The thyroid gland secretes three different hormones. One, calcitonin, is involved in blood calcium regulation. The other two, thyroxine (T4) and tri-iodothyronine (T3) are known together as thyroid hormone and are involved in growth and energy metabolism. T3 is the more active form of thyroid hormone. T4 is converted to T3 by a selenium-dependent process. Iodine, vitamin A and the amino acid tyrosine are required for the synthesis of thyroid hormone. It is thought that thyroid hormone plays a part in the conversion of beta-carotene to vitamin A since hypothyroidism, the thyroid deficiency disease, result in high levels of beta carotene in the blood (hypercarotenaemia).