What is Vitamin D3 ?

Vitamin D is a family of compounds that is essential for the proper growth and formation of teeth and bones. This test measures the level of vitamin D in the blood.

Two forms of vitamin D can be measured in the blood, 25-hydroxyvitamin D and 1,25-dihydroxy vitamin D. The 25-hydroxyvitamin D is the major form found in the blood and is the relatively inactive precursor to the active hormone, 1,25-dihydroxy vitamin D. Because of its long half-life and higher concentration, 25-hydroxyvitamin D is commonly measured to assess and monitor vitamin D status in individuals.

Vitamin D comes from two sources: endogenous, which is produced in the skin on exposure to sunlight, and exogenous, which is ingested in foods and supplements. The chemical structures of the types of vitamin D are slightly different, and they are named vitamin D2 (ergocalciferol, which comes from plants) and vitamin D3 (cholecalciferol, which comes from animals). The D2 form is found in fortified foods and in most vitamin preparations and supplements. Vitamin D3 is the form produced in the body and is also used in some supplements. Vitamin D2 and D3 are equally effective when they are converted by the liver and the kidney into the active form, 1,25-dihydroxy vitamin D.

Some tests do not distinguish the D2 and D3 forms of the vitamin and report only the total result. Newer methods, however, may report levels of both D2 and D3 and then add them together for a total level.

The main role of vitamin D is to help regulate blood levels 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 has also been shown to influence the growth and differentiation of many other tissues and to help regulate the immune system. These other functions have implicated vitamin D in other disorders, such as autoimmunity and cancer.

Based on data from the National Health and Nutrition Examination Survey, the Centers for Disease Control and Prevention (CDC) reports that two-thirds of the U.S. population has sufficient vitamin D, while roughly one-quarter are at risk of inadequate vitamin D and 8% are at risk of deficiency, as defined by the Dietary Reference Intake (DRI) set by the Institute of Medicine.

People at higher risk of deficiency include the elderly or obese people, people who don’t get enough sun exposure, people with darker skin, and people who take certain medications for long periods of time. Adequate sun exposure is typically estimated to be two periods per week of 5-20 minutes. People who do not have adequate sun exposure may obtain the vitamin D that they need from food sources or supplements.

How is it used?

A vitamin D test 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
• Help diagnose or monitor problems with parathyroid gland functioning since PTH is essential for vitamin D activation
• Screen people who are at high risk of deficiency, as recommended by the National Osteoporosis Foundation, the Institute of Medicine, and the Endocrine Society
• Help monitor the health status of individuals with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn disease, since vitamin D is a fat-soluble vitamin and is absorbed from the intestine like a fat
• Monitor people who have had gastric bypass surgery and may not be able to absorb enough vitamin D
• Help determine the effectiveness of the treatment when vitamin D, calcium, phosphorus, and/or magnesium supplementation is prescribed