Exit block in myxedema, treated effectively by thyroid hormone therapy.

A case of a 71-year-old female who developed exit block one year after implantation of a pervenous pacemaker is reported. The pacemaker had been implanted for sick sinus syndrome; the exit block was probably due to the development of severe myxedema which caused high pacing threshold. Normal pacing threshold was obtained on the seventh day of adequate thyroid hormone therapy and a follow-up one year later revealed effective control of the myxedema and no abnormalities in the function of the pacemaker system.