There are two forms of Vitamin D that can be measured in the blood - 25 hydroxy-vitamin D and 1,25 dihydroxy-vitamin D.  25 hydroxy-vitamin D is the major form of the hormone found in the blood and is the inactive component from which the active hormone, 1,25 dihyroxy Vitamin D is made.  Because of its long half life and higher concentration, 25 hydroxy-vitamin D is commonly measured to assess and monitor Vitamin D status in individuals. The test for 1,25 dihydroxy-vitamin D is used to test that the kidney is converting an appropriate amount of inactive 25-hydroxy-vitamin D to the active 1,25 dihydroxy-vitamin D.

Vitamin D comes from 2 sources:  a) endogenous – produced in the skin on exposure to sunlight and b) exogenous – ingested in foods and supplements.  The chemical structures of these types of Vitamin D are slightly different and are distinguished by subscripts.  Vitamin D₂ (ergocalciferol) is the form found in foods of animal origin and in most vitamin preparations and supplements while Vitamin D₃ (cholecalciferol) is the form produced in the body and is also used in some high dose formulations used to treat severe Vitamin D deficiency.

Both D₂ and D₃ Vitamin D are converted to the 25-hydroxy Vitamin D and then to 1,25 di-hydroxy Vitamin D.

The main role of Vitamin D is to help regulate the absorption of calcium, phosphate, and (to a lesser extent) magnesium. Vitamin D is vital for the growth and health of bone; without it, bones will be soft, malformed, and unable to repair themselves normally, resulting in the disease called rickets in children and osteomalacia in adults