Vitamin D

Vitamin D, 25 Hydroxy

Vitamin D blood tests are used to determine if bone weakness, bone malformation, or abnormal metabolism of calcium (reflected by abnormal calcium, phosphorus or PTH blood tests) is occurring as a result of a deficiency or excess of vitamin D.


Since vitamin D is fat-soluble and is absorbed from the intestine like a fat, vitamin D blood tests are sometimes used to monitor individuals with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn’s disease, to assure that they have adequate amounts of vitamin D. Vitamin D blood tests also are used to determine effectiveness of treatment when vitamin D, calcium, phosphorus, and/or magnesium supplementation is prescribed.


When is it ordered?
Either calcidiol or calcitriol tests may be ordered when a patient has an abnormal blood calcium, phosphorus, and/or magnesium level or evidence of bone disorders.


If calcium is low or the patient has symptoms of vitamin D deficiency, such as bone malformation in children (rickets) and bone weakness, softness, or fracture in adults (osteomalacia), the calcidiol test usually is ordered to identify a possible deficiency in vitamin D.


If calcium is high or the patient has a disease that might produce excess amounts of calcitriol, such as sarcoidosis or some forms of lymphoma, the calcitriol test usually is ordered.Vitamin D blood tests also may be used to help diagnose or monitor problems with parathyroid gland functioning since parathyroid hormone is essential for vitamin D activation. When vitamin D, calcium, phosphorus, or magnesium supplementation is necessary, vitamin D levels are sometimes measured to monitor treatment effectiveness.


What does the test result mean?
NOTE: A standard reference range is not available for this blood test. Because reference values are dependent on many factors, including patient age, gender, sample population, and test method, numeric test results have different meanings in different testing labs. Your lab report should include the specific reference range for your test. Lab Tests Online strongly recommends that you discuss your blood test results with your doctor. For more information on reference ranges, please read Reference Ranges and What They Mean.


Low blood levels of calcidiol may mean that you are not getting enough exposure to sunlight or enough dietary vitamin D to meet your body’s demand; that there is a problem with its absorption from the intestines; or that enough is not being converted to calcidiol in the liver (which means that it is not making it into the bloodstream). Occasionally, drugs used to treat seizures, particularly phenytoin (Dilantin), can interfere with the liver’s production of calcidiol.


High levels of calcidiol usually reflect excess supplementation from vitamin pills or other nutritional supplements.

Low levels of calcitriol are often seen in kidney disease and are one of the earliest changes to occur in persons with early kidney failure.

High levels of calcitriol may occur when there is excess parathryoid hormone or when there are diseases, such as sarcoidosis or some lymphomas, that can make calcitriol outside of the kidneys.


Is there anything else I should know?
High levels of vitamin D and calcium can lead to the calcification and damage of organs, such as the kidneys, as the body tries to lower blood calcium levels by depositing calcium phosphate compounds into the organs.


If magnesium levels are low, they can cause a low calcium level that is resistant to vitamin D and parathyroid hormone regulation. It may be necessary to supplement both magnesium and calcium to regain normal function.