Vitamin D3 is formed by the action of ultraviolet radiation from sunlight on a cholesterol derivative (7-dehydrocholesterol) in skin. A lot of people even in warm weather climates are Vitamin D deficient. Vitamin D is metabolized in the liver and then in certain kidney cells, which results in 1,25-dihydroxyvitamin D and that is what you see in blood work to determine if you have a Vitamin D deficiency.
