A 39-year-old Chinese man presented to the study hospital with right-sided gynecomastia. Underlying Graves' disease was not diagnosed until recurrent episodes of hypokalemic periodic paralysis were observed. The estradiol (E2) and progesterone levels and the E2-to-testosterone (T) (E2/T) ratio of the patient were elevated before treatment. Immediate intravenous potassium supplementation was started to reverse the paralysis. Additionally, antithyroid drugs were administered to restore a euthyroid state. After treatment, the patient gained strength. Gynecomastia regressed with a return to the euthyroid state; the E2 and progesterone levels normalized and the plasma E2/T ratio declined. In addition to the classic symptoms, some atypical symptoms of Graves' disease may also occur. One of the challenges lies in recognizing the underlying etiology. Early diagnosis and appropriate treatment can avoid unnecessary investigations and serious cardiopulmonary complications.