Parathyroid hormone (PTH) is a critical hormone in the regulation of Ca++ homeostasis. 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. Another hormone, parathyroid hormone-related protein, binds to the same receptor as parathyroid hormone and has major effects on development. Like most other protein hormones, parathyroid hormone is synthesized as a preprohormone. After intracellular processing, the mature hormone is packaged with in the Golgi into secretory vesicles, the secreted into blood by exocytosis. In renal epithelium, PTH promotes conversion of Vitamin D to its active form, lowers Ca++ excretion and increases phosphate excretion. PTH also increases hematopoietic stem cell proliferation and mobilization and induces arterial vasodilation by regulating Ca++ influx in PTH1R-expressing arterial smooth muscle.