Parathyroid hormone (
PTH),
parathormone or
parathyrin, is secreted by the
chief cells of the
parathyroid glands as a
polypeptide containing 84
amino acids, yet effective hormone-receptor interaction requires solely the 34-N-terminal amino acids. While PTH acts to increase the concentration of ionic
calcium (Ca
2+) in the
blood,
calcitonin, a hormone produced by the
parafollicular cells (C cells) of the
thyroid gland, acts to decrease ionic calcium concentration. PTH essentially acts to increase the concentration of calcium in the blood by acting upon the
parathyroid hormone 1 receptor, which is present at high levels in bone and kidney, and the
parathyroid hormone 2 receptor, which is present at high levels in the central nervous system, pancreas, testis, and placenta. PTH half-life is approximately 4 minutes. It has a molecular mass of approximately 9500
Da.