Parathyroid hormone 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 finds its major target cells in bone and kidney. Parathyroid hormone is secreted as a linear protein of 84 amino acids. Parathyroid hormone accomplishes its job by stimulating at least three processes: Mobilization of calcium from bone, Enhancing absorption of calcium from the small intestine, Suppression of calcium loss in urine.