<unk>  also known as 16α-hydroxyestradiol or as estra-1,3,5(10)-triene-3,16α,17β-triol, is a naturally occurring estrane steroid with double bonds between the C1 and C2, C3 and C4, and C5 and C10 positions and hydroxyl groups at the C3, C16α, and C17β positions. The name<unk> and the abbreviation E3 were derived from the chemical terms estrin (estra-1,3,5(10)-triene) and triol (three hydroxyl groups). Analogues A variety of analogues of<unk> are known, including both naturally occurring isomers and synthetic substituted derivatives and esters. 16β-Epiestriol (epiestriol), 17α-epiestriol, and 16β,17α-epiestriol are isomers of<unk> that are endogenous weak estrogens. Mytatrienediol (16α-methyl-16β-epiestriol 3-methyl ether) is a synthetic derivative of 16β-epiestriol that was never marketed.<unk> acetate benzoate,<unk> succinate, and<unk> tripropionate are synthetic<unk> esters that have been marketed for medical use, whereas<unk> dihexanoate,<unk> dipropionate, and<unk> triacetate have not been introduced. Quinestradol is the 3-cyclopentyl ether of<unk> and has also been marketed. Polyestriol phosphate, an ester of<unk> in the form of a polymer, has been marketed previously as well. These esters, ethers, and polymers are prodrugs of<unk>  Ethinylestriol and nilestriol are synthetic C17α ethynylated derivatives of<unk>  Ethinylestriol has not been marketed, but nilestriol, which is the 3-cyclopentyl ether of ethinylestriol and a prodrug of it, has been. Estetrol (E4), also known as 15α-hydroxyestriol, is a naturally occurring analogue of<unk> with an additional hydroxyl group, at the C15α position. It is closely related to<unk> and has similar but non-identical pharmacological properties. Like<unk>  estetrol is a relatively weak and atypical estrogen. Estetrol is under development for potential clinical use for a variety of indications, such as menopausal hormone therapy and hormonal birth control.
Estriol 
2-Hydroxyestradiol
Estradiol acetate
Estrogen 
Estrogen
11-Ketoprogesterone
Estradiol 
Estradiol