Vitamin D

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

Since vitamin D is a fat-soluble vitamin and is absorbed from the intestine like a fat, vitamin D is sometimes used to monitor individuals with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn's disease, and in patients who have had gastric bypass surgery and may not be able to absorb enough Vitamin D. Vitamin D is sometimes used to determine effectiveness of treatment when vitamin D, calcium, phosphorus, and/or magnesium supplementation is prescribed.

25-hydroxyvitamin D
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), 25-hydroxyvitamin D usually is ordered to identify a possible deficiency in vitamin D. Vitamin D deficiency is thought to be much more common than previously believed. Some studies have shown that as many of 50% of the elderly and women being treated for osteoporosis may be Vitamin D deficient. 25-hydroxyvitamin D is often ordered before an individual begins drug therapy for osteoporosis. Some osteoporosis medications now include the recommended Vitamin D dose.

1,25-dihydroxyvitamin D
If calcium is high or the patient has a disease that might produce excess amounts of vitamin D, such as sarcoidosis or some forms of lymphoma, 1,25-dihydroxyvitamin D usually is ordered. Rarely, this testing may be indicated when abnormalities of 1-alphahydroxylase are suspected.

Vitamin D levels also may be used to help diagnose or monitor problems with parathyroid gland functioning since PTH 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.

There are differences among Vitamin D methods, making a universal reference range difficult to establish. Total 25-hydroxyvitamin D (D₂ + D₃) is the correct measure of Vitamin D status. There is currently no consensus on the level which indicates deficiency.

25-hydroxyvitamin D
Low blood levels of 25-hydroxyvitamin D may mean that you are not getting enough exposure to sunlight or enough dietary vitamin D to meet your body’s demand or that there is a problem with its absorption from the intestines. Occasionally, drugs used to treat seizures, particularly phenytoin (Dilantin), can interfere with the production of 25-hydroxyvitamin D in the liver.

There is increasing evidence that vitamin D deficiency may increase the risk of some cancers, immune diseases, and cardiovascular disease.

High levels of 25-hydroxyvitamin D usually reflect excess supplementation from vitamin pills or other nutritional supplements.

1,25-dihydroxyvitamin D
Low levels of 1,25-dihydroxyvitamin D can be seen in kidney disease and are one of the earliest changes to occur in persons with early kidney failure.

High levels of 1,25-dihydroxyvitamin D may occur when there is excess parathryoid hormone or when there are diseases, such as sarcoidosis or some lymphomas, that can make 1,25-dihydroxyvitamin D outside of the kidneys.

High levels of vitamin D and calcium can lead to the calcification and damage to organs, particularly the kidneys and blood vessels.

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.