(T3) The substance which exerts the physiological action of thyroid hormone (see THYROID GLAND). It is formed in the body cells by the de-iodination of THYROXINE (tetra-iodothyronine) which is the active principle secreted by the thyroid gland. It has also been synthesised, and is now available for the treatment of hypothyroidism (see THYROID GLAND, DISEASES OF). It is three times as potent as thyroxine (T4).