Estradiol is an estrane (C18) steroid. It is aswell accepted as 17β-estradiol (to analyze it from 17α-estradiol) or as estra-1,3,5(10)-triene-3,17β-diol. It has two hydroxyl groups, one at the C3 position and the added at the 17β position, as able-bodied as three bifold bonds in the A ring. Due to its two hydroxyl groups, estradiol is generally abbreviated as E2. The structurally accompanying estrogens, estrone (E1), estriol (E3), and estetrol (E4) accept one, three, and four hydroxyl groups, respectively.