associations between serum iodine and other factors were analyzed using the Spearman rank correlation test. Receiver operating characteristic (ROC) curves were used to illustrate diagnostic ability of StI and SnbI.

RESULTS

In total, 1686 children aged 7-14 y were enrolled. The median 24-h UIC for the 2 collections was 385 and 399 μg/L, respectively. The median iodine intake was estimated to be 299 μg/d and was significantly higher in boys than in girls (316 μg/d compared with 283 μg/d; P < 0.001). StI and SnbI were both positively correlated with FT4 (ρ = 0.30, P < 0.001; and ρ = 0.21, P < 0.001), Tg (ρ = 0.21, P < 0.001; and ρ = 0.19, P < 0.001), 24-h UIC (ρ = 0.56, P < 0.001; and ρ = 0.47, P < 0.001), 24-h UIE (ρ = 0.46, P < 0.001; and ρ = 0.49, P < 0.001), urine iodine-to-creatinine ratio (ρ = 0.58, P < 0.001; and ρ = 0.62, P < 0.001), and iodine intake (ρ = 0.49, P < 0.001; and ρ = 0.53, P < 0.001). The areas under the ROC curves for StI and SnbI for the diagnosis of excessive iodine intake in children were 0.76 and 0.77, respectively. The optimal StI and SnbI threshold values for defining iodine excess in children were 101 and 56.2 μg/L, respectively.

CONCLUSIONS

Serum iodine was positively correlated with iodine intake and the serum FT4 concentration in children. It is a potential biomarker for diagnosing excessive iodine intake in children. This trial was registered at clinicaltrials.gov as NCT02915536.

Practical considerations for developmental thyroid toxicity assessments: What's working, what's not, and how can we do better?

Thyroid hormones (THs; T3 and T4) play a role in development of cardiovascular, reproductive, immune and nervous systems. Thus, interpretation of TH changes from rodent studies (during pregnancy, in fetuses, neonates, and adults) is critical in hazard characterization and risk assessment. A roundtable session at the 2017 Society of Toxicology (SOT) meeting brought together academic, industry and government scientists to share knowledge and different perspectives on technical and data interpretation issues. Data from a limited group of laboratories were compiled for technical discussions on TH measurements, including good practices for reliable serum TH data. Inter-laboratory historical control data, derived from immunoassays or mass spectrometry methods, revealed: 1) assay sensitivities vary within and across methodologies; 2) TH variability is similar across animal ages; 3) laboratories generally achieve sufficiently sensitive TH quantitation levels, although issues remain for lower levels of serum TH and TSH in fetuses and postnatal day 4 pups; thus, assay sensitivity is critical at these life stages. Best practices require detailed validation of rat serum TH measurements across ages to establish assay sensitivity and precision, and identify potential matrix effects. Finally, issues related to data interpretation for biological understanding and risk assessment were discussed, but their resolution remains elusive.

Screening the ToxCast Phase 1, Phase 2, and e1k Chemical Libraries for Inhibitors of Iodothyronine Deiodinases

Deiodinase enzymes play an essential role in converting thyroid hormones between active and inactive forms by deiodinating the pro-hormone thyroxine (T4) to the active hormone triiodothyronine (T3) and modifying T4 and T3 to inactive forms. Chemical inhibition of deiodinase activity has been identified as an important endpoint to include in screening chemicals for thyroid hormone disruption. To address the lack of data regarding chemicals that inhibit the deiodinase enzymes, we developed robust in vitro assays that utilized human deiodinase types 1, 2, and 3 and screened over 1800 unique chemicals from the U.S. EPA's ToxCast phase 1_v2, phase 2, and e1k libraries. Initial testing at a single concentration identified 411 putative deiodinase inhibitors that produced inhibition of 20% or greater in at least 1 of the 3 deiodinase assays, including chemicals that have not previously been shown to inhibit deiodinases. Of these, 228 chemicals produced enzyme inhibition of 50% or greater; these chemicals were further tested in concentration-response to determine relative potency. Comparisons across these deiodinase assays identified 81 chemicals that produced selective inhibition, with 50% inhibition or greater of only 1 of the deiodinases. This set of 3 deiodinase inhibition assays provides a significant contribution toward expanding the limited number of in vitro assays used to identify chemicals with the potential to interfere with thyroid hormone homeostasis. In addition, these results set the groundwork for development and evaluation of structure-activity relationships for deiodinase inhibition, and inform targeted selection of chemicals for further testing to identify adverse outcomes of deiodinase inhibition.

Utilization of selenocysteine in early-branching fungal phyla

Selenoproteins are a diverse group of proteins containing selenocysteine (Sec)-the twenty-first amino acid-incorporated during translation via a unique recoding mechanism^1,2. Selenoproteins fulfil essential roles in many organisms¹, yet are not ubiquitous across the tree of life^3-7. In particular, fungi were deemed devoid of selenoproteins^4,5,8. However, we show here that Sec is utilized by nine species belonging to diverse early-branching fungal phyla, as evidenced by the genomic presence of both Sec machinery and selenoproteins. Most fungal selenoproteins lack consensus Sec recoding signals (SECIS elements⁹) but exhibit other RNA structures, suggesting altered mechanisms of Sec insertion in fungi. Phylogenetic analyses support a scenario of vertical inheritance of the Sec trait within eukaryotes and fungi. Sec was then lost in numerous independent events in various fungal lineages. Notably, Sec was lost at the base of Dikarya, resulting in the absence of selenoproteins in Saccharomyces cerevisiae and other well-studied fungi. Our results indicate that, despite scattered occurrence, selenoproteins are found in all kingdoms of life.

Disruption of deiodinase type 2 in zebrafish disturbs male and female reproduction

Thyroid hormones are crucial mediators of many aspects of vertebrate life, including reproduction. The key player is the biologically active 3,5,3'-triiodothyronine (T3), whose local bio-availability is strictly regulated by deiodinase enzymes. Deiodinase type 2 (Dio2) is present in many tissues and is the main enzyme for local T3 production. To unravel its role in different physiological processes, we generated a mutant zebrafish line, completely lacking Dio2 activity. Here we focus on the reproductive phenotype studied at the level of offspring production, gametogenesis, functioning of the hypothalamic-pituitary-gonadal axis and sex steroid production. Homozygous Dio2-deficient zebrafish were hypothyroid, displayed a delay in sexual maturity, and the duration of their reproductive period was substantially shortened. Fecundity and fertilization were also severely reduced. Gamete counts pointed to a delay in oogenesis at onset of sexual maturity and later on to an accumulation of oocytes in mutant ovaries due to inhibition of ovulation. Analysis of spermatogenesis showed a strongly decreased number of spermatogonia A at onset of sexual maturity. Investigation of the hypothalamic-pituitary-gonadal axis revealed that dysregulation was largely confined to the gonads with significant upregulation of igf3, and a strong decrease in sex steroid production concomitant with alterations in gene expression in steroidogenesis/steroid signaling pathways. Rescue of the phenotype by T3 supplementation starting at 4 weeks resulted in normalization of reproductive activity in both sexes. The combined results show that reproductive function in mutants is severely hampered in both sexes, thereby linking the loss of Dio2 activity and the resulting hypothyroidism to reproductive dysfunction.

Protective effect of mulberry crude extract against nonylphenol-induced thyroid disruption by inhibiting the activity of deiodinase in rats

Nonylphenol (NP) is an endocrine-disrupting chemical (EDC) that can lead to thyroid disruption. We explored NP-induced toxicity in the rat thyroid and evaluated the mitigating effects of mulberry crude extract (MCE) on NP toxicity. First, we aimed to evaluate NP-induced thyroid disruption by dosing Sprague-Dawley (SD) rats with NP (0, 30, 90, or 270 mg/kg body weight) daily for 28 days. Second, we aimed to determine whether MCE had a detoxifying effect on NP-induced thyroid disruption by dosing SD rats with NP (270 mg/kg body weight) or/and MCE (30, 60, or 120 mg/kg body weight) daily for 28 days. We found that NP significantly inhibited free triiodothyronin (FT₃) and free thyroxine (FT₄) activity in rat serum (P < 0.05), but MCE intervention significantly increased FT₃ and FT₄ serum levels (P < 0.05). It is possible that changes in hormonal composition might trigger the TRH-TSH-TH automatic feedback loop. The activity of the three iodothyronine deiodinases increased significantly after NP-dosing (P < 0.05), but only deiodinase3 (D3) was downregulated after MCE treatment (P < 0.05). Therefore, MCE might be an effective NP-detoxification agent against thyroid disruption because it regulates D3 activity.

Transport, Metabolism, and Function of Thyroid Hormones in the Developing Mammalian Brain

Ever since the discovery of thyroid hormone deficiency as the primary cause of cretinism in the second half of the 19th century, the crucial role of thyroid hormone (TH) signaling in embryonic brain development has been established. However, the biological understanding of TH function in brain formation is far from complete, despite advances in treating thyroid function deficiency disorders. The pleiotropic nature of TH action makes it difficult to identify and study discrete roles of TH in various aspect of embryogenesis, including neurogenesis and brain maturation. These challenges notwithstanding, enormous progress has been achieved in understanding TH production and its regulation, their conversions and routes of entry into the developing mammalian brain. The endocrine environment has to adjust when an embryo ceases to rely solely on maternal source of hormones as its own thyroid gland develops and starts to produce endogenous TH. A number of mechanisms are in place to secure the proper delivery and action of TH with placenta, blood-brain interface, and choroid plexus as barriers of entry that need to selectively transport and modify these hormones thus controlling their active levels. Additionally, target cells also possess mechanisms to import, modify and bind TH to further fine-tune their action. A complex picture of a tightly regulated network of transport proteins, modifying enzymes, and receptors has emerged from the past studies. TH have been implicated in multiple processes related to brain formation in mammals-neuronal progenitor proliferation, neuronal migration, functional maturation, and survival-with their exact roles changing over developmental time. Given the plethora of effects thyroid hormones exert on various cell types at different developmental periods, the precise spatiotemporal regulation of their action is of crucial importance. In this review we summarize the current knowledge about TH delivery, conversions, and function in the developing mammalian brain. We also discuss their potential role in vertebrate brain evolution and offer future directions for research aimed at elucidating TH signaling in nervous system development.

Chronic effects of mercury on Bufo gargarizans larvae: Thyroid disruption, liver damage, oxidative stress and lipid metabolism disorder

Mercury is severely detrimental to organisms and is ubiquitous in both terrestrial and aquatic ecosystems. In the present study, we examined the effects of chronic mercury (Hg) exposure on metamorphosis, body size, thyroid microstructures, liver microstructural and ultrastructural features, and transcript levels of genes associated with lipid metabolism, oxidative stress and thyroid hormones signaling pathways of Chinese toad (Bufo gargarizans) tadpoles. Tadpoles were exposed to mercury concentrations at 0, 6, 12, 18, 24 and 30 µg/L from Gosner stage 26-42 of metamorphic climax. The present results showed that high dose mercury (24 and 30 µg/L) decelerated metamorphosis rate and inhibited body size of B. gargarizans larvae. Histological examinations have clearly exhibited that high mercury concentrations caused thyroid gland and liver damages. Moreover, degeneration and disintegration of hepatocytes, mitochondrial vacuolation, and endoplasmic reticulum breakdown were visible in the ultrastructure of liver after high dose mercury treatment. Furthermore, the larvae exposed to high dose mercury demonstrated a significant decrease in type II iodothyronine deiodinase (Dio2) and thyroid hormone receptor α and β (TRα and TRβ) mRNA levels. Transcript level of superoxide dismutase (SOD) and heat shock protein (HSP) were significantly up regulated in larvae exposed to high dose mercury, while transcript level of phospholipid hydroperoxide glutathione peroxidase (PHGPx) was significantly down regulated. Moreover, exposure to high dose mercury significantly down regulated mRNA expression of carnitine palmitoyltransferase (CPT), sterol carrier protein (SCP), acyl-CoA oxidase (ACOX) and peroxisome proliferator-activated receptor α (PPAPα), but significantly up regulated mRNA expression of fatty acid elongase (FAE), fatty acid synthetase (FAS) and Acetyl CoA Carboxylase (ACC). Therefore, we conclude that high dose mercury induced thyroid function disruption, liver oxidative stress and lipid metabolism disorder by damaging thyroid and liver cell structures and altering the expression levels of relevant genes.

Expression of deiodinase 2 (DIO2) and integrin alpha 9 (ITGA9) genes as indicators' of adaptability and their relationship with physio-biochemical parameters in Tharparkar and Karan Fries heifers during different seasons

The aim of the study was to observe the influence of ambient conditions (seasons) on expression pattern of deiodinase 2 (DIO2) and integrin alpha 9 (ITGA9) genes in peripheral blood mononuclear cells (PBMC) and their relationship with physiological and biochemical parameters of Tharparkar and Karan Fries heifers. Healthy heifers of Tharparkar and Karan Fries breed (6 each) were selected and blood samples were collected from these animals at weekly interval. These blood samples were centrifuged to separate the plasma (for biochemical parameters) and buffy coat (for total RNA isolation). The expression of DIO2 gene was down-regulated during hot humid season and up-regulated during winter season in both the breeds. The DIO2 gene showed a positive correlation with thyroid hormones (T3, T4) and negative correlation with physiological responses (RR, PR, RT, ST), cortisol hormone, antioxidant enzymes (SOD, GPx) and non-esterified fatty-acids (NEFA). The expression pattern of ITGA9 gene was opposite to DIO2 gene, i.e. up-regulated during hot humid season and down-regulated during winter season in both the breeds. ITGA9 gene expression showed a positive correlation with cortisol, antioxidant enzymes, NEFA and physiological responses, and negative correlation with thyroid hormones. The fold change in expression of DIO2 and ITGA9 genes was higher in Karan Fries than Tharparkar heifers during hot humid and winter season. The dry matter intake was lower during hot-humid season than autumn and winter season in both the breeds of heifer. The expression pattern of the DIO2 and ITGA9 genes and their relationship with physio-biochemical parameters revealed the essential role in adaptability of cattle as candidate gene. Based on the results of present study, it can be stated that Tharparkar heifers are better adapted to tropical climatic conditions than Karan Fries heifers.

Multiple nutritional factors and thyroid disease, with particular reference to autoimmune thyroid disease

Hashimoto's thyroiditis (HT) and Graves’ disease (GD) are examples of autoimmune thyroid disease (AITD), the commonest autoimmune condition. Antibodies to thyroid peroxidase (TPO), the enzyme that catalyses thyroid-hormone production and antibodies to the receptor for the thyroid-stimulating hormone, are characteristic of HT and GD, respectively. It is presently accepted that genetic susceptibility, environmental factors, including nutritional factors and immune disorders contribute to the development of AITD. Aiming to investigate the effect of iodine, iron and selenium in the risk, pathogenesis and treatment of thyroid disease, PubMed and the Cochrane Library were searched for relevant publications to provide a narrative review.Iodine: chronic exposure to excess iodine intake induces autoimmune thyroiditis, partly because highly-iodinated thyroglobulin (Tg) is more immunogenic. The recent introduction of universal salt iodisation can have a similar, although transient, effect.Iron: iron deficiency impairs thyroid metabolism. TPO is a haem enzyme that becomes active only after binding haem. AITD patients are frequently iron-deficient since autoimmune gastritis, which reduces iron absorption and coeliac disease which causes iron loss, are frequent co-morbidities. In two-thirds of women with persistent symptoms of hypothyroidism despite appropriate levothyroxine therapy, restoration of serum ferritin above 100 µg/l ameliorated symptoms.Selenium: selenoproteins are essential to thyroid action. In particular, the glutathione peroxidases remove excessive hydrogen peroxide produced there for the iodination of Tg to form thyroid hormones. There is evidence from observational studies and randomised controlled trials that selenium, probably as selenoproteins, can reduce TPO-antibody concentration, hypothyroidism and postpartum thyroiditis. Appropriate status of iodine, iron and selenium is crucial to thyroid health.

From zebrafish to human: A comparative approach to elucidate the role of the thyroid hormone transporter MCT8 during brain development

Monocarboxylate transporter 8 (MCT8) facilitates transmembrane transport of thyroid hormones (THs) ensuring their action on gene expression during vertebrate neurodevelopment. A loss of MCT8 in humans results in severe psychomotor deficits associated with the Allan-Herndon-Dudley Syndrome (AHDS). However, where and when exactly a lack of MCT8 causes the neurological manifestations remains unclear because of the varying expression pattern of MCT8 between specific brain regions and cells. Here, we elaborate on the animal models that have been generated to elucidate the mechanisms underlying MCT8-deficient brain development. The absence of a clear neurological phenotype in Mct8 knockout mice made it clear that a single species would not suffice. The evolutionary conservation of TH action on neurodevelopment as well as the components regulating TH signalling however offers the opportunity to answer different aspects of MCT8 function in brain development using different vertebrate species. Moreover, the plethora of tools for genome editing available today facilitates gene silencing in these animals as well. Studies in the recently generated mct8-deficient zebrafish and Mct8/Oatp1c1 double knockout mice have put forward the current paradigm of impaired TH uptake at the level of the blood-brain barrier during peri- and postnatal development as being the main pathophysiological mechanism of AHDS. RNAi vector-based, cell-specific induction of MCT8 knockdown in the chicken embryo points to an additional function of MCT8 at the level of the neural progenitors during early brain development. Future studies including also additional in vivo models like Xenopus or in vitro approaches such as induced pluripotent stem cells will continue to help unravelling the exact role of MCT8 in developmental events. In the end, this multispecies approach will lead to a unifying thesis regarding the cellular and molecular mechanisms responsible for the neurological phenotype in AHDS patients.