Primary active compound is T~3~ (triiodothyronine), which may be converted from T~4~ (thyroxine) and which then circulates throughout the body to influence growth and maturation of various tissues. Exact mechanism of action unknown. It is believed thyroid hormone exerts its many metabolic effects through control of DNA transcription and protein synthesis. Involved in normal metabolism, growth, and development; promotes gluconeogenesis, increases utilization and mobilization of glycogen stores and stimulates protein synthesis, increases basal metabolic rate.