Breast cancer is one of the most common cancers in China. Even though advances in the field of breast cancer therapeutics, chemotherapy remains the mainstay therapeutic modality. The cytotoxic therapies are generally associated with some immediate or otherwise delayed side effects, such as adverse effects on gastrointestinal, hepatic, renal and hematological systems. The effect of chemotherapy on endocrine system, however, is comparatively less envisaged. Several epidemiological studies show a positive association between plasma thyroid hormones levels and breast cancer risk. Thyroid dysfunction is emerging as a variably common endocrine toxicity of several anticancer drugs. Due to the scarcity of data on the functioning of thyroid gland during chemotherapy in a large scale group, the present study was aimed to investigate the incidence of thyroid abnormity in Chinese women with newly diagnosed breast cancer, and also the effects of chemotherapy on thyroid gland functions or structure in these breast cancer patients undergoing at least four cycles of chemotherapy.