A patient developed a pulmonary metastasis from papillary thyroid carcinoma. This tumor concentrated relatively little 131I, but sufficient18F-fluoro-2-deoxy-D-glucose (FDG) to be quantified and imaged by positron emission tomography. The uptake of FDG was lower on positron emission tomographic images after T4 therapy and when the serum TSH concentration was reduced to the low normal range. It may be possible to use decreases in FDG uptake by thyroid cancers, which represent declines in metabolism by the tumors, to indicate the optimum doses of T4 treatment for patients with these neoplasms. In addition, the ratio of tumor to background radioactivity was higher for FDG than for the flow agent201Tl, so that studies with FDG may be a useful scintigraphic method for locating thyroid cancers when radioiodine imaging is unsatisfactory.