Thyroid stimulating hormone
TSH is secreted from the thyrotrope cells of the anterior pituitary in response to stimulation by thyrotropin-releasing factor, a neuropeptide that is secreted by neuroendocrine neurones of the paraventricular nucleus of thehypothalamus into the hypothalamo-hypophysial portal circulation. TSH is secreted into the systemic circulation, and in turn acts on the thyroid gland resulting in secretion of the hormones thyroxine (T4) and triiodothyronine (T3). TSH secretion is inhibited by somatostatin, which is released by another population of hypothalamic neuroendocrine neurones.
The level of thyroid hormones (T3 and T4) in the blood have a negative feedback effect on TSH secretion. Thus, when levels of T3 and T4 are high, TSH production is decreased.
TSH is a 28kDa glycoprotein with two subunits, an alpha and a beta subunit. In thyrotropes, TSH is packaged in secretory vesicles that are released by calcium-dependent exocytosis (TRH mobilises intracellular calcium stores via the phospholipase C signaling pathway). It acts via a G-protein coupled receptor - the TSH receptor, which is mainly expressed in the thyroid gland but which is also present in some other tissues, including in some regions of the brain.
The α (alpha) subunit of TSH is identical to that of human chorionic gonadotropin (HCG), luteinizing hormone (LH), and follicle-stimulating hormone (FSH). The β (beta) subunit (TSHB) is unique to TSH. Both subunits are necessary for its biological activity.
TSH levels in the blood are often measured to evaluate thyroid function in patients that appear to have symptoms of hyper- or hypothyroidism, for example when a patient has an enlarged thyroid gland. Levels are usually between 0.4 and 4.0 mIU/L in people with normal thyroid function.
TSH screening is routinely performed in the USA on newborns as part of each state’s newborn screening program. The American Thyroid Association recommends that adults older than age 35 be screened for thyroid disease with a TSH test every five years, although other organizations, such as the U.S. Preventive Services Task Force, challenge this recommendation. Several organizations recommend instead screening women over 50 or those at high risk for thyroid disorders, such as pregnant and postpartum women.