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In addition to the hypophysiotropic thyrotropin‐releasing hormone (TRH)‐synthesizing neurons, a glial cell type, the tanycytes, also play a role in the regulation of the hypothalamic‐pituitary‐thyroid (HPT) axis. Tanycytes modulate the feedback regulation of the axis by regulating the local thyroid hormone availability in the median eminence where the hypophysiotropic axons terminate. Recently, we showed that tanycytes produce diacylglycerol lipase alpha (DAGLα), the synthesizing enzyme of the endocannabinoid 2‐arachidonoylglycerol (2‐AG) that inhibits the release of TRH from the hypophysiotropic terminals in median eminence explants. To determine the importance of the endocannabinoid production of tanycytes, adult male Rax‐CreERT2//DAGLαfl/fl mice were treated with tamoxifen to induce a tanycyte specific decrease of DAGLα expression (T‐DAGLα KO). The effect of this genetic manipulation on the activity of the HPT axis was determined. Tanycyte specific decrease of DAGLα expression resulted in an approximately 2‐fold increase of TSHβ mRNA level that was accompanied by increased levels of circulating free T4. The TRH mRNA level was, however, not influenced by the genetic manipulation. In addition to the effects on the HPT axis, the T‐DAGLα KO mice showed increased fat mass ratio and decreased blood glucose levels. These data indicate that when endocannabinoid release of tanycytes is decreased, the disinhibition of the TRH release induces increased TSH synthesis and higher circulating T4 levels. Thus it suggests that in wild‐type mice, tanycytes exert a tonic inhibitory effect on the TRH release of hypophysiotropic axons. Furthermore, the endocannabinoid release of tanycytes also influences glucose homeostasis and fat deposition. [ABSTRACT FROM AUTHOR]

Context: Serum TSH and thyroid hormone (TH) levels are routine markers of thyroid function. However, their diagnostic performance is limited under special conditions, e.g. in amiodarone-induced hyperthyroidism (AIH). Such cases would require the assessment of tissue TH action, which is currently unfeasible., Objective: Development of an approach that determines how well serum parameters are reflected in tissue TH action of patients., Methods: TH-responsive marker genes were identified from human hair follicles (HF) with Next Generation Sequencing, validated by qPCR. A classification model was built with these markers to assess tissue TH action and was deployed on amiodarone treated patients. The impact of amiodarone on tissue TH action was also studied in Thyroid Hormone Action Indicator (THAI) mice., Results: The classification model was validated and shown to predict tissue TH status of subjects with good performance. Serum- and HF-based TH statuses were concordant in hypothyroid and euthyroid amiodarone treated patients. In contrast, amiodarone decreased the coincidence of serum-based and HF-based TH statuses in hyperthyroid patients, indicating that AIH is not unequivocally associated with tissue hyperthyroidism. This was confirmed in the THAI model, where amiodarone prevented tissue hyperthyroidism in THAI mice despite high serum fT4., Conclusion: We developed a minimally-invasive approach using HF markers to assess tissue TH economy that could complement routine diagnostics in controversial cases. We observed that a substantial proportion of AIH patients do not develop tissue hyperthyroidism, indicating that amiodarone protects tissues from thyrotoxicosis. Assessing tissue TH action in patients with AIH may be warranted for treatment decisions., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Endocrine Society.)

Thyroid hormones (TH) play a critical role in cell metabolism and tissue function. TH economy is susceptible to endocrine disrupting chemicals (EDCs) that can disturb hormone production or action. Many environmental pollutants are EDCs, representing an emerging threat to both human health and agricultural production. This has led to an increased demand for proper test systems to examine the effects of potential EDCs. However, current methodologies face challenges. Most test systems use endogenous markers regulated by multiple, often complex regulatory processes, making it difficult to distinguish direct and indirect effects. Moreover, in vitro test systems lack the physiological complexity of EDC metabolism and pharmacokinetics in mammals. Additionally, exposure to environmental EDCs usually involves a mixture of multiple compounds, including in vivo generated metabolites, so the possibility of interactions cannot be ignored. This complexity makes EDC characterization difficult. The Thyroid Hormone Action Indicator (THAI) mouse is a transgenic model that carries a TH-responsive luciferase reporter system, enabling the assessment of tissue-specific TH action. One can evaluate the tissue-specific effects of chemicals on local TH action by quantifying luciferase reporter expression in tissue samples. Furthermore, with in vivo imaging, the THAI mouse model allows for longitudinal studies on the effects of potential EDCs in live animals. This approach provides a powerful tool for testing long-term exposure, complex treatment structures, or withdrawal, as it enables the assessment of changes in local TH action over time in the same animal. This report describes the process of in vivo imaging measurements on THAI mice. The protocol discussed here focuses on developing and imaging hyper- and hypothyroid mice, which can serve as controls. Researchers can adapt or expand the treatments presented to meet their specific needs, offering a foundational approach for further investigation.