Breast cancer is a hormone-dependent neoplasm. Conflicting results regarding the clinical correlation between breast cancer and thyroid diseases have been reported. The aim of this study was to determine the goiter prevalence, thyroid hormones and antithyroid antibody levels in patients with breast cancer. For this purpose, thyroid ultrasonography was performed and serum levels of free triiodothyronine (FT3 ), free thyroxine (FT4 ), thyroid-stimulating hormone (TSH), anti-thyroid peroxidase antibodies (anti-TPO ab) and anti-thyroglobulin antibodies (anti-TG ab) were determined in 50 operable breast cancer patients at the time of diagnosis and 30 healthy individuals as control group. Goiter prevalence was found to be significantly higher in the study group (50% vs 10%). FT3 , FT4 , and TSH levels of patients were not different compared to controls, whereas anti-TPO ab and anti-TG ab levels were significantly higher in patients than in the control group. In conclusion, goiter prevalence and thyroid antibody levels were found to be increased in breast cancer patients and thyroid functions should be monitored.