Currently PET/CT is the most commonly used and effective method to diagnose malignancy, but 18F-FDG PET/CT imaging is not routinely required for differentiated thyroid cancer. PET/CT should be considered only in the following cases: 1. Patients with DTC with negative diagnostic activity of 131I and serum Tg>10μg/L, PET/CT imaging can help to detect dedifferentiated lesions; 2. Patients with DTC with negative diagnostic activity of 131I and Tg<10μg/L, but recurrence or metastasis is suspected by ultrasound and other examinations; 3. Patients with negative visualization and elevated TgAb.