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Kidney Diseases

Low 25 hydroxy vitamin D test

03/30/2006

Question:

I very much appreciate your input.

What does a low 25 hydroxy vitamin D test for? I assume it has a relationship to the kidney, but I guess that test was not mentioned when I was in dietetics school. As I am outside a great deal, try to take the recommended amount of dairy, fats, vegetables and fruits, I doubt that it is related to my actual Vit D intake, as my bones are strong, also, doubt that absorption is a problem.

Just curious

Answer:

Vitamin D is made by the action of sunlight on skin and/or is absorbed from the diet. Once vitamin D is in the body, it travels in the bloodstream to the liver, where a hydroxyl (OH) group is attached, making it into 25-hydroxy vitamin D. Then it goes to the kidneys, receives another hydroxyl group, and becomes 1,25-dihydroxy vitamin D. The final product (1,25-dihydroxy D) is much more active than either 25-hydroxy D or plain vitamin D in helping us to absorb calcium from the intestine and to make bones.

People with kidney disease often have impaired ability to make 1,25-dihydroxy D and therefore suffer serious bone disease as a result. The bone disease is at least partly corrected (or even prevented) by giving synthetic 1,25-dihydroxy D, either orally or IV. So a low 25-hydroxy D level indicates either decreased D intake/synthesis or decreased ability of the liver to perform the first step in the metabolic process. Low 25-hydroxy D can be caused by certain medications (such as phenobarbital) and can result in low blood calcium levels and decreased bone density.

In summary, low 25-hydroxy D is more likely to be associated with a liver problem, while low 1,25-dihydroxy D is associated with kidney problems.

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Response by:

Mildred   Lam, MD Mildred Lam, MD
Associate Professor of Medicine
School of Medicine
Case Western Reserve University