Vitamin D (calciferol) is a fat-soluble steroid hormone precursor, meaning that it has no hormone activity itself, but is converted to the active hormone form through a photochemical (ultraviolet B radiation-induced) synthesis mechanism in the skin. After production in skin or consumption in food, it is converted in the liver and kidney to form 1,25 dihydroxyvitamin D, (1,25(OH)2D) the physiologically active form of vitamin D (calcitriol), which plays an important role in the maintenance of organ systems.