The different estrogens, androgens and progestogens, and their binding affinity
When we speak of estrogens, we don’t refer to only one hormone. Estrogens, as well of androgens and progestegens are three groups, each covering similar yet different hormones. Knowing about those differences is important in many ways.
Let’s speak first of binding affinity. Binding affinity designates the ability and “ease” with which one hormone will bind to its receptor. If we use the key and keyhole metaphor, you can imagine that several keys can open the same lock, but some will enter and turn with much difficulty, when others will do it quite smoothly. This is binding affinity: it is high when the key gets in without effort, and opens efficiently, and it is low when the key can open the lock, but not without effort. The different kinds of each sex hormone come with different binding affinity, which is why we will prefer some over others for efficient feminization/masculinization.
If we take a look at estrogen first, we can count four of them present in the body (at least the main ones) in differents amount: Estrone (E1), Estradiol (E2), Estriol (E3) and Estetrol (E4). But really, keep in mind only estradiol (E2), and to a lesser extent estrone (E1). Estradiol is the main active estrogen in our bodies, and it has a much higher binding affinity than estrone, so that is the one we will use for feminizing HRT. There are also a number of synthetic estrogens, developed by pharmaceutical laboratories in particular for the use in birth control pills: a common one is Ethinylestradiol (EE). As a synthetic derivative of estradiol, EE also comes with more (and more frequent) side effects. This is why it is not recommended to use birth control pills for feminizing HRT.
Note
Actually, research has shown that the common side effects of estrogen therapy that trans women are usually warned about by practitioners - and used as a reason to keep them at low doses of estrogens - such as blood clots, are mostly associated to synthetic estrogens (like EE), and to a much lesser extent to bioidentical estradiol. If you don’t have predispositions to thrombosis (comorbidities and/or genetic background), taking bio-identical estradiol - even in relatively high doses - should not be a big problem. Estrone (E1), even though bioidentical, is also associated with more frequent side effects, which is why you want to avoid oral administration of estradiol pills: indeed, most of the estradiol (E2) it contains is converted into Estrone (E1) by the liver first pass. A good way to avoid the hepatic first pass while taking oral pills is the “sublingual method”, which consists in letting the pill melt completely under the tongue. This way, estradiol is directly absorbed by the mucous membrane of the mouth, and reaches the bloodstream without passing through the liver first pass.
Let’s now have a look at androgens. We mostly know of testosterone, and it is indeed the most present androgen naturally produced by the body. However, the body is also able to convert testosterone into a much more potent form in terms of masculinizing effects: DHT (dihydrotestosterone). This conversion is made with the help of an enzyme called 5a-reductase - which is good to know since a number of antiandrogens medication (Finasteride and Dutasteride) prescribed to transfeminine people consists in inhibiting this enzyme and by doing so preventing the conversion of testosterone into its more potent form, DHT. And there are finally all the synthetic androgens that we put before in the category of anabolic steroids.
The same goes for progestogens: the main - naturally occurring - one is progesterone; but pharmaceutical labs have also created a vast amount of synthetic progestogens, lots of them - once again - for the use in birth control medication. The well known cyproterone acetate (Androcur) is one of them; and even though it is a progestogen that activates the progestogens receptors, its action (and side effects) is far from being the same as bioidentical progesterone. Another progestogen commonly found in birth control pills is Levonorgestrel.