In women of reproductive age, the primary source of circulating estrogens is the ovaries. There are three forms of estrogen circulating in our bloodstream: estradiol, estrone and estriol. The normal ratio of these three types of estrogens ideally should be 10–20%, 10–20%, and 60–80% respectively. The estrogen that accounts for most of the tissue stimulation is called estradiol. Estrone is a little bit less potent with estriol being the weakest. Brain converts estrone and estradiol to 2- and 4-hydroxylated derivatives known as catechol estrogens