Cardiac electrophysiology is affected by thyroid hormones both directly (affecting cardiac cell membrane potentials and calcium reuptake) and indirectly (upregulating gap junction protein and β-1 adrenergic receptor expression). As a result, we know that patients diagnosed with hyperthyroidism have a 3-fold increased risk of atrial fibrillation. Recent data from the Cardiovascular Health Study revealed a 2-fold increased risk of atrial fibrillation among patients with subclinical hyperthyroidism and T4 concentrations in the upper quartile of the normal range. This association also was observed in the Thyroid Studies Collaboration. The question of whether to treat patients with subclinical hyperthyroidism has not been addressed definitively, although one small observational study did demonstrate a decrease in heart rate, premature atrial contraction, and premature ventricular contraction burden and left ventricular mass on serial transthoracic echocardiography in patients treated with methimazole for subclinical hyperthyroidism.These data are relevant—though not immediately translatable—to the study of postoperative atrial fibrillation, which affects cardiothoracic surgery patients at disproportionately high rates. Efforts to decrease this complication have been the focus of innumerable studies, with only a few identified interventions successful at mitigating atrial fibrillation. Preoperative testing for and treatment of subclinical hyperthyroidism has not been applied to routine preoperative optimization, and this remains a potential modifiable risk factor for preventing perioperative atrial fibrillation.