Regulatory aspects of the human hypothalamus-pituitary-thyroid axis

Chrono-Endocrinology in Clinical Practice: A Journey from Pathophysiological to Therapeutic Aspects

This review was aimed at collecting the knowledge on the pathophysiological and clinical aspects of endocrine rhythms and their implications in clinical practice, derived from the published literature and from some personal experiences on this topic. We chose to review, according to the PRISMA guidelines, the results of original and observational studies, reviews, meta-analyses and case reports published up to March 2024. Thus, after summarizing the general aspects of biological rhythms, we will describe the characteristics of several endocrine rhythms and the consequences of their disruption, paying particular attention to the implications in clinical practice. Rhythmic endocrine secretions, like other physiological rhythms, are genetically determined and regulated by a central hypothalamic CLOCK located in the suprachiasmatic nucleus, which links the timing of the rhythms to independent clocks, in a hierarchical organization for the regulation of physiology and behavior. However, some environmental factors, such as daily cycles of light/darkness, sleep/wake, and timing of food intake, may influence the rhythm characteristics. Endocrine rhythms are involved in important physiological processes and their disruption may cause several disorders and also cancer. Thus, it is very important to prevent disruptions of endocrine rhythms and to restore a previously altered rhythm by an early corrective chronotherapy.

Maternal pre-pregnancy obesity modifies the association between first-trimester thyroid hormone sensitivity and gestational Diabetes Mellitus: a retrospective study from Northern China

BACKGROUND

Contradictory relationships have been observed between thyroid function and gestational diabetes mellitus (GDM). Previous studies have indicated that pre-pregnancy BMI (pBMI) could modify their relationships. Few studies have illustrated the role of thyroid hormone sensitivity on GDM. We aimed to explore the effect of pre-pregnancy obesity on the association between early pregnancy thyroid hormone sensitivity and GDM in euthyroid pregnant women.

METHODS

This study included 1310 women with singleton gestation. Subjects were classified into pre-pregnancy obese and non-obese subgroups by pBMI levels with a cutoff of 25 kg/m2. Sensitivity to thyroid hormone was evaluated by Thyroid Feedback Quartile-Based Index (TFQI), Chinese-referenced parametric TFQI (PTFQI), TSH Index (TSHI) and Thyrotrophic T4 Resistance Index (TT4RI). The associations between these composite indices and GDM were analyzed using multivariate regression models in the two subgroups, respectively.

RESULTS

In pre-pregnancy non-obese group, early pregnancy TFQI, PTFQI, TSHI and TT4RI levels were higher in subjects with incident GDM compared to those without GDM (all P < 0.05). By contrast, obese women with GDM exhibited lower levels of those indices (all P < 0.05). The occurrence of GDM were increased with rising TFQI, PTFQI, TSHI and TT4RI quartiles in non-obese women ( all P for trend < 0.05), while exhibited decreased trend across quartiles of those indices in obese women (all P for trend < 0.05). Further logistic analysis indicated contrary relationships between thyroid hormone sensitivity and the occurrence of GDM in the two groups, respectively. The OR of the fourth versus the first quartile of TFQI for GDM was 1.981 (95% CI 1.224, 3.207) in pre-pregnancy non-obese group, while was 0.131 (95% CI 0.036, 0.472) in pre-pregnancy obese group. PTFQI and TSHI yielded similar results.

CONCLUSIONS

The association between maternal sensitivity to thyroid hormones during early gestation and the occurrence of GDM was modified by pre-pregnancy obesity.

Influence of medications on thyroid function in dogs: An update

Erroneous thyroid function test results can occur because of drugs that alter thyroid hormone physiology in one or more aspects, including synthesis, secretion, distribution, and metabolism. Research since publication of the last review in the Journal of Veterinary Internal Medicine (JVIM) 20 years ago has evaluated the effects of amiodarone, zonisamide, inhalant anesthetics, clomipramine, trilostane, and toceranib on thyroid function tests in the dog. In addition, recent work on the effects of glucocorticoids, sulfonamides, phenobarbital, and nonsteroidal anti-inflammatory drugs will be reviewed. Awareness of these effects is necessary to avoid misdiagnosis of hypothyroidism and unnecessary treatment.

Modelling of the thyroid hormone synthesis as a part of nonlinear reaction mechanism with feedback

The synthesis of thyroid hormones in the hypothalamic-pituitary-thyroid (HPT) axis was studied. For this purpose, a reaction model for HPT axis with stoichiometric relations between the main reaction species was postulated. Using the law of mass action, this model has been transformed into a set of nonlinear ordinary differential equations. This new model has been examined by stoichiometric network analysis (SNA) with the aim to see if it possesses the ability to reproduce oscillatory ultradian dynamics founded on the internal feedback mechanism. In particular, a feedback regulation of TSH production based on the interplay between TRH, TSH, somatostatin and thyroid hormones was proposed. Besides, the ten times larger amount of produced T₄ with respect to T₃ in the thyroid gland was successfully simulated. The properties of SNA in combination with experimental results, were used to determine the unknown parameters (19 rate constants of particular reaction steps) necessary for numerical investigations. The steady-state concentrations of 15 reactive species were tuned to be consistent with the experimental data. The predictive potential of the proposed model was illustrated on numerical simulations of somatostatin influence on TSH dynamics investigated experimentally by Weeke et al. in 1975. In addition, all programs for SNA analysis were adapted for this kind of a large model. The procedure of calculating rate constants from steady-state reaction rates and very limited available experimental data was developed. For this purpose, a unique numerical method was developed to fine-tune model parameters while preserving the fixed rate ratios and using the magnitude of the experimentally known oscillation period as the only target value. The postulated model was numerically validated by perturbation simulations with somatostatin infusion and the results were compared with experiments available in literature. Finally, as far as we know, this reaction model with 15 variables is the most dimensional one that have been analysed mathematically to obtain instability region and oscillatory dynamic states. Among the existing models of thyroid homeostasis this theory represents a new class that may improve our understanding of basic physiological processes and helps to develop new therapeutic approaches. Additionally, it may pave the way to improved diagnostic methods for pituitary and thyroid disorders.

Circadian Regulation of Hormonal Timing and the Pathophysiology of Circadian Dysregulation

Circadian rhythms are endogenously generated, daily patterns of behavior and physiology that are essential for optimal health and disease prevention. Disruptions to circadian timing are associated with a host of maladies, including metabolic disease and obesity, diabetes, heart disease, cancer, and mental health disturbances. The circadian timing system is hierarchically organized, with a master circadian clock located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks throughout the CNS and periphery. The SCN receives light information via a direct retinal pathway, synchronizing the master clock to environmental time. At the cellular level, circadian rhythms are ubiquitous, with rhythms generated by interlocking, autoregulatory transcription-translation feedback loops. At the level of the SCN, tight cellular coupling maintains rhythms even in the absence of environmental input. The SCN, in turn, communicates timing information via the autonomic nervous system and hormonal signaling. This signaling couples individual cellular oscillators at the tissue level in extra-SCN brain loci and the periphery and synchronizes subordinate clocks to external time. In the modern world, circadian disruption is widespread due to limited exposure to sunlight during the day, exposure to artificial light at night, and widespread use of light-emitting electronic devices, likely contributing to an increase in the prevalence, and the progression, of a host of disease states. The present overview focuses on the circadian control of endocrine secretions, the significance of rhythms within key endocrine axes for typical, homeostatic functioning, and implications for health and disease when dysregulated. © 2022 American Physiological Society. Compr Physiol 12: 1-30, 2022.

Expression profile of circular RNAs in continuous light-induced ovarian dysfunction

PURPOSE

This study aims to elucidate the underlying relationship between the expression profiles of circular RNAs (circRNAs) and the ovarian dysfunction induced by continuous light.

METHODS

High-throughput sequencing was used to profile the transcriptome of differentially expressed circRNAs (DEcircRNAs) in rat ovary under continuous light exposure (12 h:12 h light/light cycle, L/L group) and a control cycle (12 h:12 h light/dark cycle, L/D group). Gene ontology (GO) enrichment analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, and circRNAs-microRNAs-messenger RNAs networks were performed to predict the role of DEcircRNAs in biological processes and pathways. A quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) assay was conducted to verify the high-throughput sequencing results and the expression level of circadian rhythm genes.

RESULTS

In total, 305 circRNAs were differentially expressed between the L/L and L/D groups. Among these, 211 circRNAs were up regulated, while 94 were down regulated. Eight candidate circRNAs from 305 DEcircRNAs were verified by qRT-PCR. Further bioinformatics analysis revealed that interactions between DEcircRNAs and a set of microRNAs involved in ovarian dysfunction-related pathways, such as regulation of androgen receptors, gonadotrophin releasing hormone signaling pathway, endocrine resistance, etc. Subsequently, we identified rno_circ:chr2:86868285-86964272 and rno_circ:chr1:62330221-62360073 may participate in the pathophysiology of ovarian dysfunction by constructing circRNAs-microRNAs-messenger RNAs networks. Meanwhile, constant light reduced the expression of circadian rhythm genes CLOCK, BAML1, PER1, and PER2 compared with that of controls. Caspase3 and Bax were up regulated in the L/L group compared with the L/D group, while Bcl-2 was down regulated.

CONCLUSIONS

In summary, the results reveal that the expression profiles and potential functions of DEcircRNAs in rat ovaries may play important roles in continuous light-induced ovarian dysfunction. These findings provide novel clues and molecular targets for studying the mechanisms and clinical therapy of ovarian dysfunction.

Subacute and low-dose tributyltin exposure disturbs the mammalian hypothalamus-pituitary-thyroid axis in a sex-dependent manner

Tributyltin (TBT) is an endocrine disruptor chemical (EDC) capable of altering the proper function of the hypothalamus-pituitary thyroid (HPT) axis. This study aimed to evaluate the subacute effects of TBT on the HPT axis of male and female rats. A dose of 100 ng/kg/day TBT was used in both sexes over a 15-day period, and the morphophysiology and gene expression of the HPT axis were assessed. TBT exposure increased the body weight in both sexes, while food efficiency increased - only in male rats. It was also possible to note alterations in the thyroid, with the presence of a stratified epithelium, cystic degeneration, and increased interstitial collagen deposition. A reduction in T3 and T4 levels was only observed in TBT male rats. A reduction in TSH levels was observed in TBT female rats. Evaluating mRNA expression, we observed a decrease in hepatic D1 and TRH mRNA levels in TBT female rats. An increase in D2 mRNA expression in the hypothalamus was observed in TBT male rats. Additionally, no significant changes in TRH or hepatic D1 mRNA expression in TBT male rats or in hypothalamic D1 and D2 mRNA expression in TBT female rats were observed. Thus, we can conclude that TBT has different toxicological effects on male and female rats by altering thyroid gland morphophysiology, leading to abnormal HPT axis function, and even at subacute and low doses, it may be involved in complex endocrine and metabolic disorders.

Establishment of Reference Intervals for Thyroid-Associated Hormones Using refineR Algorithm in Chinese Population at High-Altitude Areas

Objectives

Diagnosis of thyroid disease among individuals dwelling at high altitude remains a challenge. Reference intervals (RIs) for thyroid-associated hormones among Tibetans living at various high altitudes were established to improve diagnosis.

Methods

One thousand two hundred eighty-one subjects were randomly recruited from Nyingchi, Shigatse/Lhasa, and Ali of Tibet. Thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxine (FT4) were measured by the Cobas e601 electrochemiluminescence analyzer. We used multiple linear regression and variance component analysis to assess the effect of sex, age, and altitude on hormones. RIs were established by refineR algorithm and compared with those provided by the manufacturer.

Results

Serum TSH was significantly lower in males than in females, while FT3 and FT4 were higher in males. Both FT3 and FT4 decreased with increasing age. FT3 increased with altitude, while TSH and FT4 were less influenced by altitude. The RI for TSH was 0.764–5.784 μIU/ml, while for FT4, the RIs were 12.36–19.38 pmol/L in females and 14.84–20.18 pmol/L in males. The RIs for FT3 at Nyingchi, Shigatse/Lhasa, and Ali in females were 4.09–4.98, 4.31–5.45, and 4.82–5.58 pmol/L, while in males, the values were 4.82–5.41, 4.88–5.95, and 5.26–6.06 pmol/L, respectively. The obtained RIs for TSH and FT4 were generally higher, while that for FT3 was narrower than the RIs provided by Cobas.

Conclusions

Specific RIs were established for thyroid-associated hormones among Tibetans, which were significantly different from those provided by the manufacturer.

Brains in Metamorphosis: Temporal Transcriptome Dynamics in Hatchery-Reared Flatfishes

Metamorphosis is a captivating process of change during which the morphology of the larva is completely reshaped to face the new challenges of adult life. In the case of fish, this process initiated in the brain has traditionally been considered to be a critical rearing point and despite the pioneering molecular work carried out in other flatfishes, the underlying molecular basis is still relatively poorly characterized. Turbot brain transcriptome of three developmental stages (pre-metamorphic, climax of metamorphosis and post-metamorphic) were analyzed to study the gene expression dynamics throughout the metamorphic process. A total of 1570 genes were differentially expressed in the three developmental stages and we found a specific pattern of gene expression at each stage. Unexpectedly, at the climax stage of metamorphosis, we found highly expressed genes related to the immune response, while the biological pathway enrichment analysis in pre-metamorphic and post-metamorphic were related to cell differentiation and oxygen carrier activity, respectively. In addition, our results confirm the importance of thyroid stimulating hormone, increasing its expression during metamorphosis. Based on our findings, we assume that immune system activation during the climax of metamorphosis stage could be related to processes of larval tissue inflammation, resorption and replacement, as occurs in other vertebrates.

Effects of thyroxine and propylthiouracil on feeding behavior and the expression of hypothalamic appetite-regulating peptides and thyroid function in goldfish (Carassius auratus)

There is poor evidence for an association between thyroidal state, feeding and appetite regulation in fish. We assessed how an altered thyroid state influences feeding behavior, food intake and expression of hypothalamic appetite-regulating peptides (Klotho-α and Klotho-β; orexin, OX; cholecystokinin, CCK; agouti-related peptide, AgRP; cannabinoid receptor 1, CB1) in goldfish. We also measured the expressions of hypothalamic, pituitary and liver transcripts that regulate the thyroid [thyrotropin-releasing hormone (TRH), thyrotropin-releasing hormone receptor (TRH-R) type 1, thyroid stimulating hormone beta (TSHβ), deiodinases (DIO2, DIO3), UDP-glucuronosyltransferase (UGT1A1), thyroid receptor alpha and beta (TRα, TRβ)], and circulating levels of total thyroxine (tT₄) and total triiodothyronine (tT₃). Goldfish were implanted with propylthiouracil (PTU) or T₄ osmotic pumps for 12 days. T₄- treatment increased feeding behavior but not food intake, increased central TSHβ and DIO2, and hepatic DIO2 transcript expression and increased central DIO3 mRNA. Under hyperthyroid conditions, hypothalamic Klotho and CCK expressions were downregulated, suggesting an increased metabolic state and a hypothalamic response to regulate energy balance. AgRP, OX and CB1 were not affected by T₄ treatment. PTU had no effect on any of the parameters examined, suggesting it is not a sensitive thyroid inhibitor in fish. Overall, we show that unlike in mammals, hyperthyroid conditions in goldfish do not lead to an increased desire or need to consume food, furthering evidence for a weak link between the thyroid and appetite.

Comparison of Serum Thyroid Hormone Levels in Green-winged Macaws (Ara chloropterus) Using Radio and Chemiluminescent Immunoassays

This study compared the use of radioimmunoassay (RIA) and chemiluminescent immunoassay (CLIA) to measure serum thyroid hormone levels in green-winged macaws (Ara chloropterus). The sample group comprised 20 male and 13 female (n = 33) healthy, captive green-winged macaws of reproductive age but outside of breeding season. Mean free triiodothyronine (T3), total T3, and free thyroxine (T4) measured by CLIA and RIA corresponded to 5.11 and 5.08 pmol/L (3.33 and 3.31 pg/mL), 1.59 and 1.56 nmol/L (103.5 and 101.5 ng/dL), and 8.25 and 8.82 pmol/L (0.64 and 0.69 ng/dL), respectively. Mean total T4 by RIA corresponded to 6.29 nmol/L (0.49 µg/dL). Mean free T4 and thyroid-stimulating hormone (TSH) levels differed according to immunoassay method, with higher values measured by RIA compared with CLIA. Sex influence was statistically significant in immunoassay results only on free T3 levels. Free T3 levels measured by RIA were higher than levels measured by CLIA in male birds, but this difference was not found with the female bird samples. Conversely, free T3 levels measured by CLIA were higher in the female macaws compared with male birds. Comparative analysis of thyroid hormone measurements in this study revealed that RIA and CLIA are equivalent methods to measure free T4 and total T3 levels but not TSH levels. These findings support the use of CLIA for free T4 and total T3 level determinations in green-winged macaws. However, the CLIA kit used in this study provided invalid total T4 level results for the macaws sampled. Radioimmunoassay and CLIA were equally ineffective for determining TSH levels in this species.

A randomized dose-escalation study on the safety, tolerability, immunogenicity, pharmacokinetics and pharmacodynamics of a novel recombinant human albumin in healthy subjects

OBJECTIVE

Recombinant human albumin (rHA) is an alternative to human serum albumin (HSA) for treating ascites in cirrhosis patients. This study was to evaluate the safety, tolerability, immunogenicity, and pharmacokinetics/pharmacodynamics (PK/PD) of rHA in healthy subjects to guide the design for further clinical trials.

METHODS

Healthy subjects aged 18-55 years were enrolled in this double-blinded, first-in-human, placebo-controlled single ascending dose (SAD) (1.25, 5, 10, 20, or 30g) and positive-controlled multiple-dose study (3-day treatment of 10g/day for three cycles every three weeks). The safety was assessed by adverse events (AEs). Antibodies (IgE and IgD) and cytokines were analyzed for immunogenicity. Serum albumin levels and changes in plasma colloid osmotic pressure (PCOP) and hematocrit (HCT) were measured for PK/PD analysis.

RESULTS

rHA was well tolerated as all AEs were assessed as mild or moderate. No severe allergy or difference in the incidence of AEs was observed among the different cohorts in the SAD study or in the different cycles in the multiple-dose study. The incidence of AEs was similar for the rHA and HSA cohort. Antibodies or cytokines showed no changes after drug administration. As expected, serum albumin levels and PCOP increases, and HCT ratio decreases were dose-related with significant differences (p < 0.01). No differences were observed between rHA and HSA.

CONCLUSION

rHA is safe and well-tolerated in healthy Chinese subjects. rHA and HSA exhibited similar safety, tolerability, and PK/PD profiles. The results support further evaluation of rHA treatment in cirrhotic patients with ascites. The clinical trial registration numbers are CTR20191221 (http://www.chinadrugtrials.org.cn/clinicaltrials.searchlistdetail.dhtml).

Chronothyroidology: Chronobiological Aspects in Thyroid Function and Diseases

Chronobiology is the scientific discipline which considers biological phenomena in relation to time, which assumes itself biological identity. Many physiological processes are cyclically regulated by intrinsic clocks and many pathological events show a circadian time-related occurrence. Even the pituitary–thyroid axis is under the control of a central clock, and the hormones of the pituitary–thyroid axis exhibit circadian, ultradian and circannual rhythmicity. This review, after describing briefly the essential principles of chronobiology, will be focused on the results of personal experiences and of other studies on this issue, paying particular attention to those regarding the thyroid implications, appearing in the literature as reviews, metanalyses, original and observational studies until 28 February 2021 and acquired from two databases (Scopus and PubMed). The first input to biological rhythms is given by a central clock located in the suprachiasmatic nucleus (SCN), which dictates the timing from its hypothalamic site to satellite clocks that contribute in a hierarchical way to regulate the physiological rhythmicity. Disruption of the rhythmic organization can favor the onset of important disorders, including thyroid diseases. Several studies on the interrelationship between thyroid function and circadian rhythmicity demonstrated that thyroid dysfunctions may affect negatively circadian organization, disrupting TSH rhythm. Conversely, alterations of clock machinery may cause important perturbations at the cellular level, which may favor thyroid dysfunctions and also cancer.

The Circadian Clock Is Sustained in the Thyroid Gland of VIP Receptor 2 Deficient Mice

VIP/VPAC2-receptor signaling is crucial for functioning of the circadian clock in the suprachiasmatic nucleus (SCN) since the lack results in disrupted synchrony between SCN cells and altered locomotor activity, body temperature, hormone secretion and heart rhythm. Endocrine glands, including the thyroid, show daily oscillations in clock gene expression and hormone secretion, and SCN projections target neurosecretory hypothalamic thyroid-stimulating hormone (TSH)-releasing hormone cells. The aim of the study was to gain knowledge of mechanisms important for regulation of the thyroid clock by evaluating the impact of VIP/VPAC2-receptor signaling. Quantifications of mRNAs of three clock genes (Per1, Per2 and Bmal1) in thyroids of wild type (WT) and VPAC2-receptor deficient mice were done by qPCR. Tissues were taken every 4^th h during 24-h 12:12 light-dark (LD) and constant darkness (DD) periods, both genders were used. PER1 immunoreactivity was visualized on sections of both WT and VPAC2 lacking mice during a LD cycle. Finally, TSH and the thyroid hormone T4 levels were measured in the sera by commercial ELISAs. During LD, rhythmic expression of all three mRNA was found in both the WT and knockout animals. In VPAC2-receptor knockout animals, the amplitudes were approximately halved compared to the ones in the WT mice. In the WT, Per1 mRNA peaked around "sunset", Per2 mRNA followed with approximately 2 h, while Bmal1 mRNA was in antiphase with Per1. In the VPAC2 knockout mice, the phases of the mRNAs were advanced approximately 5 h compared to the WT. During DD, the phases of all the mRNAs were identical to the ones found during LD in both groups of mice. PER1 immunoreactivity was delayed compared to its mRNA and peaked during the night in follicular cells of both the thyroid and parathyroid glands in the WT animals. In WT animals, TSH was high around the transition to darkness compared to light-on, while T4 did not change during the 24 h cycle. In conclusion, sustained and identical rhythms (phases and amplitudes) of three clock genes were found in VPAC2 deficient mice during LD and DD suggesting high degree of independence of the thyroid clock from the master SCN clock.

The Role of the Thyroid Axis in Fish

In all vertebrates, the thyroid axis is an endocrine feedback system that affects growth, differentiation, and reproduction, by sensing and translating central and peripheral signals to maintain homeostasis and a proper thyroidal set-point. Fish, the most diverse group of vertebrates, rely on this system for somatic growth, metamorphosis, reproductive events, and the ability to tolerate changing environments. The vast majority of the research on the thyroid axis pertains to mammals, in particular rodents, and although some progress has been made to understand the role of this endocrine axis in non-mammalian vertebrates, including amphibians and teleost fish, major gaps in our knowledge remain regarding other groups, such as elasmobranchs and cyclostomes. In this review, we discuss the roles of the thyroid axis in fish and its contributions to growth and development, metamorphosis, reproduction, osmoregulation, as well as feeding and nutrient metabolism. We also discuss how thyroid hormones have been/can be used in aquaculture, and potential threats to the thyroid system in this regard.

Obesity: a neuroimmunometabolic perspective

Neuroimmunology and immunometabolism are burgeoning topics of study, but the intersection of these two fields is scarcely considered. This interplay is particularly prevalent within adipose tissue, where immune cells and the sympathetic nervous system (SNS) have an important role in metabolic homeostasis and pathology, namely in obesity. In the present Review, we first outline the established reciprocal adipose-SNS relationship comprising the neuroendocrine loop facilitated primarily by adipose tissue-derived leptin and SNS-derived noradrenaline. Next, we review the extensive crosstalk between adipocytes and resident innate immune cells as well as the changes that occur in these secretory and signalling pathways in obesity. Finally, we discuss the effect of SNS adrenergic signalling in immune cells and conclude with exciting new research demonstrating an immutable role for SNS-resident macrophages in modulating SNS-adipose crosstalk. We posit that the latter point constitutes the existence of a new field - neuroimmunometabolism.

Tanycytes and the Control of Thyrotropin-Releasing Hormone Flux Into Portal Capillaries

Central and peripheral mechanisms that modulate energy intake, partition and expenditure determine energy homeostasis. Thyroid hormones (TH) regulate energy expenditure through the control of basal metabolic rate and thermogenesis; they also modulate food intake. TH concentrations are regulated by the hypothalamus-pituitary-thyroid (HPT) axis, and by transport and metabolism in blood and target tissues. In mammals, hypophysiotropic thyrotropin-releasing hormone (TRH) neurons of the paraventricular nucleus of the hypothalamus integrate energy-related information. They project to the external zone of the median eminence (ME), a brain circumventricular organ rich in neuron terminal varicosities and buttons, tanycytes, other glial cells and capillaries. These capillary vessels form a portal system that links the base of the hypothalamus with the anterior pituitary. Tanycytes of the medio-basal hypothalamus express a repertoire of proteins involved in transport, sensing, and metabolism of TH; among them is type 2 deiodinase, a source of 3,3',5-triiodo-L-thyronine necessary for negative feedback on TRH neurons. Tanycytes subtypes are distinguished by position and phenotype. The end-feet of β2-tanycytes intermingle with TRH varicosities and terminals in the external layer of the ME and terminate close to the ME capillaries. Besides type 2 deiodinase, β2-tanycytes express the TRH-degrading ectoenzyme (TRH-DE); this enzyme likely controls the amount of TRH entering portal vessels. TRH-DE is rapidly upregulated by TH, contributing to TH negative feedback on HPT axis. Alterations in energy balance also regulate the expression and activity of TRH-DE in the ME, making β2-tanycytes a hub for energy-related regulation of HPT axis activity. β2-tanycytes also express TRH-R1, which mediates positive effects of TRH on TRH-DE activity and the size of β2-tanycyte end-feet contacts with the basal lamina adjacent to ME capillaries. These end-feet associations with ME capillaries, and TRH-DE activity, appear to coordinately control HPT axis activity. Thus, down-stream of neuronal control of TRH release by action potentials arrival in the external layer of the median eminence, imbricated intercellular processes may coordinate the flux of TRH into the portal capillaries. In conclusion, β2-tanycytes appear as a critical cellular element for the somatic and post-secretory control of TRH flux into portal vessels, and HPT axis regulation in mammals.

A comparison of the thyroid disruption induced by decabrominated diphenyl ethers (BDE-209) and decabromodiphenyl ethane (DBDPE) in rats

In recent years, decabromodiphenyl ethane (DBDPE), a new alternative flame retardant to the decabrominated diphenyl ethers (BDE-209), is widely used in a variety of products. Previous studies have indicated that DBDPE, like BDE-209, could disrupt thyroid function. However, compared with BDE-209, the degrees of thyrotoxicosis induced by DBDPE were not clear. In addition, the mechanism of thyrotoxicosis induced by DBDPE or BDE-209 was still under further investigation. In this study, male rats as a model were orally exposed to DBDPE or BDE-209 by 5, 50, 500 mg/kg bw/day for 28 days. Then, we assessed the thyrotoxicosis of DBDPE versus BDE-209 and explored the mechanisms of DBDPE and BDE-209-induced thyrotoxicosis. Results showed that decreased free triiodothyronine (FT3) and increased thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH) in serum were observed in both 500 mg/kg bw/day BDE-209 and DBDPE group. Decreased total thyroxine (TT4), total T3 (TT3), and free T4 (FT4) were only observed in BDE-209 group but not in DBDPE group. Histological examination and transmission electron microscope examination showed that high level exposure to BDE-209 and DBDPE both caused significant changes in histological structure and ultrastructure of the thyroid gland. Additionally, oxidative damages of thyroid gland (decreased SOD and GSH activities, and increased MDA content) were also observed in both BDE-209 and DBDPE groups. TG contents in the thyroid gland was reduced in BDE-209 group but not in DBDPE group. Both BDE-209 and DBDPE affected the expression of hypothalamic-pituitary-thyroid (HPT) axis related genes. These findings suggested that both BDE-209 and DBDPE exposure could disrupt thyroid function in the direction of hypothyroidism and the underlying mechanism was likely to be oxidative stress and perturbations of HPT axis. However, DBDPE was found to be less toxic than BDE-209.

Comparing Methods for Measurement Error Detection in Serial 24-h Hormonal Data

Measurement errors commonly occur in 24-h hormonal data and may affect the outcomes of such studies. Measurement errors often appear as outliers in such data sets; however, no well-established method is available for their automatic detection. In this study, we aimed to compare performances of different methods for outlier detection in hormonal serial data. Hormones (glucose, insulin, thyroid-stimulating hormone, cortisol, and growth hormone) were measured in blood sampled every 10 min for 24 h in 38 participants of the Leiden Longevity Study. Four methods for detecting outliers were compared: (1) eyeballing, (2) Tukey’s fences, (3) stepwise approach, and (4) the expectation-maximization (EM) algorithm. Eyeballing detects outliers based on experts’ knowledge, and the stepwise approach incorporates physiological knowledge with a statistical algorithm. Tukey’s fences and the EM algorithm are data-driven methods, using interquartile range and a mathematical algorithm to identify the underlying distribution, respectively. The performance of the methods was evaluated based on the number of outliers detected and the change in statistical outcomes after removing detected outliers. Eyeballing resulted in the lowest number of outliers detected (1.0% of all data points), followed by Tukey’s fences (2.3%), the stepwise approach (2.7%), and the EM algorithm (11.0%). In all methods, the mean hormone levels did not change materially after removing outliers. However, their minima were affected by outlier removal. Although removing outliers affected the correlation between glucose and insulin on the individual level, when averaged over all participants, none of the 4 methods influenced the correlation. Based on our results, the EM algorithm is not recommended given the high number of outliers detected, even where data points are physiologically plausible. Since Tukey’s fences is not suitable for all types of data and eyeballing is time-consuming, we recommend the stepwise approach for outlier detection, which combines physiological knowledge and an automated process.

Interpreting Elevated TSH in Older Adults

Thyroid function is most often monitored clinically through measurement of the regulatory hormone, thyrotropin (TSH). Subclinical hypothyroidism is generally defined as a TSH level above the reference range while thyroid hormone levels remain within the reference range. Elevated TSH is more common among older adults, leading to high rates of treatment, and over-treatment, in this population. However, the use of levothyroxine in older adults with mild TSH elevations has begun to be called into question by observations that demonstrate a lack of harm from not treating and a lack of benefit from treating. Importantly, these findings suggest that the existing diagnostic algorithm for subclinical hypothyroidism, based on isolated TSH elevation, may be inappropriate for older adults. Age-specific reference ranges have been suggested as a way to avoid inappropriate treatment, but that strategy continues to rely on population norms rather than disease definitions to drive clinical decisions. Recent insight into age-related variability in the underlying pathophysiology that impacts on thyroid function tests demonstrates the need for new clinical tools to allow the targeted use of therapy where it will have benefit.

Adrenal, thyroid and gonadal axes are affected at high altitude

Humans cannot live at very high altitude for reasons, which are not completely understood. Since these reasons are not restricted to cardiorespiratory changes alone, changes in the endocrine system might also be involved. Therefore, hormonal changes during prolonged hypobaric hypoxia were comprehensively assessed to determine effects of altitude and hypoxia on stress, thyroid and gonadal hypothalamus-pituitary hormone axes. Twenty-one male and 19 female participants were examined repetitively during a high-altitude expedition. Cortisol, prolactin, thyroid-stimulating hormone (TSH), fT4 and fT3 and in males follicle-stimulating hormone (FSH), luteinizing hormone (LH) and total testosterone were analysed as well as parameters of hypoxemia, such as SaO2 and paO2 at 550 m (baseline) (n = 40), during ascent at 4844 m (n = 38), 6022 m (n = 31) and 7050 m (n = 13), at 4844 m (n = 29) after acclimatization and after the expedition (n = 38). Correlation analysis of hormone concentrations with oxygen parameters and with altitude revealed statistical association in most cases only with altitude. Adrenal, thyroid and gonadal axes were affected by increasing altitude. Adrenal axis and prolactin were first supressed at 4844 m and then activated with increasing altitude; thyroid and gonadal axes were directly activated or suppressed respectively with increasing altitude. Acclimatisation at 4844 m led to normalization of adrenal and gonadal but not of thyroid axes. In conclusion, acclimatization partly leads to a normalization of the adrenal, thyroid and gonadal axes at around 5000 m. However, at higher altitude, endocrine dysregulation is pronounced and might contribute to the physical degradation found at high altitude.