Lab Value


Parathyroid hormone (PTH) acts indirectly on which organ to increase calcium levels?

Parathyroid hormone (PTH), calcitonin, and vitamin D, in concert, regulate calcium homeostasis (see the image).

Under normal physiologic circumstances, PTH is released in response to hypocalcemia. The main physiologic response elicited by PTH is to increase plasma calcium levels. PTH has its primary physiologic effects on kidney and bone. In the kidney, PTH increases calcium absorption and phosphate excretion. It also increases the activity of 1α-hydroxylase, the enzyme responsible for formation of the active form of vitamin D [1,25(OH)2D]. The site of PTH regulation of Ca2+ reabsorption is the distal tubules.

In the bone, PTH binds osteoclast receptors and increases their function while inhibiting osteoblast function, resulting in rapid calcium release from bone matrix into the extracellular compartment.

In the intestine, PTH increases the absorption of calcium indirectly via vitamin D3 activation. Cholecalciferol (vitamin D3) is produced by the effect of sunlight on 7-dehydrocholesterol in the skin. Cholecalciferol is then hydroxylated to calcifediol (25-hydroxyvitamin D), and further hydroxylated to its active form of calcitriol (1,25-dihydroxyvitamin D). This activated form results in a greater absorption of calcium and phosphate from the intestine. PTH directly increases the hydroxylation of calcifediol to calcitriol.