Of the three major estrogens (estrone [e1], estradiol [e2], estriol [e3]), is the least potent natural estrogen (E2 > E1 > E3). In men and non-pregnant women, E1 and E2 are formed from androstenedione and testosterone. E3 is synthesized from E2 and to a limited extent, from 16-alpha metabolites of estrone.
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