Parathyroid Hormone


  • Most important endocrine regulator of calcium and phosphorus concentration in extracellular fluid.

  • Secreted from cells of the parathyroid glands - four small glands situated at each pole of the thyroid gland.

  • Major target cells in bone and kidney.

  • Similar to other protein hormones, parathyroid hormone is synthesized as a preprohormone and after intracellular processing it is secreted into the blood a linear protein of 84 amino acids.

Physiologic Effects

  • Parathyroid hormone increases the calcium ion concentration in extracellular fluid as a result of an initial fall below normal.
  • In conjunction with increasing calcium concentration, the concentration of phosphate ion in blood is reduced.
  • It accomplishes its job by stimulating at least three processes:
  • Mobilization of calcium from bone probably by stimulating osteoclasts to reabsorb bone mineral, liberating calcium into blood
  • Enhancing absorption of calcium from the small intestine by indirectly stimulating the 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.
  • Stimulates tubular reabsorption of calcium and loss of phosphate ions in urine.