Liothyronine (CYTOMEL®) is a T3 thyroid hormone normally synthesized and secreted by the thyroid gland in much smaller quantities than its prohormone thyroxine (T4). Most T3 is derived from peripheral monodeiodination of T4 at the 5' position of the outer ring of the iodothyronine nucleus. The hormone finally delivered and used by the tissues is mainly T3. The mechanisms by which thyroid hormones exert their physiologic action are not well understood. These hormones enhance oxygen consumption by most tissues of the body, increase the basal metabolic rate and the metabolism of carbohydrates, lipids, and proteins. Thus, they exert a profound influence on every organ system in the body and are of particular importance in the development of the central nervous system. Thyroid hormone drugs are indicated: as the replacement or supplemental therapy in patients with hypothyroidism of any etiology; as pituitary thyroid-stimulating hormone (TSH) suppressants, in the treatment or prevention of various types of euthyroid goiters; as diagnostic agents in suppression tests to differentiate suspected mild hyperthyroidism or thyroid gland autonomy.