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