Although the utility of FDG-PET scan is usually limited to the follow up of a recurrence follow the resection of primary malignant tumors. Its usefulness may be widen to the diagnosis in some malignant neoplasms; that use mostly glucose their energy sources. Thyroid cancers use glucose for their metabolic activities and incidental focal thyroid uptake shown on FDG-PET scans may be representative of a primary tumor. Therefore, increased focal thyroid uptake shown on whole body FDG-PET scans should not be overlooked even when it is not marked, and prompt further investigation should be done to rule out a possible cancer. We present a case of thyroid cancer, incidentally detected with a PET scan in a 60-year-old-man. We also present a review of the literatures regarding the usefulness of PET scan in thyroid cancer.