Vitamin D Test

This is a test used to investigate a problem related to bone metabolism or parathyroid function. It is often used if you have an abnormal calcium, phosphorus, and/or parathyroid hormone level or if you have evidence of bone disease or bone weakness

These tests measure the concentrations of various forms of vitamin D in your blood. The term vitamin D actually refers to a number of different but related chemical compounds (termed sterols), some of which are inactive and some of which are active forms. The two most common vitamin D tests measure calcidiol (an inactive form) and calcitriol (the active form). The test for calcidiol (sometimes called 25-hydroxy-vitamin D) is used to assure that the body has adequate vitamin D supply. The test for calcitriol (sometimes called 1,25 dihydroxy-vitamin D) is used to assure that the kidney is converting an appropriate amount of calcidiol to the active hormone calcitriol.

The main role of the active hormone calcitriol is to help regulate the absorption of calcium, phosphorus, and (to a lesser extent) magnesium. Vitamin D is vital for the growth and health of bone; without it, bones will be soft, malformed, and unable to repair themselves normally, resulting in diseases called rickets in children and osteomalacia in adults.

Vitamin D comes from two sources. The body is able to form vitamin D by exposure to sunlight. This is the basis of referring to vitamin D as the sunshine vitamin -- it is formed from 7-dehydrocholesterol in the skin when the skin is exposed to light. Vitamin D also can be ingested -- either in foods or in vitamin supplements. The different compounds of vitamin D are distinguished by the use of subscript numbers. Vitamin D2 comes from diet and vitamin preparations. Vitamin D3 is endogenous (produced in the body). Vitamins D2 and D3 are slightly different chemical structures, but both lead to production of the active hormone calcitriol.

PREPARATION FOR TEST

A blood sample is obtained by inserting a needle into a vein in the arm.

NORMAL FINDINGS

Ranges for normal findings may vary among different laboratories and hospitals. You should always check with your doctor after having lab work or other tests done to discuss the meaning of your test results and whether your values are considered within normal limits.

MEANING OF TEST

Your caregiver will go over the test results with you and discuss the importance and meaning of your results, as well as treatment options and the need for additional tests, if necessary.

Reference values are dependent on many factors, including patient age, gender, sample population, and testing method. Numeric test results have different meanings in different labs. Your lab report should include the specific reference range for your test.

  • 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.

OBTAINING THE TEST RESULTS

It is your responsibility to obtain your test results. Ask the lab or department performing the test when and how you will get your results.