Objective. This study investigated whether thyroid hormone (TH) levels are correlated to cell proliferation (Ki67), in euthyroid breast cancer patients. Design and Methods. 86 newly diagnosed breast cancer patients with estrogen receptor (ER) positive tumors, who referred for surgery, were included in the study. Results. FT3, FT4, and TSH were within normal range. The gene discussed is ESR1; the disease is breast cancer.