Cardiac arrhythmias and heart rate in hyperthyroidism

Abstract

The arrhythmia profile and heart rate (HR) were analyzed by 24-hour Holter monitoring in 37 hyperthyroid patients before (triiodothyronine [T 3] hormone level = 331 ± 108 ng/dl), during (T 3 level = 202 ± 98 ng/dl) and after an antihyperthyroid therapy of 8 to 89 weeks' duration (T 3 level = 149 ± 41 ng/dl). The data were compared with those of 50 control subjects free from cardiac disease. Only 12 hyperthyroid patients (32%) had complex ventricular arrhythmias (Lown grade 3 or 4) as compared with 6 normal subjects (12%, p > 0.05). Three patients (8%) had repetitive ventricular arrhythmias (Lown grade 4A/B) as compared with 4 normal subjects (8%, p > 0.05). Supraventricular premature complexes occurred more often in hyperthyroid patients than in normal subjects before and after therapy (P < 0.001). The prevalence of supraventricular tachycardia decreased from 8 patients to 1 during therapy (p < 0.002). The HR decreased from 95 ± 13 to 79 ± 9 beats/min after therapy, but was still increased as compared with the normal subjects (72 ± 8 beats/min, p < 0.001). A day/night difference in HR > 10% was found in 32 patients (86%) and was more pronounced than in the normal group (p < 0.001). Compared with the normal HR profile, the HR curve of hyperthyroid patients was shifted to a higher level (about 20 beats/min). Serum T 3 level correlated best with HR at night in hyperthyroid patients (r = 0.74, p < 0.001). Thus, hyperthyroid patients show frequent supraventricular arrhythmias that might be reversible during therapy. However, ventricular arrhythmias are infrequent and remain essentially unchanged during therapy. The hyperthyroid state increases HR in general, but day/night differences in HR remain unchanged.