Estrogens

Bioidentical estrogens
Estrogens are the main feminizing hormones in the human body, and are often used by transgender people to induce feminization with the help of an an anti-androgen. They are produced endogenously in humans with functional ovaries in four different variants:


 * E1: Estrone a weak estrogen once available medically though no longer marketed.
 * E2: Estradiol the most potent and prevalent estrogen found in humans, and is often the variant used in HRT.
 * E3: Estriol another weak estrogen whose levels are almost undetectable in people who aren't pregnant.
 * E4: Estetrol a weak estrogen that appears during pregnancy though is otherwise not present.

Xenoestrogens
Xenoestrogens are hormones which interact with the estrogen receptors ERα and ERβ but do not appear naturally in the human body. Some common sources are in animals (such as Ethynyl estradiol found in pregnant mare urine), artificial substances (such as Bisphenol A which is used in some plastics), and plants (such as Deoxymiroestrol found in P. mirifica) where they are alternatively known as phytoestrogens.

Bioidentical hormones are often preferred over xenohormones in HRT because they are more predictable and understood, although xenoestrogens have some record of use as with Ethynyl estradiol in premarin, and P. mirifica being marketed as a breast enlargement supplement.