Which hormone increases calcium reabsorption in the distal convoluted tubule of the kidney?

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

Answer: (B)

Calcium reabsorption in the distal convoluted tubule is regulated by hormones that respond to calcium needs. Parathyroid hormone increases this reabsorption by acting on distal tubular cells to raise calcium entry from the filtrate through apical channels (like TRPV5) and to boost calcium transport into the blood via basolateral mechanisms (Ca2+-ATPase and Na+/Ca2+ exchangers). This helps restore low serum calcium by pulling more calcium back into circulation. PTH also stimulates 1α-hydroxylase in the proximal tubule, raising active vitamin D levels and increasing intestinal calcium absorption, which supports overall calcium balance.

Calcitonin lowers serum calcium and has only limited direct effect on renal calcium reabsorption in the distal tubule. Aldosterone mainly increases sodium reabsorption and potassium excretion in the distal nephron, not calcium reabsorption. Antidiuretic hormone focuses on water reabsorption in the collecting ducts, with little direct impact on calcium handling in the distal tubule.