Estradiol is highly selective for these ERs and mERs, and does not interact importantly with other steroid hormone receptors. It is far more potent as an estrogen than are other bioidentical estrogens like estrone and estriol. Given by subcutaneous injection in mice, estradiol is about 10-fold more potent than estrone and about 100-fold more potent than estriol.The ERs are expressed widely throughout the body, including in the breasts, uterus, vagina, fat, skin, bone, liver, pituitary gland, hypothalamus, and other parts of the brain. In accordance, estradiol has numerous effects throughout the body. Among other effects, estradiol produces breast development, feminization, changes in the female reproductive system, changes in liver protein synthesis, and changes in brain function. The effects of estradiol can influence health in both positive and negative ways. In addition to the aforementioned effects, estradiol has antigonadotropic effects due to its estrogenic activity, and can inhibit ovulation and suppress gonadal sex hormone production. At sufficiently high dosages, estradiol is a powerful antigonadotropin, capable of suppressing testosterone levels into the castrate/female range in men.There are differences between estradiol and other estrogens, such as non-bioidentical estrogens like natural conjugated estrogens and synthetic estrogens like ethinylestradiol and diethylstilbestrol, with implications for pharmacodynamics and pharmacokinetics as well as efficacy, tolerability, and safety. Pharmacokinetics
Estradiol can be taken by a variety of different routes of administration. These include oral, buccal, sublingual, intranasal, transdermal (gels, creams, patches), vaginal (tablets, creams, rings, suppositories), rectal, by intramuscular or subcutaneous injection (in oil or aqueous), and as a subcutaneous implant.
The higher risk of VTE with oral estradiol can be attributed to the first pass and a disproportionate effect on liver synthesis of coagulation factors. Even high doses of parenteral estradiol, such as high-dose polyestradiol phosphate, have minimal influence on coagulation factors, in contrast to oral estrogen therapy. However, sufficient doses of parenteral estradiol, for instance very high doses of estradiol valerate by intramuscular injection, can nonetheless activate coagulation, presumably increasing VTE risk.In addition to the route of administration, the type of estrogen influences VTE risk. Oral conjugated estrogens are associated with a higher risk of VTE than oral estradiol. Estradiol- and estradiol valerate-containing birth control pills are associated with a lower risk of VTE than birth control pills containing ethinylestradiol. The relative risk of VTE is thought to be highest with oral ethinylestradiol, intermediate with oral conjugated estrogens, low with oral estradiol and parenteral estradiol valerate, and very low with transdermal estradiol. Conjugated estrogens and ethinylestradiol are thought to have a higher risk of VTE than estradiol because they are resistant to hepatic metabolism and have a disproportionate influence on liver production of coagulation factors.The combination of oral or transdermal estradiol and a progestin is associated with a higher risk of VTE than estradiol alone. Dydrogesterone is associated with a lower risk than other progestins such as medroxyprogesterone acetate and norethisterone, while oral progesterone is associated with no increase in risk of VTE.