Historically women have been given too much estrogen even when they do need estrogen replacement. Excessive estrogen may help control the hot flashes for a month or two, but eventually the symptoms return. A surplus of estrogen causes the same symptoms as insufficient estrogen, just with a slight time delay before the symptoms return. Too much estrogen at first increases the number of estrogen receptors, but after a period of time the body decreases the number of receptors, so the estrogen cannot work properly no matter how much is there.
Premarin® 0.625 mg or Estrace® 0.5 mg, dosages commonly administered for years, are excessive estrogen burden for any woman. The majority of estradiol given by mouth is converted to estrone, another strong estrogen. Estradiol levels resulting from these doses may be at normal premenopausal levels, but estrone levels will be significantly higher than normal. High estrone levels lead to symptoms of estrogen dominance and significantly increase the risk of breast cancer. Premarin® contains 50% estrone, and 5-19% estradiol (along with a bunch of horse estrogens!). The estradiol in it for the most part is quickly converted to estrone, so what the patient is really receiving is a strong dose of estrone. While Premarin® produces normal premenopausal levels of estradiol, estrone levels are usually 7-10 times higher than they should be. Clinicians usually only measure the estradiol level to manage therapy.
Estrace® is bioidentical estradiol, but again the usual doses given by mouth produce too high an estrone level , usually 3-5 times higher than normal.
Topical administration of estrogen is safer and enables administration in a manner where the correct ratio of estradiol to estrone can be accomplished. Topical administration is a very efficient manner to deliver hormones, so compared to the oral route much lower doses are required. But because many practitioners have erroneously assumed that venous blood measurement can be used to determine the amount of topical dosing, doses much higher than physiologic amounts are too commonly used.