Thyroid hormones and selenium status in breast cancer

Within a case-control study of postmenopausal breast cancer patients (n = 99) and matched healthy controls (n = 105), thyroid hormone levels were compared and correlated with toenail selenium concentrations. Plasma triiodothyronine (T-3) was significantly lower in cases (1.4 +/- 0.4 nmol/l) than in controls (1.6 +/- 0.4 nmol/l), and a strong inverse relationship with breast cancer was observed with an odds ratio of 0.17 (95% CI = 0.08-0.36) in the highest compared with the lowest tertile of T-3. Plasma thyroxine and thyroid-stimulating hormone concentrations were similar between cases and controls. Plasma T-3 concentration was positively associated with toenail selenium in cases (age-adjusted regression coefficient = 0.049) and controls (age-adjusted regression coefficient = 0.036). Toenail selenium concentrations tended to be lower in cases than in controls, but the differences did not Peach statistical significance. Although the disease process per se may explain the lower plasma T-3 concentrations, it is also possible, inasmuch as these patients were in early-stage breast cancer, that selenium status may be influencing T-3 levels via changes in the activity of the selenoenzyme type I iodothyronine deiodinase.