It is well established that the skin, after exposure to sunlight, serves as an organ for the production of the hormone cholecalciferol.1 During the past 15 years, extensive research on the metabolism of vitamin D has revealed that cholecalciferol, after its formation in the skin, is transported to the liver, where it is metabolized to calcifediol (25-hydroxyvitamin D3). Calcifediol is the major circulating form of cholecalciferol and is transported to the kidney and metabolized to calcitriol (1,25-dihydroxyvitamin D3), the biologically active form of this hormone.2,3
Because vitamin D is a calcium-regulating hormone, most investigators previously believed that the principal target organs for this hormone were limited to the small intestine, bone, and kidney. Indeed, these target organs contain specific cytosolic and nuclear-binding proteins that have a high affinity and low capacity for calcitriol. After this hormone enters the cell and is bound to its cytoplasmic receptor,