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Abstract
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 (T3) was significantly lower in cases (1.4 ± 0.4 nmol/l) than in controls (1.6 ± 0.4 nmolll), and a strong inverse relationship with breast cancer was observed with an odds ratio of 0.17 (95% CI = 0.08‐O.36) in the highest compared with the lowest fertile of T3. Plasma thyroxine and thyroid‐stimulating hormone concentrations were similar between cases and controls. Plasma T3 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 reach statistical significance. Although the disease process per se may explain the lower plasma T3 concentrations, it is also possible, inasmuch as these patients were in early‐stage breast cancer, that selenium status may be influencing T3 levels via changes in the activity of the selenoenzyme type 1 iodothyronine deiodi‐nase.