Background: The relationship between thyroid autoimmunity and breast cancer is a challenging subject. We aimed to investigate this association in women with breast cancer. Methods: In this descriptive study, 41 women with newly diagnosed breast cancer before receiving any pharmacologic treatment and 38 healthy age-matched women were enrolled. Anti TPO Ab (anti-thyroid peroxidase antibodies), FT4 (free thyroxine), T3 (triiodothyronine) and TSH (thyroid-stimulating hormone) were measured in both groups. Results: The mean±SD ages in patients with breast cancer and the control group were 41.71±1.73 and 40.03±1.74 years, respectively (P=0.496). There was no statistically significant difference between the mean values of FT4 and T3 in patients with breast cancer (P=0.447) and the control group (P=0.534). The mean TSH level in patients with breast cancer was 4.9±1.7 µIU/ml which was significantly higher than healthy women (1.79±0.15 µIU/ml, P=0.004). The frequency rate of the increased Anti TPO Ab levels (higher than 35 IU/ml) in women with breast cancer was 22% which was significantly higher than the control group (%0, P=0.002), while no statistically significant difference was found between the mean Anti TPO Ab levels between the two groups (61.07±29.73 versus 9.78±0.78, P=0.21). Four cases of subclinical and/or overt hypothyroidism were found in women with breast cancer. Conclusion: Based on our findings breast cancer patients have higher rates of thyroid autoimmunity. Measurement of FT4 and T3 in all women with breast cancer is not recommended but measurement of TSH and Anti TPO Ab levels seem reasonable.