Which hormone increases intestinal calcium absorption indirectly by stimulating the kidney to produce active vitamin D?

• A. Calcitonin
• B. Growth hormone
• C. Parathyroid hormone
• D. Cortisol

Answer: (C)

Parathyroid hormone increases intestinal calcium absorption indirectly by stimulating the kidney to produce active vitamin D. It activates 1-alpha-hydroxylase in the proximal tubules, converting 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol). Calcitriol then enhances calcium (and phosphate) absorption from the gut, raising the amount that enters the circulation. So the boost in intestinal calcium comes through this calcitriol pathway, driven by PTH.

As a reminder, calcitonin lowers serum calcium by inhibiting osteoclast activity, and growth hormone and cortisol influence bone and mineral metabolism in other ways without promoting renal production of calcitriol as their primary effect.