What is being tested?

Your skin makes vitamin D when it is exposed to sunlight. You can also get vitamin D from food or supplements. The liver changes the vitamin D from the skin or food into a storage form called 25-hydroxy-vitamin D. It is this form that is usually measured in the vitamin D test.

Other forms of vitamin D can also be measured, depending on your specific circumstances.

Why would I need this test?

Your doctor might ask you to have this test if you are at risk of being vitamin D deficient. You might also need it if you have:

• abnormal levels of minerals such as calcium, phosphate or magnesium in your blood
• bone problems
• diseases that might result in, or be caused by, too much or too little vitamin D
• problems with your parathyroid gland

How to prepare for this test

No special preparation is required.

Understanding your results

You will need to discuss with your doctor what the results mean for you. Generally, a low level of 25-hydroxy-vitamin D may mean you are not getting enough vitamin D from sunlight or from food. It could also indicate problems with vitamin D absorption from your intestines, or that your liver is not making enough of this type of vitamin D.

A high level of vitamin D usually comes from taking too much in, either from pills or in food.

Abnormal levels of a type of vitamin D produced in the kidney (1,25-dihydroxy-vitamin D) can be a sign of a range of other conditions.

More information

About Vitamin D testing

Visit Pathology Tests Explained website for more information about Vitamin D testing.

About blood testing

Visit our 'Guide to blood testing' to learn more about blood tests in general with information such as:

• what to consider before having the test
• what happens during a blood test
• results accuracy
• blood tests cost