A 31-year-old Taiwanese lady presented with palpitations, anxiety, orthopnoea, peripheral oedema, diarrhoea and a swelling in her neck. She was taking no regular medication. Her paternal uncle had been treated for thyrotoxicosis. She smoked 20 cigarettes per day. Clinical examination revealed tachycardia (pulse rate between 115 and 150 beats/min), tachypnoea (respiratory rate 30 breaths/min), hypertension (BP 161/125 mmHg) and congestive cardiac failure. The thyroid gland was enlarged but examination of the eyes was normal. Investigations revealed normal renal function, hypoalbuminaemia (albumin 27 g/l) but otherwise normal liver function tests, undetectable TSH and significantly elevated fT4 55.7 pmol/l (NR 926). Atrial flutter with variable block was confirmed on electrocardiogram. The chest radiograph was consistent with significant pulmonary oedema. An echocardiogram showed dilated right and left ventricles with a left ventricular (LV) ejection fraction of 52%, (5575%), moderate to severe mitral regurgitation (MR) and moderate tricuspid regurgitation (TR). A diagnosis of thyroid storm with cardiomyopathy was made, and the patient was commenced on high dose propylthiouracil 250 mg every four hours, loop diuretic, fluid restriction, beta-blocker, ACE inhibitor and anti-coagulated with warfarin. Two weeks later she had reverted spontaneously to sinus rhythm and was no longer clinically in heart failure. She was discharged on carbimazole (initially 40 mg tds and titrated down to 5 mg od according to thyroid function tests) and ramipril 2.5 mg od. Two months later, she was not breathless on exertion and had the following results: TSH 0.06 mU/l, fT4 6.1 pmol/l, fT3 5.3 pmol/l. TSH receptor antibodies were positive, consistent with Graves disease. There was a dramatic improvement in her echocardiogram which now showed good right and left ventricular function with a LV ejection fraction of 68%, only mild MR and no TR. This case illustrates that thyrotoxic cardiomyopathy and arrhythmias may resolve rapidly with anti-thyroid medication.