RATIONALE: Heart failure is relatively common in patients with hyperthyroidism, but thyrotoxic cardiomyopathy with poor left ventricular (LV) systolic function is very rare. PATIENT CONCERNS: We experienced a representative case of a patient who presented with severe LV dysfunction related to thyroid storm and needed extracorporeal membrane oxygenation (ECMO) temporally. DIAGNOSIS: Thyrotoxic cardiomyopathy. INTERVENTIONS AND OUTCOMES: Aggressive antithyroid therapy, including steroid pulse to hyperthyroidism, leads to the dramatic improvement of cardiac function and she was successfully weaned from ECMO. LESSONS: The most outstanding feature of the current case was the rapid decrease of cardiac injury and improvement of cardiac function by strengthening antithyroid therapy, including steroid pulse, without thyroid hormone level normalization. In thyroid storm, various systemic inflammatory reactions have different time courses and among them, the cardiac phenotype emerges in most striking and critical ways.