The below quote comes from a very knowledgeable and well respected member on PM. Remember, there are varying opinions, so you need to research for yourself.
"xcellent info, but remember there is competition between Estrogen and Testosterone.
The "normal" range of E2 needs to be adjusted for the amount of test you are on. For example, if you somehow took enough of an AI to hit 50 estradiol on 1g of test, you would get all sorts of low estrogen side effects (this would require 50mg+ aromasin, no one takes that much anyway). Now if you take 20-25mg of aromasin on 1g of test, you will hit about 100 estradiol, however you will not have any symptoms of estrogentic side effects, even if you are sensitive to them.
Competition between testosterone and estrogen (estradiol) is well known to endocrinologists, and some organic chemists, however it is a vague area. A great example of this effect is in postmenopausal women, when their ovaries stop producing estrogens, they get androgenic side effects from the testosterone produced by their adrenal gland, even though this is the same amount that was produced before menopause.
This competition is thought to be due to co-binding factors at the AR and ER, however again, this is a vague area. I have extensively questioned a rather famous endocrinologist I once met on this issue, Patrick Arnold, and a very smart organic chemist I know, all with very little information forthcoming."