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Vitamin D

Formal name: 25-hydroxy-vitamin D; calcidiol; 1,25 dihydroxy-vitamin D; calcitriol
Related tests: Calcium; phosphate; PTH; magnesium

At a Glance

Why Get Tested?

To investigate a problem related to bone metabolism or parathyroid function

When to Get Tested?

Your doctor may request a Vitamin D measurement if you have an abnormal calcium, phosphate, and/or parathyroid hormone level, as part of the investigation of some forms of bone disease or occasionally if you have kidney failure or disease of the gastrointestinal tract that may result in malabsorption

Sample Required?

A blood sample drawn from a vein in your arm

The Test Sample

What is being tested?

These tests measure the concentrations of various forms of vitamin D in your blood. The term vitamin D refers to a number of different but related chemical compounds (termed sterols), some of which are inactive and some of which are active forms. The two most common vitamin D tests measure 25-hydroxy-vitamin D (an inactive form) and 1,25 dihydroxy-vitamin D (the active form). The test for 25-hydroxy-vitamin is used to check that the body has an adequate supply of vitamin D. The test for 1,25 dihydroxy-vitamin D is used to test that the kidney is converting an appropriate amount of 25-hydroxy-vitamin D to the active 1,25 dihydroxy-vitamin D.

The main role of the active hormone is to help regulate the absorption of calcium, phosphate, and (to a lesser extent) magnesium, . Vitamin D is vital for the growth and health of bone; without it, bones will be soft, malformed, and unable to repair themselves normally, resulting in the disease called rickets in children and osteomalacia in adults.

Vitamin D comes from two sources. The body is able to form vitamin D by exposure to sunlight. This is why vitamin D is sometimes described as the sunshine vitamin - it is formed from 7-dehydrocholesterol in the skin when the skin is exposed to light. Vitamin D also can be ingested - either in foods or in vitamin supplements. The different compounds of vitamin D are distinguished by the use of subscript numbers. Vitamin D2 comes from diet and vitamin preparations. Vitamin D3 is produced in the body. Vitamins D2 and D3 have slightly different chemical structures, but both lead to production of the active hormone 1,25 dihydroxy-vitamin D.

Vitamin D3 as formed in the skin and vitamin D2 supplied in the diet are inactive forms that are converted by the liver into yet another inactive form called 25-hydroxy-vitamin D - this is the main form of vitamin D stored in the body and measured in the circulation. 25-hydroxy-vitamin D is converted to the active form 1,25 dihydroxy-vitamin D by the kidney. About 99.9% of vitamin D in the blood is 25-hydroxy-vitamin D and 0.1% is 1,25 dihydroxy-vitamin D.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

The Test

How is it used?

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

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When is it requested?

Either 25 hydroxy-vitamin D or 1,25 dihydroxy-vitamin D tests may be requested when a patient has an abnormal blood calcium, phosphate, and/or magnesium level or evidence of bone disorders.

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 also 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.

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What does the test result mean?

Looking for reference ranges?

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 (Dilantin), 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 are often 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.

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Is there anything else I should know?

High levels of vitamin D and calcium can lead to calcification and damage of organs, 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.

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Common Questions

1.  Are there other uses for vitamin D?

Yes, there is a topical form of vitamin D cream that is used to treat psoriasis. Research is being done in other areas, including the potential use of vitamin D to help control autoimmune conditions.

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2.  Is vitamin D a necessary component of calcium supplements?

Since absorption of calcium is dependent on vitamin D, many manufacturers of calcium supplements add vitamin D to ensure optimal calcium uptake. If you have adequate amounts of vitamin D from other sources the additional vitamin D is not necessary. The amount of vitamin D in these tablets is not likely to lead to excess vitamin D or be harmful either.

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.