Her ECG reveals atrial fibrillation with a ventricular response of 160 beats/min. The radial pulse rate of 115 beaats/min detected on physical examination reflects the pulse deficit that is common when the ventricular response to atrial fibrillation is rapid. Her TSH level was very low (0.1) and her T4 level was high, greater than 10mg/dl, indicating hyperthyroidism, which, in this case, was iatrogenic and the cause of the atrial fibrillation. Following the administration of a beta-adrenergic blocking agent, the ventricular rate slowed and the rhythm spontaneously reverted to sinus. The dose of thyroxin was reduced and the rhythm has since remained sinus.