Background: Atrial fibrillation occurs in 515% of hyperthyroid patients. On the other hand, amiodarone is a highly effective antiarrhythmic agent. Despite antiarrhythmic treatment, iodine-induced hyperthyroidism via amiodarone may be associated with recurrence of initial arrhythmias.
Aims: To assess if amiodarone maintained its antiarrhythmic efficacy in the presence of amiodarone-induced hyperthyroidism (AIT).
Patients and methods: 40 patients with AIT, 19 M/21F, aged 27-78 years, 15 from iodine deficient areas, initially received amiodarone for prevention of atrial fibrillation (n=26), of ventricular tachycardia or ventricular premature beats (n=14). Control group consists of 46 patients with common hyperthyroidism, matched for age and gender with study group. TSH, total T3, total T4, free T4 were measured by microenzymatic immunoassay. Resting 12-lead surface ECG assessed cardiac rhythm. Local Ethical Committee approval has been obtained for this study.
Results: Despite significantly lower T3 levels in AIT (225.8±33.2 ng/dl) versus common hyperthyroidism (334.7±25.7 ng/dl, P=0.01), recurrence of paroxysmal atrial fibrillation occurred in 13 out 20 patients (65%), significantly more frequent (P< 0.001) than in common hyperthyroidism, where the relevant rates were 13 out 46 patients (28.3%). Chronic atrial fibrillation was already present at diagnosis of AIT in 6 patients. Ventricular arrhythmias recurrence occurred in 2 out 14 patients (14.3%) in study group and in 1 out 46 patients in control group (2.1%), P<0.001; however, in AIT recurrence of ventricular arrhythmias is significantly rare than of atrial fibrillation (P<0.001).
Conclusion: Amiodarone antiarrhythmic efficacy is surpassed in AIT by the increased arrhythmic susceptibility of both atrial and ventricular myocardial tissue; atria seem to be more sensitive than ventriculi to minimally increased thyroid hormones levels.