Triiodothyronine, C15H12I3NO4, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate. Synonyms: T3 (Sodium salt), Liothyronine, O-(4-Hydroxy-3-iodophenyl)-3' 5' -diiodo-L-tyrosine sodium salt.