Subacute Thyroiditis Presenting as a Focal Lesion on [¹⁸F] Fluorodeoxyglucose Whole-Body Positron-Emission Tomography/CT

SUMMARY: A 52-year-old man presented with subacute thyroiditis and showed an intensely hypermetabolic thyroid lesion on ¹⁸F-FDG PET/CT. The patient also had coexisting chronic lymphocytic thyroiditis. The lesion of subacute thyroiditis revealed an SUV_max of 9.1 g/mL and was ill-defined, heterogeneous, markedly hypoechoic, and hypovascular on US. Biopsy demonstrated histologic findings of subacute thyroiditis, which should be included in the differential diagnosis of an intensely hypermetabolic thyroid lesion on ¹⁸F-FDG PET/CT.