Parathyroid hormone

Parathyroid hormone (PTH, hormone parathyroid) hormone ) is a linear polypeptide of 84 amino acids that increases the permeability of cells to calcium and phosphate ions. It regulates the level of calcium in the blood. It is produced by the main cells of the parathyroid glands.

Parathyroid hormone is synthesized from a larger precursor (115 AMK) and is not stored in cells. That is why it is renewed several times an hour. The indicator for synthesis is the level of calcium in the blood, when hypocalcemia stimulates and hypercalcemia inhibits parathyroid hormone synthesis. The secretion of parathyroid hormone is also inhibited by calcitriol. PTH synthesis is also stimulated by an increased concentration of phosphates. Magnesium has a similar effect on PTH secretion as calcium, but its effect is milder.

The target tissues of PTH are kidney, bone (and teeth), and intestine. These structures are important for calcium regulation; they contain specific receptors that are subject to down-regulation. Calcium is strictly regulated in the body.

Effects
PTH acts in three ways on target tissues:


 * 1) Bone resorption after binding of the hormone to osteoblast receptors, calcium is released from the bone fluid thanks to the calcium pump. Osteoblasts mediate the effect of the hormone on osteoclasts, which thus resorb bone. With a long-lasting effect, the bone thins and at the same time osteoblastic processes are activated to maintain bone density. But resorption prevails over new formation.
 * 2) Kidney – díky PTH  increases calcium reabsorption in the ascending limb of the loop of Henle, distal tubule and collecting duct. Increases tubular resorption of magnesium. Conversely, phosphate excretion is increased by decreasing reabsorption in the proximal tubule.
 * 3) Intestine – renal 1-hydroxylase is stimulated by means of parathormone, which changes calcidiol into calcitriol. This ensures increased absorption of both calcium, and phosphates from the intestines.

It has the opposite effects of calcitonin, a thyroid hormone,, lowering blood calcium levels. Hypoparathyroidism  causes tetanic convulsions. On the contrary, hyperthyroidism can mean the risk of osteoporosis.

Links

 * ws: Parathormon

Related Articles

 * Calcium
 * Phosphate
 * Vitamin D
 * Disorders of calcium phosphate metabolism