Vitamin D exists in four different forms: cholecalciferol, calcifediol, calcitriol and ergocalciferol.
Cholecalciferol: This form is also called vitamin D3, and it's made from cholesterol in your body when your skin is exposed to ultraviolet light. For most of us, it takes about 5 to 30 minutes of sun exposure two days each week to make sufficient amounts of vitamin D. Of course, that can vary based on weather conditions and time of year. Cholecalciferol is not biologically active; it has to travel through your bloodstream to the liver where is it converted into another form of vitamin D called calcifediol.
Calcifediol: The storage form of vitamin D is called 25-hydroxyvitamin D, or calcifediol. It's also the form of vitamin D that's measured in blood tests when your health care provider wants to determine when you have symptoms of vitamin D deficiency. If your calcidiol levels are low, you may not be making enough vitamin D, which can lead to a softening and weakening of your bones. In children, this is called rickets and in adults, it's called osteomalacia. Osteomalacia can lead to osteoporosis.
Calcitriol: Your kidneys take calcifediol and convert it to the biologically active form of vitamin D called 1,25-hydroxyvitamin D, or calcitriol. This is the form that promotes calcium absorption and helps balance the blood levels. It also has a role in normal cell growth, and nerve and muscle function. Calcitriol is also necessary for a healthy immune system and may help to reduce inflammation. Your body regulates your blood levels of calcitriol very closely, so it isn't a good form for testing or monitoring vitamin D deficiency. In fact, calcitriol levels may remain normal while calcifediol levels begin to drop.
Ergocalciferol: Vitamin D2, or ergocalciferol, is similar to cholecalciferol, but it's the form of vitamin D synthesized in plants. Your liver can convert ergocalciferol to calcifediol. It isn't converted to calcidiol as efficiently as cholecalciferol, but it appears to be sufficient for use as a dietary supplement. Some people prefer vitamin D3, and cholecalciferol is also available as a supplement.
Office of Dietary Supplements, National Institutes of Health. "Dietary Supplements Fact Sheet: Vitamin D." Accessed June 3, 2011. http://ods.od.nih.gov/factsheets/vitamind/.