Patients with differentiated thyroid cancer (DTC) have a relatively good overall prognosis, but 20% of them develop local recurrences and 10% distant metastases with relatively high mortality (8%). The current prognostic systems lack the ability to predict outcome of patients when discovered to harbor metastases years later. The introduction of 18-F-fluoro-deoxyglucose (FDG) positron emission tomography (PET) and PET/computed tomography (CT) for evaluating patients with DTC has added new information on the disease. This review will summarize the data published so far on the use of FDG in DTC. The most appropriate indication to FDG-PET/CT is in evaluating patients with high thyroglobulin level during follow-up, mainly when radioiodine whole body scan is negative. In this setting, a high accuracy has been reported leading to change in management of patients. A clear relationship has been demonstrated between FDG-PET/CT accuracy and thyroglobulin level, with better results obtained in thyroid stimulating hormone (TSH) stimulated condition. Moreover, FDG imaging provides potentially relevant information on tumour biology, and scan results may enable to prognostically stratify DTC patients with local or distant recurrences. Based on literature data, FDG-PET/CT does not have routine clinical indication in the initial staging of DTC patients, as well as in evaluating cytologically indeterminated thyroid nodules.