Blood Test
A simple blood test called "25-hydroxy vitamin D" measures circulating vitamin D and provides a fair assessment of the body's vitamin D status. The NIH recommends that the level should be 21 ng/ml or higher, but some experts suggest that blood levels in the range of 30 to 50 ng/ml are preferred. There is no evidence that levels above 50 provide any added benefit and vitamin D toxicity becomes a concern with higher levels. Levels tend to fluctuate with the seasons (lower in winter/higher in summer) due to incidental sun exposure. A doctor might order the blood test to help in determining the amount of vitamin D supplementation that will provide optimum vitamin D status year round.