Previous studies showed higher testosterone levels are associated with higher risk of breast cancer in premenopausal women, but literature is scant and inconsistent.

In a prospective nested case-control study of 104 premenopausal women with incident breast cancer and 225 matched controls, all characterized by regular menstrual cycles throughout their lifetime, we measured the concentration of estradiol, total and free testosterone (FT), progesterone, sex hormone-binding globulin (SHBG), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in blood samples collected on the 20-24th day of their cycle.

In logistic regression models, the multivariate odds ratios (OR) of invasive breast cancer for women in the highest tertile of circulating free testosterone compared with the lowest was 2.43 (95% confidence interval [95% CI], 1.15-5.10; Ptrend = 0.03), while for total testosterone the association had the same direction but was not statistically significant (OR, 1.27; 95% CI= 0.62-2.61; Ptrend = 0.51]. Endogenous Progesterone was not statistically associated with breast cancer (OR, 1.16, 95% CI, 0.60-2.27; Ptrend = 0.75), nor were the other considered hormones.

Consistent with previous prospective studies in premenopausal women and our own earlier investigation, we observed that higher levels of free testosterone are positively associated with breast cancer risk in women with regular menstrual cycles throughout their lifetime. There was no evidence of risk associated with the other endogenous sex steroids.