Parathyroid Hormone

Single most important regulator of calcium and phosphate homeostasis in the body.

Effects:
Direct effects on bone and kidneys
Indirect effects on intestines (mediated viaVitD)

Effects on Bone:
- complex interaction activating osteoblasts and osteoclasts
- act indrectly on osteoclasts but have direct ligands on osteoblasts
- acts first to mobilize minerals from areas of rapid equilibrium
- prolonged PTH exposure results in further bone mineralization as lysozomal and hydrolytic enzymes are synthesized

Effects on Kidney:
- PTH has 3 effects on kidneys
- increases production of alpha-hydroxolase, resulting in increased hydroxylation of cholecalciferol to calciferol.
- increases reabsorption of calcium in distal nephron (loop of henle and proximal tubule Ca reabsorption is linked to Na and not influenced by PTH)
- prevents reabsorption of both phosphate and bicarbonate

Effects on Intestine:
- indirect and results in increased calcium absorption by increased VitD hydroxylation in kidney

Feedback inhibition is regulated by calcium levels.
- increased calcium normally results in inhibition of PTH secretion
- decreased calcium levels result in increased PTH secretion