Vitamin D is a group of fat-soluble compounds, which are derived from cholesterol. In blood the vitamin and its metabolites are bound to vitamin D binding protein. The two major circulating forms of vitamin D are 25-hydroxyvitamin D2 [synonym: ergocalciferol; abbreviation 25(OH)D2] and 25-hydroxyvitamin D3 [synonym: cholecalciferol; abbreviation 25(OH)D3] with the latter being the predominant species under physiological circumstances. However, in some individuals significant amounts of 25(OH)D2 can be found, especially when they are supplemented with vitamin D2 supplements. For clinical purpose the 25(OH)D concentration is considered the best measure of vitamin D status although it is itself biologically inactive. Its concentration is about three orders of magnitude higher than that of the active metabolite 1,25(OH)2D and it has a long half-life in the circulation of 3 weeks. Furthermore, it correlates better with parathyroid hormone levels (PTH), bone mineral density, bone turnover markers, fracture risk, and treatment effects.