['Although studies analyzing the effect of thyroid supplementation on postoperative morbidity and mortality from cardiac surgery have been inconclusive, they suggest a role in the prevention of postoperative atrial fibrillation. To further explore this relationship we conducted a retrospective study to determine whether abnormalities in routine preoperative thyroid function studies correlate with the incidence of postoperative atrial fibrillation.', 'From May 2004 until July 2011, 821 patients with complete thyroid function testing performed preoperatively underwent cardiac surgery. Preoperative, intraoperative, and postoperative laboratory, clinical and hemodynamic data including postoperative electrocardiogram monitoring were retrospectively evaluated.', 'Mean age was 65.7 years and 36% (294) of patients were female. Mean preoperative ejection fraction was 48.6% and 18% (100) had clinical heart failure. Ninety percent (682) of patients were euthyroid and 10% (77) were hypothyroid. Atrial fibrillation occurred significantly more frequently in hypothyroid patients (33.4% vs. 22.5%; p =\u2009.033). In multivariable analysis, increasing thyroid stimulating hormone (TSH) level (OR: 1.11; CI: 1.01 to 1.22; p =\u2009.030) was an independent predictor of postoperative atrial fibrillation. Beta blocker use within 24 hours prior to operation was protective (OR: .54; CI: .35 to .83; p =\u2009.005). Length of stay was significantly longer in patients with postoperative atrial fibrillation (9.1 vs. 6.5 days; p <\u2009.001).'] Is preoperative hypothyroidism a risk factor for postoperative atrial fibrillation in cardiac surgical patients?