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The page below is a sample from the LabCE course Vitamin D. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Vitamin D Forms

Vitamin D has several different forms. Two of these forms are important to humans: 1) vitamin D2, which is made by plants, and 2) vitamin D3, which is made by human skin when exposed to sunlight.
Foods may be fortified with vitamin D2 or D3 to help people reach their daily consumption requirements. Vitamin D (both D2 and D3) is considered biologically inactive until it goes through two enzyme mediated hydroxylation reactions. The first enzymatic hydroxylation reaction takes place in the liver mediated by the enzyme 25-hydroxylase which forms 25-hydroxyvitamin D3 [25(OH)D3]. The second reaction takes place in the kidney, mediated by 1α-hydroxylase, which converts 25(OH)D3 to the biologically active hormone, calcitriol. Vitamin D2 and D3 are equally effective when they are converted by the liver and the kidney into the active form, 1,25 forms. Vitamin D3 is what is made endogenously (this is usually what is measured in the laboratory).