<unk> is a naturally occurring estrane steroid. It is also known as 17β-estradiol (to distinguish 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 other at the C17β position, as well as three double bonds in the A ring (the estra-1,3,5(10)-triene core). Due to its two hydroxyl groups,<unk> is often abbreviated as E2. The structurally related estrogens, estrone (E1), estriol (E3), and estetrol (E4) have one, three, and four hydroxyl groups, respectively. Hemihydrate A hemihydrate form of<unk> <unk> hemihydrate, is widely used medically under a large number of brand names similarly to<unk>  In terms of activity and bioequivalence,<unk> and its hemihydrate are identical, with the only disparities being an approximate 3% difference in potency by weight (due to the presence of water molecules in the hemihydrate form of the substance) and a slower rate of release with certain formulations of the hemihydrate. This is because<unk> hemihydrate is more hydrated than anhydrous<unk>  and for this reason, is more insoluble in water in comparison, which results in slower absorption rates with specific formulations of the drug such as vaginal tablets.<unk> hemihydrate has also been shown to result in less systemic absorption as a vaginal tablet formulation relative to other topical<unk> formulations such as vaginal creams.<unk> hemihydrate is used in place of<unk> in some<unk> products. Derivatives A variety of C17β and/or C3 ester prodrugs of<unk>  such as<unk> acetate,<unk> benzoate,<unk> cypionate,<unk> dipropionate,<unk> enantate,<unk> undecylate,<unk> valerate, and polyestradiol phosphate (an<unk> ester in polymeric form), among many others, have been developed and introduced for medical use as estrogens. Estramustine phosphate is also an<unk> ester, but with a nitrogen mustard moiety attached, and is used as a cytostatic antineoplastic agent in the treatment of prostate cancer. Cloxestradiol acetate and promestriene are ether prodrugs of<unk> that have been introduced for medical use as estrogens as well, although they are little known and rarely used. Synthetic derivatives of<unk> used as estrogens include ethinylestradiol, ethinylestradiol sulfonate, mestranol, methylestradiol, moxestrol, and quinestrol, all of which are 17α-substituted<unk> derivatives. Synthetic derivatives of<unk> used in scientific research include 8β-VE2 and 16α-LE2.
Estradiol 
Estrogen
Estrogen 
Alfatradiol
Androgen
17α-Estradiol
Progesterone 
Progesterone