According to a study published in the Journal of the National Cancer Institute, high levels of circulating sex hormones, including estrogens and androgens, may be associated with breast cancer risk for premenopausal women.

Heather Eliassen, Sc.D., of Brigham and Women's Hospital in Boston, and colleagues collected blood samples from 18,521 premenopausal women in the Nurses Health Study II during certain phases of their menstrual cycles. Of these women, 197 developed breast cancer.

The authors observed that women with high levels of estradiol, a type of estrogen, during the pre-ovulatory phase of their menstrual cycle had increased risks of breast cancer. The risks of invasive and estrogen- and progesterone-positive breast were highest. High levels of testosterone and androstenedione in both the pre- and post-ovulatory phases of the menstrual cycle were also associated with an increased risk of breast cancer overall.

SOURCE:
Journal of the National Cancer Institute, October 4, 2006