In a 51-year-old woman with bilateral lower extremity weakness of more than a year's duration who had previously undergone surgery and chemotherapy for endometrial cancer, [18F]FDG PET/CT showed diffusely increased [18F]FDG uptake in whole-body skeletal muscle with an SUVmax of 5.7. Both thyroid lobes were enlarged with decreased density and diffusely increased [18F]FDG uptake (SUVmax 3.1), with no evidence of tumor recurrence or metastasis. Laboratory tests confirmed elevated levels of thyroid hormones and TSH <0.01 mIU/l. Thyroid ultrasound revealed diffuse enlargement with heterogeneous echotexture and increased vascularity. Electromyography indicated damage to peripheral nerves with suspicion of myogenic involvement. The clinical diagnosis was hyperthyroid myopathy, which initially presented as myasthenia gravis. After treatment with methimazole, leukogen tablets and propranolol, the patient recovered. Hyperthyroid myopathy should be considered in muscle weakness with diffuse muscle hypermetabolism and abnormal thyroid metabolism on [18F]FDG PET/CT.