Calcium Imbalances

Calcium is not simply a number supplied by food. Serum calcium is continuously adjusted through intestinal absorption, renal tubular reabsorption, and bone turnover. Parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D coordinate these systems so that the ionized, physiologically active fraction remains within a narrow range. A fall in ionized calcium is detected by the calcium-sensing receptor on parathyroid cells. PTH secretion rises. The kidneys retain more calcium, activate vitamin D, and alter phosphate handling; bone releases calcium when necessary; and active vitamin D increases intestinal calcium absorption. The kidneys therefore influence calcium in two directions: they conserve filtered calcium, but they also activate vitamin D and excrete phosphate. Kidney disease can disrupt both functions. The laboratory result must be interpreted correctly before a nurse labels a patient hypocalcemic. Total serum calcium includes calcium bound to albumin as well as ionized calcium. Hypoalbuminemia can lower the total result while the active ionized fraction remains normal. Use an ionized calcium level or the laboratory's albumin-adjusted total calcium rather than reacting to an unadjusted total calcium in isolation. The speed of the change matters....