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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 26–13).

eFigure 26–13.

Main actions of PTH and 1,25-(OH)2D in the maintenance of calcium and phosphate homeostasis. (Modified and reproduced, with permission, from Chandrasoma P, Taylor CE. Concise Pathology, 2nd ed. Copyright © 1994 by The McGraw-Hill Companies, Inc.)

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