Vitamin D3 has several forms:
- Cholecalciferol, (sometimes called calciol) which is an inactive, unhydroxylated form of vitamin D3)
- Calcidiol (also called 25-hydroxyvitamin D3), which is the form measured in the blood to assess vitamin D status
- Calcitriol (also called 1,25-dihydroxyvitamin D3), which is the active form of D3.
7-Dehydrocholesterol is the precursor of vitamin D3 and forms cholecalciferol only after being exposed to solar UV radiation.
Cholecalciferol is then hydroxylated in the liver to become calcidiol (25-hydroxyvitamin D3).
Next, calcidiol is again hydroxylated, this time in the kidney, and becomes calcitriol (1,25-dihydroxyvitamin D3). Calcitriol is the most active hormone form of vitamin D3.