What is being tested?
There are two forms of Vitamin D that can be measured in the blood, 25 hydroxy Vitamin D and 1,25 dihydroxy Vitamin D. 25 hydroxy Vitamin D (25 OH Vitamin D) is the major form of the hormone found in the blood and is the inactive precursor to the active hormone, 1,25 dihyroxy Vitamin D. Because of its long half-life and higher concentration, 25 OH Vitamin 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 structure of these types of Vitamin D are slightly different and are distinguished by subscripts. Vitamin D₂ is the form found in foods of animal origin and in most vitamin preparations and supplements. It is often listed as Vitamin D (ergocalciferol) on vitamin bottles. Vitamin D₃ is the form produced in the body and is also used in some high dose formulations used to treat severe Vitamin D deficiency. Vitamin D₃ is sometimes referred to as cholecalciferol.

Both D₂ and D₃ Vitamin D are converted to the 25 OH Vitamin D and then to 1,25 di OH Vitamin D.

Many tests for 25 OH Vitamin D do not distinguish D₂ and D₃ forms of the vitamin and report only a total Vitamin D. Newer methods however, may report levels of both Vitamin D₂ and D₃.

Sources and Forms of Vitamin D

The main role of Vitamin D 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.

How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.