FDG PET can detect thyroid cancer in patients referred for exploration of a different cancer. Because of its lack of specificity, however, this modality is not indicated for examination of thyroid nodules: ultrasonography and fine needle biopsy with cytology allow histological diagnosis, which can be completed by iodine-123 scintigraphy when an autonomous nodule is suspected. No information is currently available about the utility of FDG PET in preoperative staging. In follow-up of patients undergoing thyroidectomy for adenocarcinoma, FDG PET is useful for detecting recurrence in cases where serum thyroglobulin levels rise and iodine-131 scintigraphy is negative: surgical resection may be appropriate. Nonetheless FDG PET should be performed more widely and earlier: the initial presence of foci positive for FDG is a major predictor of shorter survival, and most cancer lesions take up either iodine or FDG. In follow-up of medullary carcinoma, FDG PET detects residual tissue better than any other scintigraphic procedures, especially when serum levels of CEA (carcinoembryonic antigen) are rising rapidly. FDOPA PET seems to have better sensitivity than FDG-PET and may be useful in occult recurrence, as three case reports indicate.