Supra-His complete atrioventricular block in a patient with subclinical hyperthyroidism

Subclinical hyperthyroidism is a difficult entity to diagnose because of silent clinical features and it may be easily underdiagnosed unless it is suspected and thyroid hormone levels are examined. Although atrioventricular (AV) conduction abnormalities such as complete heart block may occasionally be seen in hyperthyroidism, its association with subclinical hyperthyroidism has not been reported previously. We report on a 50-year-old female patient who did not have any systemic or cardiovascular disease or history of drug use that could affect AV conduction and presented with presyncope and complete heart block with narrow QRS complexes. Thyroid function tests showed subclinical hyperthyroidism and an electrophysiological study showed the supra-His level as the site of complete AV block. After initiation of antithyroid treatment (propylthiouracil), the patient’s rhythm improved to second-degree AV block on the third day and returned to normal sinus rhythm on the fourth day.