{"type":"root","children":[{"type":"paragraph","children":[{"type":"text","value":"Parathyroid hormone (PTH), parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands as a polypeptide containing 84 amino acids. PTH is released when blood calcium levels are low. It stimulates the release of calcium from bones into the bloodstream, increases calcium absorption in the intestines (through activation of vitamin D to calcitrol), and reduces calcium excretion by the kidneys. PTH increases the concentration of calcium in the blood by acting upon the parathyroid hormone 1 receptor (high levels in bone and kidney) and the parathyroid hormone 2 receptor (high levels in the central nervous system, pancreas, testis, and placenta). PTH half-life is approximately 4 minutes. Excess PTH production can lead to hypercalcemia while insufficient PTH production can lead to hypocalcemia."}]}]}