Reentrant Atrioventricular Nodal Tachycardia Induced by Levothyroxine

S INUS tachycardia, atrial premature beats and, in some cases, atrial fibrillation are frequent complications of overt hyperthyroidism (1-3).Only recently these alterations of cardiac rhythm also have been reported in patients with subclinical hyperthyroidism.This condition may be the consequence of levothyroxine (L-T 4 ) treatment (exogenous hyperthyroidism) or may be of endogenous origin, usually because of autonomously functioning thyroid adenoma or multinodular goiter.Subclinical hyperthyroidism induced by exogenous administration of L-T 4 has been shown to increase the average 24-h heart rate and the number of atrial premature beats.Palpitations occur in some patients, and they may be alleviated by the administration of a ␤-adrenergic blocking drug (4, 5).Sawin et al. (6), in a large group of 2007 people 60 yr of age or older, documented a 3-fold higher risk of atrial fibrillation in the presence of low serum TSH concentrations caused by endogenous or exogenous subclinical thyrotoxicosis (6).Our report shows the possibility that thyroid hormones may also induce other kinds of supraventricular arrhythmias not yet described in hyperthyroid patients, such as reentrant atrioventricular (A-V) nodal tachycardia.