A 31-year-old man underwent a whole-body F-18 fluorodeoxyglucose (FDG) positron emission tomography (PET) scan for cancer screening. There was no history of diabetes, thyroid disease, or noticeable exercise before imaging. The results demonstrated diffusely increased uptake of F-18 FDG in the thyroid and thymus. In addition, there was symmetrically increased uptake of F-18 FDG in skeletal muscles, especially in the psoas and limb muscles. The concurrent appearance of these characteristics led to a diagnosis of Graves' disease.