The thyroid hormones are two hormones, T3 and T4, produced by the thyroid gland that play the major role in the regulation of metabolism. When released, they increase the metabolic rate, affect the use of proteins, fat and carbohydrates, regulate the synthesis of proteins, regulate the growth of long bones, increase the body's sensitivity to some other hormones, most notably adrenaline, and produce body heat which tends to raise body temperature. Both hormones contain iodine, which is vital to the synthesis of the hormones.