Parathyroid hormone (PTH), which is secreted by the parathyroid glands, increases osteoclastic activity in bone, increases the renal tubular reabsorption of calcium, and stimulates the synthesis of 1,25-dihydroxycholecalciferol by the kidney. Meanwhile, PTH inhibits the reabsorption of phosphate and bicarbonate by the renal tubule. All of these effects cause a net increase in serum calcium (eFigure 26–11).
Main actions of PTH and 1,25-(OH)2D in the maintenance of calcium and phosphate homeostasis. (Reproduced with permission from Chandrasoma P, Taylor CE. Concise Pathology, 2nd ed. McGraw-Hill Companies Inc.; 1994.)