The human PTH (parathyroid hormone) is synthesized and secreted into the bloodstream as an 84-amino acid mature hormone. PTH cDNA encodes the 115 amino acid preprohormone, which includes 25 amino acids in the signal peptide and six as propeptide. It is an essential hormone in the regulation of calcium homeostasis. PTH acts to raise the extracellular calcium concentration, promotes the absorption of calcium by the intestine, mobilizes calcium salts from the bones, and increases the tendency of the kidney to recover calcium from the urine. PTH also acts to stimulate bone formation to increase bone mass and strength in both humans and rats. PTH is mainly expressed in the parathyroid gland and its secretion is enhanced by low calcium levels and is inhibited by FGF-23. PTH binds and stimulates PTH1R, which activates adenylate cyclase and increases cAMP production. In osteoblasts, PTH promotes the secretion of TRANCE/RANKL and periostin through PTH1R, which promotes the release of calcium from the bone. However, in osteocytes, PTH binding to PTH1R promotes bone formation and IGF-1 production. Excess parathyroid hormone (hyperparathyroidism) results in hypercalcemia and bone loss. High serum levels of PTH and calcium are associated with the occurrence of hypertension. PTH has been used in the treatment of osteoporosis.