Triiodothyronine is one of two tyrosine-based hormones secreted by the thyroid gland, the other being thyroxine (T₄). Triiodothyronine (T₃), as its name suggests, has three iodine atoms within its molecular structure. Although the thyroid gland produces about 20 times less triiodothyronine than thyroxine, triiodothyronine is the more powerful hormone by a factor of three or four. It affects almost every process in the body, including body temperature, growth, and heart rate.