Vitamin D, 1,25 Dihydroxy

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Also known as: 25-hydroxy-vitamin D (Calcidiol, Calcifidiol) and 1,25 dihydroxy-vitamin D (Calcitriol).

25 OH Vitamin D blood tests are used to determine if bone weakness, bone malformation, or abnormal metabolism of calcium (reflected by abnormal calcium, phosphorus 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 individuals with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn's disease, to assure that they have adequate amounts of vitamin D.

The Vitamin D blood test is sometimes used to determine effectiveness of treatment when vitamin D, calcium, phosphorus, and/or magnesium supplementation

Its concentration is measured by means of analysis of a blood sample drawn from the vein in the arm.

Purpose of the test

25 OH Vitamin D test

If calcium 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 OH Vitamin D test usually is ordered to identify a possible deficiency in vitamin D.

1,25 di OH Vitamin D test

If calcium is high or the patient has a disease that might produce excess amounts of Vitamin D, such as sarcoidosis or some forms of lymphoma, the 1,25 di OH Vitamin D test usually is ordered.

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.

Reference range values
15-75 ng/L

Abnormal findings

25 OH Vitamin D test

Low blood test levels of 25 hydroxy Vitamin D may mean that patient is not getting enough exposure to sunlight or enough dietary vitamin D to meet body's demand or that there is a problem with its absorption from the intestines. Occasionally, drugs used to treat seizures, particularly phenytoin (Dilantin), can interfere with the production of 25 OH Vitamin D in the liver.

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

1,25 di OH Vitamin D test

Low blood test levels of 1,25 di OH Vitamin D can be seen in kidney disease and are one of the earliest changes to occur in persons with early kidney failure.

High blood test levels of 1,25 di OH 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 di OH Vitamin D outside of the kidneys. This is not a required fasting blood test.

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