Vitamin D is a fat-soluble hormone that is mainly synthesized in the skin after exposure to ultraviolet rays from sunlight (in the form of vitamin D3), and, to a lesser extent, is derived from dietary intake in the form of either vitamin D2 or D3 (the main sources of vitamin D are fatty fish, fish oils, egg yolks, cheese, and vitamin D-fortified foods). After vitamin D is produced in the skin or absorbed through the gastrointestinal tract, it is transported to the liver by vitamin D-binding protein (VDBP). In the liver vitamin D is converted to 25 hydroxy vitamin D (25(OH)D), which is monitored to evaluate vitamin D status because of its half-life of 2–3 weeks. Next, 25 hydroxy vitamin D is transported to the kidneys, where it is finally converted to its active form, 1,25 dihydroxyvitamin D (1,25(OH)2D).