The main role of vitamin D is to help regulate the absorption of calcium, phosphate and (to a lesser extent) magnesium from the gut. 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 the disease called rickets in children and osteomalacia in adults. Vitamin D also plays an important role in musculoskeletal health. There are two…The main role of vitamin D is to help regulate the absorption of calcium, phosphate and (to a lesser extent) magnesium from the gut. 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 the disease called rickets in children and osteomalacia in adults. Vitamin D also plays an important role in musculoskeletal health. There are two forms of vitamin D that can be measured in the blood – 25 hydroxyvitamin D and 1,25 dihydroxyvitamin D. 25 hydroxyvitamin D is the major form of vitamin D and is the relatively inactive component from which the active hormone, 1,25 dihydroxyvitamin D is made. 25 hydroxyvitamin D is the most useful indicator of vitamin D status in individuals, and is the form most commonly measured. It is the best way of estimating vitamin D status. This is because 25 hydroxyvitamin D remains in the blood longer and is present at much higher concentrations than 1,25 dihydroxyvitamin D. Occasionally it may be necessary to measure 1,25 dihydroxyvitamin D to find out whether the kidney is converting an appropriate amount of inactive 25-hydroxyvitamin D to the active 1,25 dihydroxyvitamin D. Vitamin D comes from 2 sources: It is 1) produced in the skin following exposure to sunlight (endogenous source 80%-90% of the total) and 2) ingested from foods and supplements (exogenous source 10%-20% of the total). The chemical structures of these types of vitamin D are slightly different. Vitamin D2 (ergocalciferol) is the form found in foods of vegetable origin and vitamin D3 (cholecalciferol) is the form produced in the skin and is also found in foods of animal origin. Both forms of vitamin D may be present in vitamin preparations and supplements. For most people, the majority (up to 90%) of vitamin D is formed following the action of sunlight upon the skin. Both vitamin D2 and vitamin D3 are converted to 25-hydroxyvitamin D and then to 1,25 dihydroxyvitamin D. Some vitamin D blood tests do not distinguish D2 and D3 forms of the vitamin and the result therefore gives an indication of the total amount of vitamin D present within the bloodstream. How is the sample collected for testing? A blood sample is obtained by inserting a needle into a vein in the arm. Is any test preparation needed to ensure the quality of the sample? No test preparation is needed. See MoreSee Less
