Abstract
Thyroid nodular lesions and thyroiditis are commonly observed in population over thirties. Since the whole-body F18-fluorodeoxyglucose positron emission tomography (PET-CT) system is widely utilized in clinical application, the detection of incidental thyroid uptake has increased significantly. Unexpected thyroid uptake on PET-CT can be divided into three distinguishable patterns: focal, diffuse, or diffuse plus focal. Focal uptake, incidentaloma, is clinically significant because of its high risk of malignancy at the range of 25-50%. Therefore, the cytologic diagnosis should be additionally performed in these patients. Diffuse pattern of thyroid uptake is very likely secondary to thyroiditis and/or hypothyroidism. Although the clinical meaning of diffuse plus focal uptake of thyroid observed on PET-CT is not clear, the risk of malignancy was increased when it compared with diffuse uptake pattern only. However, further evaluation is required to define its clinical significance.