Thyroid-stimulating hormone (TSH), also known as thyrotropin, is a glycoprotein composed of α and β subunits. The structure of the α subunit is shared with follicle-stimulating hormone (FSH), luteinizing hormone (LH), and human chorionic gonadotropin (HCG), but the β subunit is unique to TSH and responsible for its bioactivity.

TSH stimulates the thyroid gland to synthesis and release of thyroid hormone. As with other trophic hormones, the production and secretion of TSH in the thyrotrophs is influenced by negative feedback regulation by thyroid hormone and a short feedback loop by TSH in the pituitary gland.

TSH concentrations are measured as part of a first-line thyroid function test in patients suspected of having hyperthyroidism or hypothyroidism of thyroid hormones. Interpretation of the results depends on both the TSH and T4 concentrations. In some situations measurement of T3 may also be useful.