A case of transient 2:1 atrioventricular block, resolved by thyroxine supplementation for subclinical hypothyroidism.

A 42-year-old man was admitted to our hospital with palpitation attacks. Holter ECG showed 2:1 atrioventricular block and bradycardia with the minimum heart rate of 44 beats/min. There was a possible indication of electrophysiological study and cardiac pacemaker implantation. Laboratory data on admission revealed elevated thyrotropin level, with normal thyroxine level. To rule out functional atrioventricular block, we tried 2 weeks of the thyroxine supplementation, and Holter ECG showed improved heart rate without any atrioventricular block or long pause. We experienced that subclinical hypothyroidism caused severe bradycardia and 2:1 atrioventricular block, and that thyroxine supplementation completely improved these conditions.