Objective: The association between thyroid function and breast cancer is controversial and has been studied for more than 50 years in both animals and humans with mixed results. The type of relationship between hypothyroidism and the risk of breast cancer remains to be elucidated. Subclinical hypothyroidism (SCH) may have a role in breast cancer development. In this study, the prevalence of SCH was investigated in patients with newly diagnosed breast cancer at a single center. Material and Methods: A total of 910 breast cancer patients were screened, and 403 women (mean age, 55.7±10.7 years) were included for the analysis. The staging of breast cancer patients was based on The Staging Manual of the American Joint Committee on Cancer. SCH was defined as elevated TSH (normal range: 0.25-4.55 μIU/mL) with normal free T4 (normal range: 11.5-22.7 pmol/L) and normal free T3 (normal range: 3.5-6.5 pmol/L) concentrations and these were used to compare SCH-positive (n=46) and SCH-negative (n=357) patients. Results: The prevalence of SCH among newly diagnosed breast cancer patients was 11.4%. SCH-positive and SCH-negative patients were comparable in terms of menopausal status and tumor grade. Conclusion: The rate of SCH among newly diagnosed breast cancer patients in this study indicates that SCH may be a coincidental condition in breast cancer patients.