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Vitamin D Testing

The Mayo Medical Laboratories Difference


Updated: September 2013
Published: February 2009

Vitamin D is vital for strong bones. It also has important, emerging roles in immune function and cancer prevention. Deficiencies at any stage of life can have devastating consequences. Similarly, vitamin D toxicity resulting from overmedication can cause serious hypercalcemia. Vitamin D consists of 2 bioequivalent forms:

  • Vitamin D2: obtained from vegetable sources (dietary sources, supplements);
  • Vitamin D3: derived from both endogenous (synthesized from cholesterol through sun exposure) and exogenous (animal diet) sources.

Until recently, vitamin D replacement in the United States consisted exclusively of 25-hydroxy-vitamin D2. During the last 1 to 2 years an increasing proportion of 25-hydroxy-vitamin D3 replacement is being used, in particular in high dose prescription formulations.

Vitamin D deficiency is more common than previously believed, especially among adolescents, women, and the elderly. For example, studies have shown that more than 50% of the institutionalized elderly and an equal proportion of women of any age undergoing treatment for osteoporosis have inadequate levels of vitamin D.1 While treatment with vitamin supplementation is easy and inexpensive, many affected individuals go undiagnosed and untreated.

The total 25-hydroxyvitamin D (25-OH-VitD) level (the sum of 25-OH-vitamin D2 and 25-OH-vitamin D3) is the appropriate indicator of vitamin D body stores. Although there is no universal consensus about a treatment cut point, studies suggest 25 to 35 ng/mL as the minimal concentration of 25-OH-VitD needed to avoid the adverse effects of deficiency.2,3,4 By contrast, population reference ranges do not correspond with healthy ranges. In northern latitude locations in particular, one-third of the population may have vitamin D levels less than 25 ng/mL at the end of winter.


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