Parathyroid hormone, or parathormone, is a hormone secreted by the parathyroid glands as a polypeptide which contains 84 amino acids. It acts to increase the concentration of calcium (Ca2+) in the blood, whereas calcitonin, which is a hormone secreted by the parafollicular cells of the thyroid gland, acts to decrease calcium concentration. The parathyroid hormone causes the increase of the concentration of calcium in the blood by acting upon parathyroid hormone receptor in three parts of the body: bone, kidney, and intestine. The parathyroid hormone half-life is approximately 4 minutes and has a molecular mass of 9.4 kDa.
If calcium ion concentrations in extracellular fluid fall below normal, parathyroid hormone brings them back within the normal range. In conjunction with increasing calcium concentration, the concentration of phosphate ion in blood is reduced. Parathyroid hormone accomplishes its job by stimulating at least three processes:
1) Mobilization of calcium from bone: although the mechanisms remain obscure, a well-documented effect of parathyroid hormone is to stimulate osteoclasts to reabsorb bone mineral, liberating calcium into blood.
2) Enhancing absorption of calcium from the small intestine: Facilitating calcium absorption from the small intestine would clearly serve to elevate blood levels of calcium. Parathyroid hormone stimulates this process, but indirectly by stimulating production of the active form of vitamin D in the kidney. Vitamin D induces synthesis of a calcium-binding protein in intestinal epithelial cells that facilitates efficient absorption of calcium into blood.
3) Suppression of calcium loss in urine: In addition to stimulating fluxes of calcium into blood from bone and intestine, parathyroid hormone puts a brake on excretion of calcium in urine, thus conserving calcium in blood. This effect is mediated by stimulating tubular reabsorption of calcium. Another effect of parathyroid hormone on the kidney is to stimulate loss of phosphate ions in urine.