I may be wrong, but I understand vitamin D2 is a vitamin, but "vitamin D3" is in fact a hormone. A vitamin is a building block, which by itself does nothing; a

I may be wrong, but I understand vitamin D2 is a vitamin, but "vitamin D3" is in fact a hormone. A vitamin is a building block, which by itself does nothing; a hormone is a directive, which instructs the body to do something - adrenaline, for example. Vitamin D2 is a precursor / building block for the hormone D3.