what is the function of parathyroid hormone?
Parathyroid hormone (PTH) is secreted by the parathyroid glands. It has several actions, but perhaps the most important is to defend against hypocalcemia. Parathyroid cells sense decreases in plasma Ca and, in response, release preformed PTH into the circulation. PTH increases plasma Ca within minutes by increasing renal and intestinal absorption of Ca and by rapidly mobilizing Ca and PO4 from bone (bone resorption). Renal Ca excretion generally parallels Na excretion and is influenced by many of the same factors that govern Na transport in the proximal tubule. However, PTH enhances distal tubular Ca reabsorption independently of Na. PTH also decreases renal PO4 reabsorption and thus increases renal PO4 losses. Renal PO4 loss prevents the solubility product of Ca and PO4 from being exceeded in plasma as Ca levels rise in response to PTH.
PTH also increases plasma Ca by stimulating conversion of vitamin D (see Vitamin Deficiency, Dependency, and Toxicity: Vitamin D) to its most active form, 1,25(OH)2D. This form of vitamin D increases the percentage of dietary Ca absorbed by the intestine. Despite increased Ca absorption, long-term increases in PTH secretion generally result in further bone resorption by inhibiting osteoblastic function and promoting osteoclastic activity. PTH and vitamin D both function as important regulators of bone growth and bone remodeling
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