Background/aim: To determine the prevalence of malignancy in thyroid incidentalomas (TIs) detected by fluorine-18 fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT). Materials and methods: 18F-FDG PET/CT images were evaluated prospectively for the presence of thyroid uptake. The patients with a TI were evaluated by an endocrinologist according to the predefined diagnostic algorithm. The final diagnosis was obtained clinically and/or by pathology. Results: TI was detected in 4.2% of 4204 patients. A malignant thyroid nodule was diagnosed in 29% and 33% of the focal and diffuse-focal uptake groups, respectively. However, no malignancy was detected in the diffuse uptake group. The standardized maximum uptake values (SUVmax) of the nodules were significantly higher in patients with thyroid malignancy than in patients with benign nodules (P = 0.006). The calculated cut-off value of SUVmax for malignancy was 3.5. In 2 patients in whom the cytopathological diagnosis was benign, malignancy was diagnosed after total thyroidectomy. Conclusion: A malignant nodule was present in one-third of the patients with focal or diffuse-focal uptake. A SUVmax value of 3.5 was considered as a cut-off value for the differentiation of a malignant lesion. Benign cytology in fine-needle aspiration biopsy for 2 patients underestimated a thyroid malignancy.