Feedback mechanisms maintaining extracellular calcium concentrations within a narrow, physiologic range (8.9-10.1 mg/dL [2.2-2.5 mM]). A decrease in extracellular fluid (ECF) calcium (Ca2+) triggers an increase in parathyroid hormone (PTH) secretion (1) via the calcium sensor receptor on parathyroid cells. PTH, in turn, results in increased tubular reabsorption of calcium by the kidney (2) and resorption of calcium from bone (2) and also stimulates renal 1,25(OH)₂D production (3). 1,25(OH)₂D, in turn, acts principally on the intestine to increase calcium absorption (4). Collectively, these homeostatic mechanisms serve to restore serum calcium levels to normal.