A 52-year-old woman underwent whole-body F-18 FDG PET imaging for cancer screening. Diffuse F-18 FDG uptake was seen in the thyroid gland. A nodule with elevated F18 FDG uptake was seen protruding posteriorly in the right thyroid lobe. Ultrasonography also showed a thyroid nodule. Surgery was performed and histologically, the nodule was composed of atrophic thyroid tissue, fibrous stroma, and lymphoid cell infiltration; it was consistent with chronic thyroiditis. The patient had high antimicrosomal antibody titers. Thus, chronic thyroiditis may be found incidentally during FDG PET studies.