Vitamin D tests

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25-hydroxy vitamin D tests are used to see if bone weakness, bone malformation, or abnormal metabolism of calcium (reflected by abnormal calcium, phosphate or PTH tests) is occurring as a result of a deficiency or excess of vitamin D.

Since vitamin D is a fat-soluble vitamin and is absorbed from the intestine like a fat, vitamin D tests are sometimes used to monitor and assure individuals with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn's disease that they have adequate amounts of vitamin D. Vitamin D is also sometimes used to find out about the effectiveness of treatment with vitamin D, calcium, phosphate, and/or magnesium supplementation.

When is it requested?

If calcium concentration is low or the patient has symptoms of vitamin D deficiency, such as bone malformation in children (rickets) and bone weakness, softness, or fracture in adults (osteomalacia), the 25 hydroxy-vitamin D test usually is requested to identify a possible deficiency in vitamin D. Vitamin D deficiency is thought to be much more common than previously believed.  Some studies have shown that as many of 50% of the elderly and women being treated for osteoporosis may be vitamin D deficient.

If calcium concentration is high or the patient has a disease that might produce excess amounts of 1,25 dihydroxy-vitamin D, such as sarcoidosis or some forms of lymphoma, the 1,25 dihydroxy-vitamin D test may be requested.

Vitamin D tests also may be used to help diagnose or monitor problems with parathyroid gland functioning since parathyroid hormone is essential for vitamin D activation. When vitamin D, calcium, phosphorus, or magnesium supplementation is necessary, vitamin D levels are sometimes measured to monitor treatment effectiveness.