Cardiovascular manifestations of iatrogenic hyperthyroidism in two dogs.

Two dogs were diagnosed with iatrogenic thyrotoxicosis (1 definitive, 1 presumptive). Both showed physical examination findings of agitation, tachypnea, and tachycardia. Sinus tachycardia with supraventricular ectopy was diagnosed in one case, and syncope and atrial flutter was present in the other. Both dogs had concurrent cardiac disease that might have contributed to the severity of their clinical signs. Excessive thyroid hormone supplementation in humans causes supraventricular arrhythmias including sinus tachycardia, supraventricular tachycardia, atrial fibrillation, and atrial flutter. Clinical signs and rhythm abnormalities resolved in both dogs with resolution of the thyrotoxicosis.