Absence of Significant Estrogenic Effects in the Postmenopausal Population

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Estrogens are categorized according to their structure into two groups: Steroidal and nonsteroidal. Each group contains both natural and synthetic compounds. The steroidal estrogens contain the same basic structure of 18 carbons. The three principal natural steroids — estrone, estradiol, and estriol — are considered the classic estrogens because they were the first to be isolated.