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
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.
- 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.