Men's Osteoporosis Support GroupVitamin D: when to test? There is a recent Osteoporosis Issues on this site that is dedicated to some of the controversies of vitamin D. However, I want to discuss when to test vitamin D levels, which was not mentioned in that article. Note: Unfortunately vitamin D levels are measured in multiple formats. To convert nmol/L to ng/mL divide by 2.5, and multiply ng/mL by 2.5 to convert ng/mL to nmol/L. Multiply mcg by 40 to get IU measurement. J Am Med Dir Assoc. 2010 Feb;11(2):128-31. Epub 2010 Jan 12. Testing for vitamin d deficiency in veterans-is there a seasonal bias? Peiris AN and others. PMID: 20142068. This study involved 9447 veterans in a Tennessee VA medical facility and found, "Vitamin D testing occurred more frequently in September, October, November, and December, whereas the lowest levels of 25(OH) vitamin D levels were found in January, February, and March." Additionally, they found, "Although an argument could be made to check for peak 25(OH) vitamin D levels in September and trough levels in March, the seasonal contribution to vitamin D deficiency is overshadowed by ongoing vitamin D deficiency throughout the year." And a point they noted that I had not seen before, "Thus, it may be prudent to test for vitamin D deficiency in patients presenting with fatigue, myalgias, and arthralgias regardless of the season of presentation." Editor's comments. It appears that the ideal vitamin D testing would occur twice yearly. To measure the maximum (peak) level, test in September and for the lowest level (trough), test in March. And, according to Peiris and others, test whenever fatigue, myalgias or arthralgias occur. And I would suggest, that if you are taking adequate doses of vitamin D supplements throughout the year (even in the summer when you are also getting sunshine), you will always be above the minimum normal serum vitamin D level of 30 ng/mL (75 nmol/L). So trough, e.g., would be around 30 ng/mL and peak might go as high as 90 or 100 ng/mL, especially if you get much sun exposure in the summer. The "normal" range of serum vitamin D on lab results that I have done is 30-100 ng/mL. In a previous Update I've referenced research by Heaney RP that suggested toxicity would not occur with serum vitamin D levels below 500 nmol/L, which would require daily vitamin D intakes of about 20,000 IU/day. A 1999 study by Veith R, with free full text, also suggested that, "Published cases of vitamin D toxicity with hypercalcemia, for which the 25(OH)D concentration and vitamin D dose are known, all involve intake of > or = 1000 microg (40000 IU)/d." So whether the toxicity dose is 40,000 or 20,000 IU/day, the safety margin appears quite wide considering any recommended daily dose I've seen.
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