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25-Hydroxyvitamin D [25(OH)D]

Test code(s) 17306

This FAQ has been retired. It was effective 11/09/2012 to 04/14/2015.

To view the current Vitamin D Testing FAQ click here.

Question 1. What does vitamin D do?

Vitamin D is necessary for proper function of most of the organs and tissues of the body but one of its major effects is on calcium metabolism and bone health. Vitamin D is essential for calcium absorption from the intestines. Vitamin D deficiency results in inadequate calcium deposition in bones and can lead to osteoporosis in adults and rickets in children.

There is increasing evidence that vitamin D may play a role outside of its impact on calcium metabolism. Studies suggest that low levels of vitamin D may be associated with increased risk of some cancers, type 2 diabetes mellitus, multiple sclerosis, cardiovascular disease, rheumatoid arthritis, depression, Alzheimer’s disease, infections, preeclampsia, cesarean deliveries and neurocognitive dysfunction.

Question 2. Is there more than one form of vitamin D?

Yes. There are two forms of vitamin D – vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Vitamin D3 is the naturally occurring form of vitamin D in humans. It is made in the skin following exposure to sunlight (UVB). Vitamin D3 can also be found in certain fish and cod liver oil. Vitamin D2 is found in some vegetables, including mushrooms, and in many fortified foods (milk, yogurt, etc.). Both vitamin D2 and D3 are available over the counter. Only vitamin D2 (Drisdol®) is available by prescription in the US.

Vitamin D2  should not be confused with 1,25-dihydroxyvitamin D [1,25(OH)2D]  which is the biologically active form of the vitamin.

Question 3. What test should be used for determining a patient’s vitamin D status?

The appropriate test is the serum 25-hydroxyvitamin D [25(OH)D] test.

Although 1,25(OH)2D is the active metabolite of vitamin D, serum levels of 1,25(OH)2D do not reflect the body’s stores and are not useful for determining vitamin D status.        

Question 4. What is the normal range (ie, reference interval) for 25(OH)D?

This is somewhat controversial. A recent report by the Institute of Medicine recommended using 20 – 50 ng/mL, but many experts in the field, including the Endocrine Society (J Clin Endocrine Metab. 2011;96:1911-1930) believe a total 25(OH)D interval of 30-100 ng/mL reflects vitamin D sufficiency.

Currently, there are no reference intervals for the 2 forms of 25(OH)D, 25(OH)D2 and 25(OH)D3.

Question 5. How much vitamin D is needed for good health?

This is also somewhat controversial. The recent Institute of Medicine report concluded that healthy adults < 70 years old need 600 IU/day and those over 70 years old about 800 IU/day. The Endocrine Society (J Clin Endocrinol Metab. 2011;96:1911-1930) currently recommends the following dosing schedule for routine use:

•          Age 0 to 1 year: 400 to 1,000 IU/day

•          Age 1 to 18 years: 600 to 1,000 IU/day

•          All adults over age 18: 1,500 to 2,000 IU/day

•          Pregnant or nursing women under age 18: 600 to 1,000 IU/day

•          Pregnant or nursing women over age 18: 1,500 to 2,000 IU/day

There is a general agreement that doses up to 4000 IU/day are safe.

These recommendations are for healthy adults. Some people may need more vitamin D, including those with:

•          Osteoporosis

•          Documented severe deficiency

•          Obesity

•          Malabsorption syndromes

•          Medications that affect vitamin D metabolism

The recommended intake level for these people should be set by their physician. Once vitamin D therapy is initiated, the physicians may monitor improvement in levels by measuring the patient’s 25(OH)D level.

This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.

Document: FAQS.03 Revision: 1