1. The terms "vitamin D2" and "vitamin D3" do not refer to the hydroxylation status of vitamin D--either D2 or D3 can exist in the (1,25-OH), (25-OH), and unhydroxylated varieties--rather, they refer to whether or not the vitamin D derives from animal (D3, also called cholecalciferol) or plant (D2, also called ergocalciferol).
2. Ergocalciferol (Vit D2) is the least expensive, is the type of vitamin D with which milk is fortified, and can be given in large enough oral doses to correct vitamin D deficiency relatively rapidly (e.g., 50000 units of ergocalciferol po qweek x 12 weeks).
3. Cholecalciferol (Vit D3) is better absorbed orally than Vit D2, but is not available as a high dose form like D2 (e.g., 400-800 units po qd of cholecalciferol).
4. Calcitriol refers to the activated form of vitamin D, which is hydroxylated at both the (1) and (25) positions, and may be either D2 or D3; the medication calcitriol is the D3 form.
5. Cholecalciferol (VitD3) is produced in the human skin in response to UV light and is initially unhydroxylated at both the (1) and (25) position. Hydroxylation occurs at the (25) position in the liver (an efficient process which generally occurs even in the setting of advanced liver disease) and occurs at the (1) position in the kidney.
6. Paricalcitol (Zemplar) is a vitamin D2 analogue which is hydroxylated at both (1) and (25) positions.
7. doxercalciferol (Hectorol) is another vitamin D2 analogue which is hydroxylated only at the (1) position.