25-OH Vitamin D3 (Vitamin D)

What is 25-OH vitamin D3?

Vitamin D is a fat-soluble vitamin that supports the absorption of calcium and phosphorus from the intestines and is essential for bone and muscle health. Most of the vitamin D in the body is produced in the skin through the sun's UVB rays; a smaller portion comes from oily fish, egg yolks and fortified foods. D3, or cholecalciferol, is the form produced in the skin and the form most commonly used in supplements.

Vitamin D made in the skin or taken in with food is first converted in the liver into 25-hydroxyvitamin D, also called calcidiol. Because this form circulates in the blood for weeks, it is the indicator that best reflects the body's vitamin D stores and the preferred measurement when deficiency is being investigated. The active hormone, 1,25-dihydroxyvitamin D, is formed in the kidneys; however, because it is short-lived in the blood, it is not used to assess the body's stores. The test is performed on blood serum using immunoassay or chromatographic methods, and the result is reported in ng/mL.

When is the 25-OH vitamin D3 test ordered?

Your doctor may order this test when you have symptoms that suggest vitamin D deficiency or when you are at risk of deficiency. Deficiency is more common in people who get little sun, who wear clothing that covers most of the body, who are older, who are overweight or who have absorption problems. Once treatment has started, the test may be repeated at intervals to see whether the level has become adequate.

During periods of increased need, such as pregnancy and breastfeeding, infancy and adolescence, your doctor may also check your vitamin D level if they consider it necessary. In people found to have low bone density on a bone density scan or who have frequent fractures, the test is ordered as part of the treatment plan. People taking medicines that affect vitamin D metabolism, such as epilepsy medicines or long-term cortisone, are also among the groups monitored regularly.

• Bone pain, muscle weakness or muscle cramps
• Persistent fatigue and lack of energy
• Suspected bone loss (osteoporosis) or treatment planning
• Absorption disorders such as celiac disease or inflammatory bowel disease
• Abnormal calcium, phosphorus or parathyroid hormone results
• Monitoring during vitamin D treatment

How should you prepare for the 25-OH vitamin D3 test?

You do not need to fast for this test; the blood sample can be given at any time of day. Because the vitamin D level changes slowly over days and weeks, your last meal does not noticeably affect the result. If you take supplements, do not stop them on your own; it is enough to tell your doctor what dose you take and how long you have been taking it.

High-dose biotin, that is, vitamin B7, can interfere with some immunoassay methods, so mention it separately if you take this supplement. Blood is usually drawn with a fine needle from a vein on the inner side of the arm, and the procedure takes a few minutes. A Curavita nurse can take this sample at your home or workplace; the tube is then delivered to the laboratory for analysis.

What does the 25-OH vitamin D3 result mean?

The symptoms of deficiency are often vague, so in many people a low level is discovered during a routine check-up. A low result is most often linked to insufficient sunlight, low dietary intake or problems with absorption from the intestines. Long-standing, marked deficiency is associated with softening of the bones and bone loss in adults, and with rickets in children. Kidney and liver diseases and certain medicines can also lower the level by affecting how vitamin D is processed.

A high result, on the other hand, is usually due to taking more supplements than needed over a long period and can lead to raised calcium levels in the blood. The cut-off values used to define deficiency and sufficiency may differ between guidelines; reference ranges also vary by laboratory, age and sex. This page shows the reference range of the laboratory that analyzes your sample. What your result means for you should be interpreted by your doctor, together with your symptoms and your other lab tests.