PTH influences calcium homeostasis primarily by increasing calcium reabsorption in which organ?

• A. Kidneys
• B. Liver
• C. Spleen
• D. Pancreas

Answer: (A)

Calcium balance is kept by PTH raising serum calcium through several avenues, with renal reabsorption being a direct and key mechanism. PTH acts on the kidneys to increase calcium reabsorption, especially in the distal tubule, reclaiming calcium from the filtrate so less is lost in urine. This rapid, renal conservation helps lift calcium levels effectively. PTH also promotes production of active vitamin D in the kidney, which increases intestinal calcium absorption, and it stimulates bone resorption to release calcium. The liver participates in vitamin D metabolism upstream, but the direct site of calcium reabsorption driven by PTH is the kidneys, while the spleen and pancreas do not play a role in this process.