Vitamin D
At a Glance
Why Get Tested?
To investigate a problem related to bone metabolism or parathyroid function, possible vitamin D deficiency, malabsorption and to monitor some patients taking vitamin D.
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 taken from a vein in your arm
The Test Sample
What is being tested?
There are two forms of vitamin D that can be measured in the blood - 25 hydroxy-vitamin D and 1,25 dihydroxy-vitamin D. 25 hydroxy-vitamin D is the major form of the hormone found in the blood and is the inactive component from which the active hormone, 1,25 dihyroxy vitamin D is made. Because 25 hydroxy-vitamin D stays for a long time in the blood and is at a higher concentration than 1,25 dihydroxy-vitamin D, 25 hydroxy-vitamin D is commonly measured to find out about vitamin D status in individuals. The test for 1,25 dihydroxy-vitamin D is used to find out whether the kidney is converting an appropriate amount of inactive 25-hydroxy-vitamin D to the active 1,25 dihydroxy-vitamin D.
Vitamin D comes from 2 sources: a) produced in the skin on exposure to sunlight (endogenous) and b) that present in foods and supplements (exogenous). The chemical structures of these types of vitamin D are slightly different and are distinguished by the subscripts in their chemical name. Vitamin D2 (ergocalciferol) is the form found in foods of animal origin and in most vitamin preparations and supplements while vitamin D3 (cholecalciferol) is the form produced in the body and is also used in some high dose formulations used to treat severe vitamin D deficiency.
Both D2 and D3 vitamin D are converted to the 25-hydroxy vitamin D and then to 1,25 di-hydroxy vitamin D.The main role of vitamin D 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 has also been implicated in the regulation of autoimmunity.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm. No test preparation is needed.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
The Test
How is it used?
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.
What does the test result mean?
Low blood levels of 25 hydroxy-vitamin D may mean that you are not getting enough exposure to sunlight or enough vitamin D in your food 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 getting into the bloodstream). Occasionally, drugs used to treat fits, particularly phenytoin (epanutin), can interfere with the liver's production of 25 hydroxy-vitamin D.
There is some evidence that vitamin D deficiency may increase the risk of some cancers, immune diseases and cardiovascular disease.
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 people with early kidney disease.
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.
Is there anything else I should know?
High levels of vitamin D and calcium can lead to the calcification and damage to organs particularly the kidneys and blood vessels.
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.
Common Questions
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Article Sources
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.
