4-Hydroxyestradiol (4-OHE2), also known as estra-1,3,5(10)-triene-3,4,17ò-triol, is an endogenous, naturally occurring catechol estrogen and a minor metabolite of estradiol. It is estrogenic, similarly to many other hydroxylated estrogen metabolites such as 2-hydroxyestradiol, 16ñ-hydroxyestrone, estriol (16ñ-hydroxyestradiol), and 4-hydroxyestrone but unlike 2-hydroxyestrone.