Which form is considered the storage 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: (A)

Vitamin D is stored in the body as 25-hydroxyvitamin D, a form produced in the liver by adding a hydroxyl group at the 25 position. This storage form, calcidiol, acts as the reservoir from which the active hormone can be made in the kidney as needed. There are two versions depending on the vitamin D source: 25-hydroxyvitamin D2 from ergocalciferol (vitamin D2) and 25-hydroxyvitamin D3 from cholecalciferol (vitamin D3). Clinically we measure total 25(OH)D to assess vitamin D status, since it reflects both sources. The other form listed, 1,25-dihydroxyvitamin D, is the active hormone, not the storage pool. Therefore, the storage form corresponding to the D2-derived pathway is 25(OH) vitamin D2.