Kim EY, et al. - Among 62,546 Korean women aged ≥ 40 years, researchers assessed breast cancer risk in relation to serum thyrotropin (TSH) and thyroid hormone concentrations in this cohort study. Participants were free of breast cancer at baseline and had measured free thyroxine (FT4) and TSH levels. They identified 834 incident breast cancers during a median follow-up of 4.8 years. An increased risk of incident breast cancer was observed in relation to abnormally high serum FT4 level vs normal FT4 level. In postmenopausal women vs in premenopausal women, this link tended to be stronger. Within the euthyroid range, a lower risk of breast cancer was observed in relation to the highest TSH tertile vs the lowest TSH tertile. In the development of breast cancer, the possible contribution of thyroid function within both the abnormal and euthyroid ranges was suggested.