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PET/computed tomography (CT) using fluorine-18 fluorodeoxyglucose (18F-FDG) has been used in the diagnosis of recurrence and metastases of differentiated thyroid cancer (DTC) in cases of negative whole-body scan (WBS) despite elevated concentrations of stimulated thyroglobulin (Tg).To assess the utility of PET/CT in the detection of recurrence among patients with DTC with increased Tg levels and negative results of WBS.PET/CT results were retrospectively analyzed in patients with DTC with increased Tg and negative results of WBS as well as negative cervical ultrasonography and chest radiography. PET-CT was performed 1–2 weeks after recent diagnostics under conditions of endogenous or exogenous thyroid-stimulating hormone stimulation. PET/CT was performed using a Discovery ST scanner 1 h after an intravenously 18F-FDG injection (activity 4–5 MBq/kg). To determine the cutoff value of Tg, receiver operating characteristic curves were analyzed.Sixty-nine patients with DTC (48 women, 21 men) aged 22–83 years (mean 50.9±17.5 years) were qualified. In 44 patients (63.8%), PET/CT indicated lesions of DTC. Thirty (43.5%) patients had 18F-FDG positive findings. In the remaining 14 patients (20.3%), lesions were found in CT only. Patients with a positive PET/CT scan had significantly higher Tg values than patients with a negative PET/CT (mean 143.8 vs. 26.5 ng/ml, P=0.03). The cutoff value of Tg concentration measured with the receiver operating characteristic analysis was 32.9 ng/ml.PET/CT is a useful tool in the detection of recurrence among thyroid cancer patients in cases of conflicting results of standard procedures, particularly for those with high Tg levels and negative WBS. The probability of obtaining a positive PET-CT result increases with the level of Tg.