Cardiac arrhythmias and heart rate in hyperthyroidism.

The arrhythmia profile and heart rate (HR) were analyzed by 24-hour Holter monitoring in 37 hyperthyroid patients before (triiodothyronine [T3] hormone level = 331 +/- 108 ng/dl), during (T3 level = 202 +/- 98 ng/dl) and after an antihyperthyroid therapy of 8 to 89 weeks' duration (T3 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 greater than 0.05). Three patients (8%) had repetitive ventricular arrhythmias (Lown grade 4A/B) as compared with 4 normal subjects (8%, p greater than 0.05). Supraventricular premature complexes occurred more often in hyperthyroid patients than in normal subjects before and after therapy (p less than 0.001). The prevalence of supraventricular tachycardia decreased from 8 patients to 1 during therapy (p less than 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 less than 0.001). A day/night difference in HR greater than 10% was found in 32 patients (86%) and was more pronounced than in the normal group (p less than 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 T3 level correlated best with HR at night in hyperthyroid patients (r = 0.74, p less than 0.001). Thus, hyperthyroid patients show frequent supraventricular arrhythmias that might be reversible during therapy.(ABSTRACT TRUNCATED AT 250 WORDS)