Objective：To discuss changes of thyroid functions among patients with breast cancer on initial diagnosis and during the chemotherapy. Methods：Thyroid functions of 112 initially diagnosed patients with breast cancer and 235 patients with benign breast diseases were detected and compared. Thyroid functions of 93 patients with breast cancer between prechemotherapy and postchemo-therapy were detected and analyzed. Thyroid functions of 120 cases between two consecutive prechemotherapies were measured and analyzed. Results：Incidence of hypothyroidism was 21.4% in patients with breast cancer and 7.2% in those with benign breast diseases. Levels of free triiodothyronine（FT3） were lower in patients with breast cancer than in those with benign breast diseases（t=-2.036，P=0.042）. Thyroid functions were significantly lower in postchemotherapy than in prechemotherapy triiodothyronine（T3）：t=13.427，P<0.001；tetraiodothyronine（T4）：t=3.732，P<0.001；FT3：Z=-10.264，P<0.001. No significant difference in thyroid function between two prechemotherapies was found（T3：t=0.170，P=0.865；T4：t=1.458，P=0.147；FT3：t=0.180，P=0.858；FT4：t=-0.973，P=0.333；thyroid stimulating hormone（TSH）：t=-0.877，P=0.383）. Conclusions：Hypothyroidism is often accompanied with patients who is initially diag－nosed as breast cancer. Thyroid functions decrease during chemotherapy but recover before the next course chemotherapy.