Purpose: To compare the diagnostic accuracy of ultrasonography (US), F18-fluoro-deoxyglucose positron emission tomography/CT (PET/CT), contrast enhanced CT (CECT), serum anti-thyroid antibody for detecting Hashimoto's thyroiditis in thyroid cancer patients who underwent neck surgery. Materials and Methods: A total of 160 patients with suspicious for thyroid cancer, who had previously undergone US guided needle aspiration of thyroid, were evaluated with the use of US, PET/CT, CECT and serum anti-thyroid antibody. The four studies were performed within two months before neck surgery. Hashimoto's thyroiditis was confirmed by histopathological results. The diagnostic accuracy of US, PET/CT, CECT and serum anti-thyroid antibody were calculated statistically. Results: Hashimoto's thyroiditis was diagnosed in 51 out of the 150 patients, following neck surgery. The sensitivity, specificity, positive predictive value, negative predictive value and accuracy of US were 76.5%, 92.9%, 84.8%, 88.5%, and 87.3%, respectively. The corresponding values of PET/CT were 37.3%, 96.0%, 82.6%, 74.8%, and 76.0%, and CECT were 62.7%, 89.9%, 76.2%, 82.4%, and 80.7%, and serum anti-thyroid antibody level were 90.2%, 93.9%, 88.5%, 94.9%, and 92.7%, respectively. McNemar test revealed significant difference among PET/CT and others, but no significant differences among US, CECT and serum anti-thyroid antibody. Overall, serum anti-thyroid antibody showed most accurate diagnostic performance. Conclusion: In detecting Hashimoto's thyroiditis, serum anti-thyroid antibody showed higher diagnostic accuracy than others. US also showed relatively high diagnostic accuracy. [ABSTRACT FROM AUTHOR]