Parathyroid Hormone (PTH) is the most important endocrine regulator of calcium and phosphorus concentration in extraCellular fluid. This hormone is secreted from cells of the parathyroid glands and targets cells in bone and kidney. Like other protein hormones, parathyroid hormone is synthesized as a preprohormone. After intraCellular processing, the mature hormone is packaged within the Golgi into secretary vesicles and secreted into blood by exocytosis. Recombinant human PTH is a 9 kDa protein consisting of 84 amino acid residues.