Which organs are involved in converting vitamin D to its active calcitriol form?

• A. Liver and kidneys
• B. Pancreas and spleen
• C. Heart and lungs
• D. Skin and bones

Answer: (A)

Activation of vitamin D to calcitriol happens through two hydroxylation steps, performed mainly by two organs. First, the liver adds a hydroxyl group to vitamin D to make 25-hydroxyvitamin D, the major circulating form. Then the kidneys add another hydroxyl group to form 1,25-dihydroxyvitamin D, calcitriol, the active hormone. This renal step is tightly regulated by parathyroid hormone and calcium levels, linking vitamin D activation to calcium homeostasis. The skin simply produces the vitamin D precursor after sun exposure, and bones respond to calcitriol rather than performing its activation. So the organs responsible for producing active calcitriol are the liver and kidneys.