Which organ converts 25(OH) vitamin D to its active form?

• A. Skin
• B. Liver
• C. Kidney
• D. Intestine

Answer: (C)

Activating vitamin D to its hormonal form happens in the kidney. Vitamin D from skin or diet is first converted in the liver to 25-hydroxyvitamin D, a circulating storage form. The kidney then adds a hydroxyl group at the 1 position via the enzyme 1-alpha-hydroxylase, producing 1,25-dihydroxyvitamin D (calcitriol), the biologically active hormone. Calcitriol acts to increase calcium and phosphate absorption from the gut and supports calcium balance, with its production in the kidney tightly regulated by parathyroid hormone and calcium/phosphate levels. The skin and liver are important for providing precursor forms, but the activation to the active hormone is the kidney’s job.