PTH is synthesized by the parathyroid glands and stored within the glands themselves. In response to low concentrations of serum calcium, PTH is released, and the synthesis of additional supplies is initiated.
PTH exerts its effects on calcium and phosphate metabolism in the bones and the kidneys. In bone, PTH stimulates the release of calcium and phosphate in response to low serum calcium levels. Paradoxically, it is not the boneresorbing osteoclasts but the bone-forming osteoblasts that have receptors for PTH. Osteoblasts, in turn, signal osteoclasts (via means not completely understood) to increase bone resorption and mobilize calcium and phosphate stores. In the kidneys, PTH stimulates the resorption of calcium, decreases phosphate resorption, and promotes the hydroxylation of inactive vitamin D into its biologically active form.