A new prospective Australian study has reported that high levels of circulating steroid hormones increase the risk of breast cancer in postmenopausal women. This confirms findings of previous studies. The study also examined whether the associations varied by hormone receptor status, age at diagnosis, or tumor grade. The study had an average follow-up period of 9.2 years and included 197 breast cancer cases that developed during this period, plus a group of cancer-free controls for comparison. Concentrations of total estrone sulfate, estradiol, testosterone, DHEA sulfate, androstenedione, and sex hormone binding globulin were measured in blood plasma collected at baseline; free plasma estradiol concentrations were calculated.

Compared to women in the lowest fourth of circulating hormones, women in the highest fourth had a higher risk of breast cancer. The increase in risk was greatest for high levels of estrone sulfate, free estradiol, and testosterone. Increases in risk were also observed for androstenedione, total estradiol, and DHEA sulfate, but these results were not statistically significant. Reduced risk of breast cancer was found for high levels of sex hormone binding globulin. These associations did not vary by tumor grade, estrogen receptor status, or progesterone receptor status. The breast cancer risk associated with estrogen and androgen levels (especially testosterone) were found to be higher at older ages. The authors conclude that endogenous steroid hormone levels are associated with postmenopausal breast cancer risk, confirming previous study results, and that these associations are independent of tumor grade and hormone receptor status and might increase with age.