A polypeptide hormone (84 amino acid residues) secreted by the PARATHYROID GLANDS which performs the essential role of maintaining intracellular CALCIUM levels in the body. Parathyroid hormone increases intracellular calcium by promoting the release of CALCIUM from BONE, increases the intestinal absorption of calcium, increases the renal tubular reabsorption of calcium, and increases the renal excretion of phosphates.