In 27 examinations of 24 patients with differentiated thyroid carcinoma an alternating pattern of metastases with either 131I- or FDG-uptake was found. In the follow-up of these patients this flip-flop pattern was seen in 89% (17/19) of patients with metastases and uptake of 131I or FDG as described here as uptake types 1 and 2 (type 1: FDG-positive and 131I-negative; type 2: FDG-negative and 131I-positive). In 4 patients a mixed type was observed (uptake type 3), i.e. a combination of metastases with uptake types 1 and 2 in the same patient. Metastases of papillary or follicular thyroid carcinoma without uptake of iodine have all been found to be FDG-positive in patients with an increase of thyroglobulin and with negative diagnostic results from other imaging modalities, and were histologically confirmed by surgery. False-negative or false-positive cases were not observed in this study. The FDG uptake showed an inverse proportionality to iodine uptake and to tumor differentiation. Increased glucose metabolism is a sign of higher malignancy.