PURPOSE: This study was to investigate the clinical significance of diffusely increased F-18 FDG uptake in the thyroid gland as an incidental finding on F-18 FDG PET/CT imaging in patients with breast carcinoma. METHODS: One hundred four patients with breast carcinoma who had no prior history of thyroid disease were enrolled. All patients underwent F-18 FDG PET/CT, ultrasound and thyroid function test (TFT-TSH, FT4, and T3), anti-TPO antibody test within 2 weeks. Also we checked estrogen (ER) and progesterone receptors (PR). We classified all patients into subgroups according to the existence and degree of F-18 FDG uptake in the thyroid gland, and evaluated the difference between subgroups. RESULTS: Of the 104 patients, 42 (40.4%) subjects showed diffusely increased thyroid uptakes. There was no significant difference in rate of abnormality in TFT and thyroid US, and existence of anti-TPO antibody and ER/PR between two groups. Of 42 patient who showed diffuse uptake, 12 (28.5%), 13 (31.0%), and 17 (40.5%) subjects demonstrated hypointense, isointense, and hyperintense thyroid uptake compared with activity of mediastinal blood pool. Thirteen (76.4%) of 17 subjects in the hyperintense thyroid uptake group revealed abnormality in various tests (US, TFT, and anti-TPO antibody). The rate of abnormality in this group was significantly different with the other two groups (p=0.002). CONCLUSION: Our data suggested that the rate of diffuse thyroid uptakes on F-18 FDG PET/CT imaging of patients with breast carcinoma was higher than healthy subjects. In case of someone who had no prior thyroid disease showed diffuse thyroid uptakes more than activity of mediastinal blood pool on F-18 FDG PET/CT imaging, it should be considered further evaluation about the thyroid gland.