• Ghada N Farhat, Steven R Cummings, Rowan T Chlebowski, Neeta Parimi, Jane A Cauley, Thomas E Rohan, Alison J Huang, Mara Vitolins, F Allan Hubbell, Joann E Manson, Barbara B Cochrane, Dorothy S Lane, and Jennifer S Lee.
• San Francisco Coordinating Center, California Pacific Medical Center Research Institute, San Francisco, CA 94107, USA. gfarhat@sfcc-cpmc.net
• J. Natl. Cancer Inst. 2011 Apr 6; 103 (7): 562-70.

BackgroundEndogenous sex hormone levels are associated with risks of breast cancer overall and estrogen receptor (ER)-positive breast tumors; however, their associations with ER-negative tumors remain unclear.MethodsIn a case-cohort study within the Women's Health Initiative Observational Study among postmenopausal women aged 50-79 years, we examined associations between endogenous testosterone and estradiol levels and the risks of ER-negative and ER-positive breast cancers. Serum levels of bioavailable testosterone and estradiol were assessed at the baseline visit in 317 invasive breast cancer case subjects and in a subcohort of 594 women. Bioavailable sex hormone levels were calculated using the total hormone level and the sex hormone-binding globulin concentration (measured by radioimmunoassays and a chemiluminescent immunoassay, respectively). Cox proportional hazards regression was used for statistical analysis. All statistical tests were two-sided.ResultThe unadjusted absolute rates of ER-negative breast cancer for testosterone quartiles 1-4 were 0.34, 0.20, 0.23, and 0.21 per 10,000 person-years, respectively. Compared with women in the lowest quartile of testosterone level, those in quartile 2 had a 56% lower risk of ER-negative cancer (hazard ratio [HR] = 0.44, 95% confidence interval [CI] = 0.23 to 0.85), those in quartile 3 had a 45% lower risk (HR = 0.55, 95% CI = 0.30 to 1.01), and those in quartile 4 had a 49% lower risk (HR = 0.51, 95% CI = 0.28 to 0.94), independent of other risk factors. Estradiol level was not associated with ER-negative breast cancer. ER-positive breast cancer risk increased with higher testosterone levels (P(trend) = .04), but this trend was not statistically significant after adjustment for estradiol (P(trend) = .15). ER-positive cancer risk was approximately twofold higher in women with estradiol levels in quartiles 2-4 compared with women in quartile 1, independent of risk factors.ConclusionHigher serum levels of bioavailable testosterone are associated with lower risks of ER-negative breast cancer in postmenopausal women.

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