Estriol

Estriol is a G-protein coupled estrogen receptor antagonist. It is one of the three main estrogens produced by the human body. It can be a weak or strong estrogen depending on if it is given acutely or chronically when given to immature animals, but is an antagonist when given in combination with estradiol. It may play a role in the development of breast cancer, but based on in vitro research does appear to act as an antagonist to the G-protein coupled estrogen receptor