In this retrospective study, we sought to investigate the prevalence and the clinical significance of thyroid uptake discovered by FDG PET/CT within the thyroid gland during imaging investigation of non-thyroid disorders. This study included 5,877 subjects with no previous history of thyroid malignancy referred to our PET/CT center (1,596 for cancer screening and 4,281 for primary staging or restaging purposes) from June 2006 to April 2009. All of the patients had at least 12 months of follow-up or were censored on the date of the last follow-up. The disease status was analyzed for correlation with the maximum standardized uptake value (SUVmax). Receiver operating characteristics (ROC) curves were constructed to determine the optimal cutoff values for SUVmax. In the screening group (n=1,596), 55 patients (4%) had thyroid FDG uptake. Twenty-two subjects were referred for cytological or histological assessment and 4 (18%) were diagnosed with thyroid carcinoma. In the primary staging/restaging group (n=4,281), 165 patients (4%) had thyroid FDG-avid sites. Thirty-three patients were referred for cytological or histological assessment and 4 (12%) were found to have thyroid carcinoma. Of the 8 malignant lesions, 7 were papillary thyroid carcinoma, and 1 medullary thyroid carcinoma. Surgical resection was performed in seven out of eight patients. Three patients had lymph node metastasis (one case detected by preoperative PET/CT) and three bilateral malignant diseases (all false negative in PET/CT scans). Of the 47 benign lesions, 35 were nodular goiter or nodular hyperplasia, 6 multinodular goiter, and 6 autoimmune thyroid disease. The average value of SUVmax of malignant thyroid lesions was significantly higher than that of benign thyroid lesions (8.2±4.5 vs. 5.6±3.2, P=0.048). However, we were unable to identify an optimal SUVmax cutoff because the total area under the curve was small. The prevalence of thyroid uptake on FDG PET/CT in this study is 3.7%, which is consistent with previous observations. Of patients who received verification by cytology or histology, 14% (8/55) were found to have thyroid malignancies. However, we were unable to establish an optimal SUVmax cutoff value to differentiate benign from malignant lesions. FDG PET/CT scans were false negative in a significant proportion of patients with thyroid carcinoma and regional lymph node metastases or separate tumor nodule(s) in a contralateral lobe. Further diagnostic tests (thyroid ultrasound/fine needle aspiration) are recommended in presence of focal thyroid uptake detected by FDG PET/CT scans.