The main feminizing effects of estrogen depend on sex-hormone receptors being present in cells and hormone molecules being free to bind to them to affect gene expression. Since sex-hormones also regulate receptor expression[1], increasing the amount of receptors or hormone available for binding is thus a desirable goal for feminizing HRT.[2]
Estradiol has a pathway of being synthesized from testosterone in the body through an enzyme called aromatase which allows for certain levels of testosterone to circulate and still be converted to estrogen.
