Graves' disease is characterized by the presence of circulating autoantibodies that stimulate the TSH receptor, inducing hyperthyroidism and goiter. Hashimoto's thyroiditis is an autoimmune disease leading to thyroid tissue destruction by cell and antibody-mediated immune processes. The occurrence of Hashimoto's thyroiditis following Graves' disease has been rarely reported. Its pathogenesis is not clear. Herein, we report the case of a 40-year-old woman who was referred to our department for thyrotoxicosis. Laboratory tests revealed overt hyperthyroidism. Thyroid scintigraphy showed an enlarged gland with diffusely increased tracer uptake, confirming the diagnosis of Graves's disease. The patient was treated with propranolol and thiamazole. Two months later, she received radioactive iodine therapy. Three years and 9 months later, the patient presented with hypothyroidism and very high levels of thyroperoxidase antibodies consistent with the diagnosis of Hashimoto's thyroiditis. She was treated with levothyroxine. The shift from Graves' disease to Hashimoto's thyroiditis was reported in the literature. However, its pathogenesis has not been clearly elucidated.