TSH

Thyroid-stimulating hormone (TSH) is a pituitary glycoprotein hormone that regulates the thyroid gland. It is a heterodimer composed of a common alpha subunit shared with follicle-stimulating hormone, luteinizing hormone, and human chorionic gonadotropin, and a unique beta subunit that provides receptor specificity. TSH binds to the TSH receptor on thyroid follicular cells, activating signaling pathways that stimulate the synthesis and release of thyroid hormones and promote growth of the thyroid gland. These hormones, primarily thyroxine (T4) and to a lesser extent triiodothyronine (T3), regulate metabolic rate and influence multiple organ systems.

TSH secretion is controlled by thyrotropin-releasing hormone (TRH) from the hypothalamus and is subject to negative feedback by circulating T4 and T3. The hypothalamic-pituitary-thyroid axis operates in a pulsatile manner, ensuring coordinated thyroid hormone production.

Clinically, TSH is the primary screening test for thyroid function. Elevated TSH suggests hypothyroidism, while suppressed (low) TSH suggests hyperthyroidism. In pregnancy, TSH levels can shift due to hormonal changes; in newborns, TSH is used for congenital hypothyroidism screening. Interpretation usually includes measurement of free T4 to distinguish causes. Reference ranges vary by assay and population.

Factors that affect TSH testing include non-thyroidal illness, certain medications (e.g., dopamine, glucocorticoids, amiodarone), and biotin interference with some immunoassays. In treatment, TSH is used to monitor and guide therapy, such as levothyroxine dosing in hypothyroidism or in thyroid cancer, where TSH suppression may be employed to reduce recurrence risk.