Unopposed oestrogen means oestrogen that is not opposed by progesterone, associated with increased cancer risks, including endometrial cancer. Unopposed oestrogen can be caused by oestrogen-only hormone therapy, obesity, and anovulatory menstrual cycles.
The ovary has small cells that secrete oestrogen, but not progesterone.
Menopausal women are exposed to more unopposed oestrogen, since during menstruating years the corpus luteum produces progesterone and oestrogen. After menopause, this doesn’t happen.
A small amount of oestrogen is produced by the conversion of androstenedione into oestrone by an enzyme called aromatase, found in fat tissue. This is why obese women are at greater risk of unopposed oestrogen and its related conditions.
Unopposed oestrogen can result in relative oestrogen excess and oestrogen dominance.