Vitamin D3 is produced in the skin after exposure to ultraviolet B light from the sun. Vitamin D3 is metabolised sequential in the liver into 25-hydroxy-vitamin D [25(OH)D], which is the storage form of vitamin D in the body, and then in the kidney into the steroid hormone, 1a,25-dihydroxyvitamin 1a,25-dihydroxyvitamin D [1,25(OH)2D].