Clinical Significance

Parathyroid hormone (PTH) is synthesized by the cheif cells of the parathyroid glands and stored into dense neuroendrocrine-type secretory granules, awaiting secretion. Intact PTH is an 84 amino acid polypeptide with a molecular mass of approximately 9.43 kilodaltons. After secretion PTH undergoes rapid proteolysis to generate various circulating C-terminal fragments. Some of these fragments re-enter the bloodstream and are cleared primarily by glomerular filtration, an important route for PTH clearance. The intact and biologically active peptide has a half-life in the circulation of less than 5 minutes.

PTH plays a crucial role in maintaining clacium homeostasis and its measurement is an important aid in the diagnosis of calcium related disorders. In healthy individuals, PTH secretion responds to small alterations in plasma ionized calcium concentration within seconds. Abnormally low ionized calcium concentrations trigger PTH secretion, whereas rising levels of extracellular clacium reduce PTH secretion through a negative feedback mechanism.

PTH regulates calcium levels by concerted effect on three principal organs: bone, intestinal mucosa, and kidney. The effect of PTH on intestinal calcium is indirect, resulting from renal production of the intestinally active vitamin D metabolite, 1,25-dihydroxyvitamin D. In the kidney, PTH stimulates calcium reabsorption and inhibition of phosphate reabsorption from the renal tubules. Eventually PTH promotes oseoclastic bone resorption and release of calcium and phophate from bone.

_{Source: Access PTHIO IFU #A16972, 12/17/15}