Vitamin D

Print this article
Share this page:
Also known as: Ergocalciferol (vitamin D2); Cholecalciferol (vitamin D3); Calcidiol (25-hydroxyvitamin D [25(OH)D]); Calcifidiol (25-hydroxy-vitamin D); Calcitriol (1,25 dihydroxyvitamin D [1,25(OH)(2)D])
Formal name: 25-hydroxy-vitamin D (Calcidiol); 1,25 dihydroxy-vitamin D (Calcitriol)

The Test Sample

What is being tested?

The main role of vitamin D is to help regulate the absorption of calcium, phosphate, and (to a lesser extent) magnesium from the gut. 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 also plays an important role in musculoskeletal health.

There are two forms of vitamin D that can be measured in the blood - 25 hydroxyvitamin D and 1,25 dihydroxyvitamin D. 25 hydroxyvitamin D is the major form of vitamin D and is the relatively inactive component from which the active hormone, 1,25 dihydroxyvitamin D is made. 25 hydroxyvitamin D is the most useful indicator of vitamin D status in individuals, and is the form most commonly measured. This is because 25 hydroxyvitamin D remains in the blood longer and is present at much higher concentrations than 1,25 dihydroxyvitamin D. Occasionally it may be necessary to measure 1,25 dihydroxyvitamin D to find out whether the kidney is converting an appropriate amount of inactive 25-hydroxyvitamin D to the active 1,25 dihydroxyvitamin D.

Vitamin D comes from 2 sources: It is 1) produced in the skin following exposure to sunlight (endogenous source) and 2) ingested from foods and supplements (exogenous source). The chemical structures of these types of vitamin D are slightly different. Vitamin D2 (ergocalciferol) is the form found in foods of vegetable origin and vitamin D3 (cholecalciferol) is the form produced in the skin and is also found in foods of animal origin. Both forms of vitamin D may be present in vitamin preparations and supplements. For most people, the majority (up to 90%) of vitamin D is formed following the action of sunlight upon the skin. Both vitamin D2 and vitamin D3 are converted to 25-hydroxyvitamin D and then to 1,25 dihydroxyvitamin D.

Some vitamin D blood tests do not distinguish D2 and D3 forms of the vitamin and the result therefore gives an indication of the total amount of vitamin D present within the bloodstream.

How is the sample collected for testing?

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

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.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.