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 tests are also are sometimes used to determine the effectiveness of treatment when vitamin D, calcium, phosphate, and/or magnesium supplementation is prescribed.

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.

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.

Low blood levels of 25 hydroxy-vitamin D may mean that you are not getting enough exposure to sunlight or enough dietary vitamin D to meet your body's demand; that there is a problem with its absorption from the intestines; or that enough is not being converted to 25 hydroxy-vitamin D in the liver (which means that it is not making it into the bloodstream). Occasionally, drugs used to treat seizures, particularly phenytoin (epanutin), can interfere with the liver's production of 25 hydroxy-vitamin D.

High levels of 25 hydroxy-vitamin D usually reflect excess supplementation from vitamin pills or other nutritional supplements.

Low levels of 1,25 dihydroxy-vitamin D can be seen in kidney disease and are one of the earliest changes to occur in persons with early kidney failure.

High levels of 1,25 dihydroxy-vitamin D may occur when there is excess parathryoid hormone or when there are diseases, such as sarcoidosis or some lymphomas, that can make 1,25 dihydroxy-vitamin D outside of the kidneys.

High levels of vitamin D and calcium can lead to calcification and damage oforgans, such as the kidneys, as the body tries to lower blood calcium levels by depositing calcium phosphate compounds into the organs.

If magnesium levels are low, they can cause a low calcium level that is resistant to vitamin D and parathyroid hormone regulation. It may be necessary to supplement both magnesium and calcium to regain normal function.