Topic overview

Start with the big picture

Ionized calcium is the physiologically active form of calcium in plasma. PTH is released when serum calcium is low; it raises calcium through effects on bone and the kidney, while promoting phosphate excretion. It also stimulates renal activation of vitamin D. Calcitriol increases intestinal absorption of both calcium and phosphate. Calcitonin is released in response to high calcium and inhibits osteoclasts, although its role in adults is minor. Renal handling helps explain these patterns: PTH reduces phosphate reabsorption in the proximal tubule and enhances calcium reabsorption in the distal convoluted tubule. The lesson also connects hormone imbalances with clinical patterns, including calcium and phosphate changes in parathyroid disorders and bone disease associated with vitamin D deficiency.