A thyroid nodule is an isolated lesion within the thyroid gland that can be distinguished radiologically from the surrounding thyroid tissue. Some palpable lesions do not have corresponding imaging abnormalities, and such abnormalities do not meet the strict definition of a thyroid nodule. Nonpalpable nodules detected by ultrasound or other imaging are named incidental nodules or incidental tumors. Nonpalpable nodules have the same rate of malignancy as palpable nodules of the same size. Generally, only nodules >1 cm in diameter need to be evaluated because these nodules have a high potential for malignancy. Sometimes nodules <1 cm in diameter should also be evaluated when ultrasound findings are suspicious or accompanied by lymphadenopathy, a history of head and neck radiation exposure, or a family history of thyroid cancer (one or more first-degree relatives with thyroid cancer). However, some nodules <1 cm in diameter without these risk factors do eventually become malignant or even lead to death. This is of course rare, and going to the trouble of diagnosing and treating all small thyroid tumors simply to prevent this would do more harm than good from a cost/benefit perspective. About 1-2% of patients who have 18FDG-PET imaging for some reason have incidental findings of thyroid nodules, and 33% of these positive 18FDG scans will be malignant and progress rapidly enough to warrant aggressive evaluation of such lesions. If diffuse 18FDG uptake is found in the thyroid tissue, it is indicative of an underlying autoimmune thyroiditis.