The Meaning of Low Vitamin D 25 Hydroxy and Blood Tests

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An Overview of Vitamin D

If a physician suspects someone might have a low vitamin D hydroxy level, tested as the level of calcidiol in the blood, the person’s blood is removed using venipucture, and is tested to detect the level of vitamin D hydroxy within the blood. This level is necessary for evaluating the amount of Vitamin D the body is receiving through exposure to sunlight and the person’s diet.

Vitamin D hydroxy (or Vitamin D) is a fat-soluble vitamin naturally occurring in certain foods such as dairy products, fish, and eggs. It exists in two forms: Vitamin D2 and Vitamin D3. The cells of the skin can produce Vitamin D3 when the naturally occurring compound 7-dehydrocholesterol is exposed to ultraviolet light from the sun. When eaten, Vitamin D is absorbed along with fat in the diet, and is taken to the liver for storage. While in the liver, it is converted to calcidiol or 25-hydroxy Vitamin D. However, this is not the active form of this vitamin, and when it is carried in the bloodstream to the kidneys or parts of the immune system, Vitamin D is converted into the hormone calcitriol, which can then be used by the body.

Why Is The Test Ordered?

The vitamin D levels in blood test results are obtained after a physician has requested the test to determine the level of blood vitamin D. The test is performed on blood that is drawn to assess a person’s levels of the vitamin as obtained from dietary sources. It is also used to assess skin function and if the amount of conversion of to 7-dehydrocholesterol to vitamin D is occurring.

In some other instances, 25-vitamin D hydroxy levels in the blood are tested to check that systems involved in absorption or fat are working effectively. The body is only able absorb fat-soluble vitamins at the cellular level if there are no diseases or conditions present to hinder this. For example, a person undiagnosed with Crohn’s disease might have problems with absorbing fat-soluble vitamins.

A third reason why a physician might want to order a vitamin D hydroxy blood test is to monitor calcium metabolism. Vitamin D, in combination with parathyroid hormone, is a major component for controlling the levels of blood calcium.

How is The Test Performed?

When testing for calcidiol levels in the blood, the physician will request that a sample of blood be taken from the person in a process known as venipuncture. Venipuncture is done in a physician’s office or in a laboratory setting, and it involves blood being drawn from the forearm right above the elbow. The nurse or laboratory technician taking the blood sample will first clean the skin above the area.

A torniquet is then applied to upper arm so the veins are more visible, and a hypodermic needle is inserted. The sample is collected within a plastic tube, and tested.

Blood Test Results For Vitamin D

The vitamin D hydroxy level in the blood can tell the physician a lot of information about various organs such as the liver and kidney. However, this is just one test used in the diagnosis, and further tests will be necessary to determine exactly what’s causing an abnormal blood level.

When a low vitamin D 25 hydroxy and blood tests are received, the first step for the physician will be assess the person’s diet and see if they are eating enough food containing vitamin D. If they are, then it will be necessary to review the other steps in the process such as absorption at the cellular level or conversion in the liver and kidney.

Usually with a low vitamin D 25 hydroxy and blood tests confirming the low level of calcidiol, the physician will be concerned about any liver or kidney disease, especially kidney disease if no other nutrients and hormones produced by the liver are affected.

If the level is abnormal, then treatment of the underlying cause of the will most likely bring the calcidiol level back within normal range.


Print Source: Davidson, Stanley & C. Haslett. 2002. “Davidson’s Principles and Practice of Medicine.” Churchill Livingstone, Edinburgh.

Print Source: Cotran R, Kumar V, and Robbins, SL. 1999. Robbins Pathologic Basis of Disease, 6th Ed. W.B. Saunders Company, Philadelphia.

Web Source: Lab Tests Online, American Association For Clinical Chemistry. “Vitamin D: The Test.” 2010. Available: