What is the active form of vitamin D?

• A. 25(OH) vitamin D2
• B. 1,25-(OH) vitamin D3
• C. 25(OH) vitamin D3
• D. 1,25-(OH) vitamin D2

Answer: (B)

Active vitamin D is 1,25-dihydroxy vitamin D3, also called calcitriol. Vitamin D from skin or diet is first converted in the liver to 25-hydroxy vitamin D, a storage/pre-cursor form. The kidney then converts that to the active hormone by adding a hydroxyl group at the 1 position via 1α-hydroxylase. This active form binds the vitamin D receptor and promotes calcium and phosphate absorption from the gut, supports bone mineralization, and is regulated by PTH (rising when calcium is low). The 25-hydroxy form is typically measured to assess vitamin D status, not the active hormone. While vitamin D2 can be activated, the physiologically dominant active form in humans is the D3-derived calcitriol.