From there, they’ll likely test your estradiol (E2) since it’s the main form of estrogen tested, though estrone (E1) and ...

Levels of the ovarian-derived 17b estradiol (E2), as ovarian follicles cease production, plunge as menopause nears, and estrogen in the form of estrone (E1) becomes the predominant form.

and route of ET prescribed will be additive to that which is endogenously produced and will influence the hepatic synthesis of SHBG and therefore the bioavailability of the prescribed estrogen ...