Holter ECG recordings before and after surgical treatment of hyperthyroidism: circadian pattern of heart rate and arrhythmias.

Holter ECG recordings obtained form 11 hyperthyroid patients (mean age 29.2 +/- 11.4 years) before and after surgical treatment were analyzed regarding heart rate (HR) and arrhythmias. The mean 24-hour HR significantly decreased form 104 +/- 8.8 bpm before to 76.8 +/- 8.9 bpm after the treatment (p less than 0.001), with the most pronounced reduction while asleep form 0:00 AM to 6:00 AM. HR variability also significantly decreased (p less than 0.02), but the circadian rhythm of HR did not change before and after the treatment. Serum T3 level correlated with the mean HR (r = 0.63, p less than 0.05) in hyperthyroid state. The percent changes in T3 (% delta T3) best correlated with the percent changes in the mean HR (% delta mean HR) (r = 0.84, p less than 0.01). The frequencies of supraventricular and ventricular premature contractions significantly decreased after the surgical treatment (p less than 0.01). Normal circadian pattern of hourly HR before the treatment highly suggests that normal adrenergic responsiveness is maintained in hyperthyroidism.