Assessment and interpretation of circulating 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D in the clinical environment.

The unique cis-triene structure of vitamin D and related metabolites makes it susceptible to oxidation, ultraviolet (UV) light-induced conformational changes, heat-induced conformational changes, and attacks by free radicals. Vitamin D(2) is much less bioactive than vitamin D(3) in humans. Metabolic activation and inactivation of vitamin D are well characterized and result in a plethora of metabolites, of which only 25-hydroxyvitamin D (25(OH)D) and 1,25-dihydroxyvitamin D (1,25(OH)(2)D) provide any clinically relevant information. 25(OH)D(2) and 25(OH)D(3) are commonly known as calcifediol and the 1,25(OH)(2)D metabolites as calcitriol. In this review the current state of the science on the clinical assessment of circulating 25(OH)D and 1,25(OH)(2)D is described.