Thyroid-stimulating Hormone

Thyroid-stimulating hormone (TSH) is a glycoprotein produced by the thyrotrope cells of the anterior pituitary gland. TSH, luteinizing hormone (LH) and follicle stimulating hormone (FSH), as well as the placental hormone chorionic gonadotropin (CG), consist of a heterodimer of two noncovalently linked subunits, a and b. The b subunit is unique to each and confers specificity of action while the a-subunit is common to all four glycoprotein hormones. Each TSH subunit is encoded by a separate gene located on a different chromosome and is transcribed in a coordinated manner responsive mainly to the stimulatory effect of hypothalamic thyrotropin- releasing hormone (TRH) and the inhibitory effect of thyroid hormone. Production of bioactive TSH involves a process of cotranslational glycosylation and folding that enables combination between the nascent α and β subunits. TSH is stored in secretory granules and released into the circulation in a regulated manner responsive mainly to the stimulatory effect of TRH. Circulating TSH binds to specific cell-surface receptors on the thyroid gland where it stimulates the production of thyroid hormones, L-thyroxine (T4) and L-triiodothyronine (T3), which act on multiple organs and tissues to modulate many metabolic processes as well as result in a negative inhibition of TSH output. The introduction of sensitive TSH assays has allowed accurate measurement of the level of circulating TSH and has led to the recognition of abnormal production of TSH related with abnormal function of the thyroid gland reflecting in a wide range of metabolic derangements.