We do know that estriol has estrogenic properties though considered to be weaker than estradiol. However the weakness is overcome by using higher dosages. The fact that estriol is metabolized into 16-hydroxyestrone also causes concern. Estradiol and estrone are produced predominantly in the ovaries and adrenal glands, though fat cells and the liver add to the total amount. Estradiol is the most potent estrogen that occurs naturally in the human body.
Estrone is another estrogen that occurs naturally in the human body.
Estriol is produced predominantly in the liver as a metabolite of estrone and estradiol. Estriol is one of many metabolites of estradiol and estrone. The fact that estriol can be used to decrease symptoms of estrogen deficiency is because estriol does bind to estrogen receptors . It is generally accepted as common knowledge that estriol does not bind to cells as strongly as estradiol does. Our data do not support an antiestrogenic role for estriol in human breast cancer.