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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).
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