How to test for Vitamin D deficiency?

It has been recognized that practitioners become confused and tend to misunderstand vitamin D testing. There are two major options to test for vitamin D deficiency: 1,25-dihydroxyvitamin D and 25-hydroxyvitamin D.

1,25-dihydroxyvitamin D is the active form of vitamin D and this is the main reason why practitioners believe that testing it for vitamin D deficiency or excess is the correct option. However, 1,25-dihydroxyvitamin D is highly regulated by parathyroid hormone (PTH) and vitamin D levels. In vitamin D deficiency for instance, the calcium levels go down and cause a compensatory elevation of parathyroid hormone. When PTH is elevated, it stimulates an increase in the levels of 1,25-dihydroxyvitamin D. Thus in vitamin D deficiency the measured levels of 1,25-dihydroxyvitamin D would be increased in the blood instead of decreased making it a poor choice to assess for vitamin D stores.

To assess for vitamin D stores (excess and deficiency), 25-dihydroxyvitamin D is the best option.

Indications for vitamin D deficiency testing:

Testing should be done only in patients at high risk for developing vitamin D deficiency. For example, older homebound or institutionalized individuals, those with limited sun exposure, obesity, dark skin, osteoporosis, malabsorption.

Should vitamin D deficiency screening be performed in asymptomatic adults?

Answer: No. The USPSTF concludes that the current evidence is insufficient to assess the balance of benefits and harms of screening for vitamin D deficiency in asymptomatic adults.

In summary: