Since this is a subject that comes up time and time again, I decided to start this thread so we can easily refer people to this information.
From the beginning...
Estradiol is an estrogen. It is known on blood tests as E2. Many people (even doctors) simply call it estrogen.
Why it Matters
There are basically two reasons we care about estradiol.
The first is that E2 is a powerful testosterone receptor antagonist. What this means is that estradiol binds to androgen receptors and renders them useless. When testosterone binds to an androgen receptor, it activates the receptor and you get the effect you're looking for. When estradiol binds to that receptor, it blocks testosterone from binding, yet it does not active the receptor, so nothing happens.
This means that if your estradiol is high, no matter how much testosterone you have, it isn't helping you as it should because too many of your androgen receptors are blocked by estradiol and your free testosterone has no where to go. Testosterone can't do you any good if it doesn't have receptors available to activate.
It can even get worse... because high levels of estradiol can cause the downregulation of androgen receptors. This means that your body may respond to higher levels of estradiol by creating fewer androgen receptors as cells are replaced in normal regeneration. In other words, not only does estradiol block the available androgen receptors, it causes your body to produce fewer of them in the future! This is one reason why raising testosterone levels may not have any immediate effect. It may be that your receptors have downregulated and so you'll need to lower estradiol and increase testosterone in order to get your body to upregulate again and this takes time.
The second reason we care about estradiol is that you also have estrogen receptors and estradiol binds to them and causes them to activate. This is fine if you want to grow man boobs, store fat on your belly, and have an enlarged prostate, but not so good if you want to look and feel like a man.
Where It Comes From
Brushing aside the highly controversial subject of environmental estrogens, the primary pathway for estradiol production is via the conversion of testosterone by aromatase. This means that the aromatase enzyme binds to testosterone and chemically converts it to estradiol. Think about that for a minute. Realize that this conversion is a double whammy. In one process you're losing T and gaining E. Obviously this is not good.
This is where aromatase inhibitors (AI) like anastrozole (brand name Arimidex - aka "adex" etc.) come into the picture. They bind to the aromatase enzyme and prevent it from converting your testosterone to estradiol. They do not work directly on estradiol nor on estrogen receptors. It is SERM's like Clomid and Nolvadex that bind to estrogen receptors. SERM's and AI's are different animals, so don't get them confused.
So an AI gives you the double whammy in reverse. It prevents the loss of testosterone to conversion and consequently lowers your estradiol which helps keep your androgen receptors available for testosterone.
E Follows T
Here's where doctors fuck up. They forget that E follows T. As your T levels go up, so will your E levels. If you're not doing something to control your E (like taking an AI) then you aren't really going to get anywhere with higher T levels because your higher E levels are just going to cancel out any beneficial effect from the T. Like I said, your T can't do you any good if all your androgen receptors are bound with estradiol.
For those of us with "age related" low T, this is a serious issue because our bodies are naturally trying to keep T low and E high by pumping out aromatase.
If you're younger and have low T for some other reason, aromatase may not be as much of an issue, but it still matters.
Fat produces estrogen and aromatase; even in men. If you're carrying extra fat, one of the best things you can do to help your hormone balance is to lose the fat.
In Range does not equal Normal
One of the problems you're likely to face is the problem of doctors believing that any blood test value that is "in range" is "normal" and therefor fine. I wish it was that simple.
Let's look at testosterone values. The range for Quest is 241-847 ng/dL. But those values are derived simply by looking at the values of everyone who has a blood test for testosterone. What's normal for a younger man is to be in the higher end of the range. What's normal for an older man is to be in the lower end of the range. Well low T might be "normal" but that doesn't mean it's good! I may be an older guy, but why should I be happy with low T simply because it's normal? ALL men of all ages should be in the higher end of the range if they want to feel good and perform athletically and sexually as men.
The same holds true for estradiol. The range is 13-54 pg/mL but "normal" young men are at the low end of the range and that's where you want to be as well.
Trust me, if your T is 250 and your E is 50, you may be "in range" for both values, but you aren't going to feel good or have anywhere near the athletic and sexual performance that would have if your T was 800 and your E was 15.
All populations exhibit variation. The average height for men might be 5' 10" but we all know guys who are much taller and much shorter. This same variation applies to T levels, E levels, and reactions to various therapies. In other words, while the principals I've outlined hold true in general, how they specifically apply to you will vary. Be smart and deal with it.
Ok, that's enough from me for now. This is just a basic primer; there's a lot more to this subject so use this as a springboard to do more research on your own.
I'm sure others here will have significant things to say as well.