Which hormone increases calcium reabsorption in the distal tubule?

• A. Calcitonin
• B. Parathyroid hormone (PTH)
• C. Aldosterone
• D. Antidiuretic hormone

Answer: (B)

Parathyroid hormone drives calcium reabsorption in the distal tubule. When the body needs to conserve calcium, PTH binds to receptors on distal tubule cells and activates a signaling cascade that increases the presence and activity of TRPV5 calcium channels on the apical (lumen-facing) membrane. That lets more Ca2+ enter the cells from the tubular fluid. Inside the cells, calcium is bound by calbindin to be safely transported to the basolateral side, where Ca2+-ATPase and the NCX exchanger move calcium into the interstitial fluid and back into the bloodstream. The result is greater calcium reabsorption from the distal nephron, helping maintain or raise serum calcium.

Calcitonin mainly lowers calcium by reducing bone resorption and has little direct effect on renal calcium reabsorption. Aldosterone focuses on sodium handling and has limited direct impact on distal tubular calcium reabsorption, and antidiuretic hormone regulates water reabsorption in the collecting ducts rather than calcium handling.