Periodic paralysis is a rare complication of hyperthyroidism. Patients of East Asian descent are most commonly affected. Presentation is characterized by recurrent episodes of painless, abrupt-onset weakness, with laboratory evaluation characterized by profound hypokalemia. Underlying hyperthyroidism may not be clinically evident, but differentiation from the familial variant is critical due to differing treatment pathways. ⋯ We describe the presentation of a 22-year-old man with recurrent relapsing-remitting weakness with undiagnosed hyperthyroidism. WHY SHOULD AN EMERGENCY PHYSICIAN BE AWARE OF THIS?: In patients with acute-onset paralysis with significant hypokalemia, or relapsing-remitting symptoms, hyperthyroidism should be suspected. Obese patients are at an especially increased risk due to underlying insulin resistance, which enhances basal sodium-potassium ATPase function. Hypokalemia is functional in nature. Nonselective β-blockers (such as propranolol) should be considered first line, as they simultaneously decrease ATPase activity, limit insulin secretion, and address the underlying disorder. Administration of > 50 mEq of exogenous potassium places patients at risk of dysrhythmias from rebound hyperkalemia.