Thyroxine (T4) is a hormone produced by the thyroid gland that exists in the body as free T4 and as bound T4 (bound to protein). A high free T4 result may indicate an overactive thyroid gland (hyperthyroidism), while a low free T4 result may indicate an underactive thyroid gland (hypothyroidism). Also known as FT4, Free Thyroxine, Eltroxin.
