Higher total and free estradiol levels in the early follicular phase of the menstrual cycle, and higher total and free testosterone levels in the early follicular and midluteal phases increase a premenopausal woman's risk of breast cancer, according to a prospective case–control study of over 18,000 premenopausal women nested within the Nurses' Health Study II.
While experts have known that higher levels of endogenous sex steroid hormones increase the risk of breast cancer in postmenopausal women, an association in younger women has been difficult to demonstrate because hormone levels vary cyclically.
But researchers found that women in the highest quartiles of follicular total and free estradiol levels had approximately twice the risk of breast cancer as women in the lowest quartiles, with the strongest associations for invasive disease and for estrogen and progesterone receptor-positive (ER+/PR+) tumors. Similarly, women with the highest levels of total and free testosterone and androstenedione in both the early follicular and midluteal phases of the menstrual cycle were twice as likely to develop invasive disease as women with the lowest amounts and about three times as likely to develop ER+/PR+ cancers.
Eliassen AH, Missmer SA, Tworoger SS, et al. Endogenous steroid hormone concentrations and risk of breast cancer among premenopausal women. J Natl Cancer Inst. 2006;98:1406-1415.