Vitamin D is actually a group of prohormones, all fat soluble, whereby ergocalciferol, or Vitamin D2 and cholecalciferol, also known as Vitamin D3 are the most common forms. The Vitamin D3 form is made when human skin is exposed to UV B radiation. The active form of Vitamin D is calcitrol and it is required in the body to manage phosphorus and calcium levels, but, among other functions, to affect anti tumor activity, immunomodulatory functions and much more. Vitamin D, or more accurately calcitrol, is being bound to a carrier protein, which allows it to be transported throughout the body to the various destinations. The Vitamin D receptors, steroid and thyroid hormone receptors, can be found in most organs, including the brain, skin, heart, breast, prostate and gonads.