The thyroid secretes the hormones thyroxine (T4), triiodothyronine (T3) and calcitonin.
T3 and T4 are iodinated substances derived from the amino acid tyrosine. They act to increase the cellular metabolic rate of the body (cellular respiration, metabolism of proteins and lipids, etc.). Calcitonin inhibits the release of calcium cations by the bones thus controlling the blood level of calcium.
