Vitamin D is essential for good bone health and possibly other important functions, including protection against autoimmune diseases and cancers. Vitamin D is stored in the body as an inactive form of 25 hydroxyvitamin D2 and D3. This form must be converted into 1,25 dihydroxyvitamin D2 or D3 in order to act on tissues. This conversion is done, primarily, in the kidneys and, secondarily, in other body cells. In the vast majority of cases, blood levels of 25 hydroxyvitamin D accurately represent the activity of vitamin D in the tissues. There are rarer situations where a problem with the conversion of vitamin D to 1,25 dihydroxyvitamin D interferes with interpretation of the results: an absence or deficiency of the kidney enzyme responsible for the conversion (1 alpha hydroxylase), abnormal presence of other cell types with 1 alpha hydroxylase. Results are expressed in picomoles of 1,25 dihydroxyvitamin D per litre of blood (pmol/L).
Despite normal vitamin D reserves, below normal 1,25 dihydroxyvitamin D levels (kidney disease) lead to vitamin D deficiency in the body. High 1,25 dihydroxyvitamin D levels (sarcoidosis, other forms of granulomatosis) can lead to increased calcium levels equivalent to vitamin D intoxication.