Thyroid hormones (thyroxine T4 and triiodothyronine T3) regulate many cellular processes in virtually every type of tissue. The diverse effects of thyroid hormone include regulation of growth, control of basal metabolic rate, enhanced myocardial contractility, and functional differentiation of the central nervous system. The synthesis of thyroid hormones is controlled by hypothalamic thyrotrophin-releasing hormone (TRH) and pituitary thyroid-stimulating hormone (TSH), and in turn, T4 and T3 regulate TRH and TSH production as part of a negative feedback loop.