Atrioventricular and sinoatrial block in thyrotoxic crisis.

A 55 year old woman in thyrotoxic crisis developed atrial fibrillation, atrioventricular block, and sinoatrial block in rapid succession. All of these abnormalities resolved completely after antithyroid treatment. This course of events illustrates the profound effect of thyroid hormones on cardiac function. In view of the potential aggravation of atrioventricular conduction disturbance by beta adrenergic blocking agents, thyrotoxic patients should be carefully screened for electrocardiographic evidence of conduction disturbance before the administration of such drugs.