Hyperthyroidism is a frequently misdiagnosed cause of atrial fibrillation. Difficulties associated with diagnosis are related to physiologic idiosyncrasies, concomitant illnesses, and medication regimens that may alter thyroid function and thyroid function tests. These factors are described. Nurses have a key role in assisting patients to manage hyperthyroidism and atrial fibrillation through case finding, assessment, education, and psychological support.