Follicular cell-derived differentiated thyroid carcinoma has a fairly good prognosis; however, the probability of recurrence is high, which increases the morbidity and mortality rates significantly. The most common agent used in patients with differentiated carcinoma for both diagnostic and therapeutic purposes is radioiodine; however, it has been reported that thyroid tumor cells lose the ability to take up iodine because of de-differentiation. Positron emission tomography (PET) using 18-F-fluorodeoxyglucose (FDG) has proven useful in patients with differentiated thyroid carcinoma presenting serologic evidence of persistent disease but negative whole-body radioiodine scanning. There is an on-going debate on the role of PET in revealing the malignancy in thyroid nodules preoperatively. As a functional imaging modality, FDG-PET imaging may have some limitations because of its finite spatial resolution, especially in patients with minimal cervical adenopathy or small pulmonary metastasis; however its accuracy may improve using fusion imaging with either computed tomography (PET-CT) or magnetic resonance imaging (PET-MR imaging).