Estrogens actually refers to a group of related hormones, each with a unique profile of activity. Under normal circumstances, a woman's circulating estrogen levels fluctuate based on her menstrual cycle. For Hormone Replacement Therapy, these hormones are often prescribed in combination to re-establish a normal physiologic balance. The three main estrogens produced in female humans are:

• Estrone -E1- (10-20% of circulating estrogens) is the primary estrogen produced after menopause.
• Estradiol -E2- (10-30% of circulating estrogens) is the most potent and major secretory product of the ovary, and the predominant estrogen produced before menopause.
• Estriol -E3- (60-80% of circulating estrogens).

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