Long-term ECG recordings in thyroxine-substituted hypothyroid subjects

. Bradyarrhythmias are sometimes observed in hypothyroid patients. This phenomenon could either be due to the disease process with structural changes of the heart or to an abnormal hormone level. The purpose of the present paper was to study the prevalence of bradyarrhythimas in hypothyroid patients on long-term substitution therapy. Nineteen subjects were studied 9–53 months after they had reached the final thyroxine substitution level. Ambulatory ECG registrations were performed during 48 hours. Ventricular ectopic beats were observed in 10 of the 19 patients, but no bradyarrhythmias were detected. The results support the notion that arrhythmias found in hypothyroid patients are at least mainly due to the low thyroid hormone level.