Normal Serum Vitamin D Levels

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Abstract
Kratz et al. report in the October 7 issue1 the revised laboratory reference values for tests commonly ordered at the Massachusetts General Hospital. We noticed that the normative range for 25-hydroxyvitamin D is below what is now the recommended range.2,3 Historical, “normative” data for circulating 25-hydroxyvitamin D levels were based on samples from sun-deprived human subjects, who appeared to be free of disease, with normal circulating 25-hydroxyvitamin D levels assessed by plotting a Gaussian distribution.4 There are many reasons why this method is inaccurate, including such factors as race, lifestyle, use or nonuse of sunscreen, age, and geographic latitude, as well as inappropriately low recommended dietary intake of vitamin D. As a result, investigators have begun to define nutritional vitamin D deficiency by using various biomarkers for circulating 25-hydroxyvitamin D levels, including calcium homeostatic indicators, such as parathyroid hormone, calcium absorption, and bone-mineral density.2,4 Noncalcium homeostatic factors, such as insulin resistance and beta-cell function, have been added to the list of 25-hydroxyvitamin D biomarkers.5 With the use of data from these biomarkers, vitamin D deficiency should be defined as circulating levels of 25-hydroxyvitamin D that are less than 32 ng per milliliter (80 nmol per liter).2