Physiology, Thyroid Stimulating Hormone

Thyroid-stimulating hormone, also known as TSH, is a glycoprotein hormone produced by the anterior pituitary. It is the primary stimulus for thyroid hormone production by the thyroid gland. It also exerts growth effects on thyroid follicular cells, leading to enlargement of the thyroid. The hypothalamic-pituitary axis regulates TSH release. Specifically, neurons in the hypothalamus release TRH, or thyroid-releasing hormone, which stimulates thyrotrophs of the anterior pituitary to secrete TSH. TSH, in turn, stimulates thyroid follicular cells to release thyroid hormones in the form of T3 or T4. Triiodothyronine, or T3, is the active form of thyroid hormone. Though it represents only 20% of the released hormone, the majority of T3 comes from the peripheral conversion of T4 to T3. Tetraiodothyronine, also known as thyroxine or T4, constitutes more than 80% of the secreted hormone. When released into the circulation, it forms T3 through the process of de-iodination. T4 and T3 can then exert negative feedback on the anterior pituitary,y with high levels of T3/T4 decreasing TSH secretion and low levels of T3/T4 increasing TSH release. In this review, we discuss the physiology, biochemistry, and clinical relevance of TSH.