Hyperthyroidism Masquerading as Frequent Premature Ventricular Contractions with Bigeminy

Syncope is a transient loss of consciousness with a broad differential diagnosis, among which cardiac etiologies are particularly concerning due to their association with increased morbidity and mortality and their frequent lack of prodromal symptoms. Hyperthyroidism is well recognized for its cardiovascular effects, most commonly causing sinus tachycardia and supraventricular arrhythmias such as atrial fibrillation. Ventricular arrhythmias, however, are rare and infrequently reported as a presenting manifestation of thyrotoxicosis. We report the case of a 63-year-old woman who presented with sudden syncope without warning signs, raising concern for cardiac syncope. Initial electrocardiography revealed frequent premature ventricular contractions (PVCs), including interpolated PVCs mimicking ventricular bigeminy, supporting a presumed cardiogenic etiology. The patient was admitted for further cardiac evaluation. Comprehensive diagnostic workup ultimately revealed severe thyrotoxicosis, with markedly elevated free triiodothyronine and thyroxine levels, suppressed thyroid-stimulating hormone, and positive thyroid receptor antibodies, consistent with Graves’ disease. The ventricular ectopy and syncopal episode were attributed to untreated hyperthyroidism. Initiation of antithyroid therapy resulted in clinical stabilization, and no malignant arrhythmias were identified on subsequent monitoring. This case underscores the importance of considering thyroid dysfunction in patients presenting with syncope and ventricular ectopy, even when electrocardiographic findings suggest a primary cardiac cause. Early recognition and treatment of thyrotoxicosis are essential to prevent potentially life-threatening arrhythmias and other cardiovascular complications.