Testing for Vitamin D Pearls

Kennel KA et al. Vitamin D deficiency in adults : when to test and how to treat.  may Clin Proc 2010: 85; 753-758

1.  Terminology:  D2 = ergocalciferol is obtained from vegetables and oral supplements.  D3= cholecalciferol is obtained from UVB sunlight, oily fish and some fortified foods such as milk and bread.  25 (OH) D= calidiol, contains D2 and D3.  1,25 (OH)2D= calcitriol and is converted in kidney and other tissues by the one alpha hydroxylase gene. 

2.   Deficiency is caused by lack of exposure to sunlight, dietary deficiency or malabsorption.  Measuring calcidiol is best measurement of body stores of 25 (OH) D total vitamin D and is best test for deficiency, whereas 25 (OH)  D  D2 and D3 is useful for monitoring to detect noncompliance, or malabsorption. In general the D content of food is low, and D levels come from sunlight and supplements.

3.  1,25 (OH)D can be falsely normal in vitamin D deficient patients due to hyperparathyroidism and thus should not be measured.

4.  Reference ranges may vary based on geographic location, season, ethnic background, age. 

5.  Vitamin D toxicity has never been reported with a level less than 80, and usually requires over 140.   Fear of toxicity is overblown.  This is because calcitriol, the renal 1,25 OH D, feedbacks directly limiting its production via 24 hydroxylase gene.  Calcitriol also feeds back on the PTH gene.  The alternative result is inert metabolites of Vit D, including 24,25 calcidiol and 1,24,25 calcitriol.

6.  Used but not clinically validated for severe Vitamin D Deficiency:  loading dose of 50,000 weekly for 2-3 months, or tiw for one month.  A minimum total dose of 600,000 iu predicts increasing the level to normal (>30).  A lower dose is used for moderate deficiency.  Maintenance of 800-2000 iu is needed to prevent slideback. 

7.  Powder D3 does NOT clog feeding tubes unlike D2.