Elevated 18F-FDG uptake in skeletal muscles and thymus: a clue for the diagnosis of Gravesʼ disease

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Abstract
The aim of this study was to evaluate the appearance of Graves' disease in whole-body 2-[18F]fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) imaging.Twenty patients (three men, 17 women; mean age +/- SD, 37.2 +/- 12.1 years) with newly diagnosed Graves' disease without anti-thyroid drug treatment took part in the study. Secondly, a control group, 15 examinees (three men, 12 women; mean age +/- SD, 45.3 +/- 13 years) each with a history of Graves' disease and post-medical treatment were recruited. This group showed normal thyroid function. Finally, a normal group, 32 physical check-up examinees with non-specific medical histories and normal thyroid functions, were recruited. Whole-body FDG PET was performed on all subjects. The intensity of FDG uptake in the thyroid, thymus and muscles region was graded subjectively on a five-point scale.Among the 20 patients with Graves' disease, only six (30%) showed thyroid uptake of FDG, 15 (75%) showed thymus uptake of FDG, and up to 19 (95%) showed symmetrical increased uptake of FDG in skeletal muscles. In particular, the skeletal muscle uptake of FDG in the psoas and rectus abdominis muscles was 19/20 (95%) and 12/20 (60%), respectively. In the control group, three of the 15 examinees showed thyroid uptake of FDG. In the normal group, four of the 32 examinees had faintly visualized thyroid uptake of FDG. In thymus and skeletal muscles, there was no substantially increased FDG uptake in control and normal group examinees.In patients with Graves' disease, the thyroid uptake of FDG is not uniformly increased. Symmetrically increased uptake of FDG in the skeletal muscles and thymus is a clue for the diagnosis of Graves' disease. The uptake of FDG in skeletal muscles is more specific in the psoas and rectus abdominis muscles. An increment of muscle FDG uptake may be responsible for the high peripheral glucose utilization seen in Graves' disease.