Metabolism Freedom of expression beyond the thyroid: the thyroid- stimulating hormone receptor in the adipocyte

Introduction The TSH receptor (TSHR) enables thyrocytes to recognize and respond to TSH secreted by pituitary thyrotrophs. It regulates thyrocyte proliferation as well as thyroid hormone production and secretion. Expression of functional TSHR occurs outside of the thyroid gland, and there is ongoing interest in understanding what role TSHR may play in these extra-thyroidal sites. In this regard, adipose tissue has been studied extensively in rodent models and humans. TSHR has been linked to lipolytic responses, as well as to production of adipokines, bioactive factors synthesized and secreted by adipocytes that influence a number of physiological processes such as inflammation and insulin sensitivity. Studies related to TSHR expression in the skeleton and the liver will also be presented. Conclusion Evidence continues to accumulate pointing to a role for TSHR beyond the thyroid gland. In adipose tissue, this receptor may be involved in triggering inflammation, insulin resistance, and cardiovascular disease. TSHR has also been implicated in some aspects of bone remodelling and in hepatic cholesterol metabolism. Overall, the studies reviewed here make a persuasive case to consider the impact of TSHR signalling in physiological systems beyond the thyroid gland.