Hormone, triiodothyronine: A hormone made by the thyroid gland. It has three iodine molecules attached to its molecular structure. Iodine is naturally found in seafood, bread, seaweed, and ordinary table salt.
Triiodothyronine is the most powerful thyroid hormone, and affects almost every process in the body, including body temperature, growth, and heart rate. Also known as T3, liothyronine.