BACKGROUND: Some studies have proposed that subclinical hypothyroidism (SCH) has 
adverse effects on the cardiovascular system, but little is known about the 
effect on patients undergoing cardiovascular operations. We examined the 
influence of preoperative SCH on postoperative outcome in patients undergoing 
coronary artery bypass grafting (CABG).
METHODS: Among patients who underwent CABG between July 2005 and June 2007 at 
Seoul National University Bundang Hospital, 224 with normal thyroid function and 
36 with SCH were enrolled. Preoperative risks and postoperative outcomes were 
evaluated prospectively without thyroid hormone replacement.
RESULTS: There were no significant differences in primary outcomes (major 
adverse cardiovascular events) and secondary outcomes such as wound problems, 
mediastinitis, leg infection, respiratory complications, delirium, or 
reoperation during the same hospitalization. However, patients with SCH had a 
higher incidence of postoperative atrial fibrillation than those with normal 
thyroid function after adjustment for age, gender, body mass index, and other 
independent variables such as emergency operation, the use of cardiopulmonary 
bypass, combined valvular operation, preoperative creatinine levels, left 
ventricular systolic dysfunction, and nonuse of beta-blockers (45.5% vs 29%; 
odds ratio, 2.552; 95% confidence interval, 1.117 to 5.830; p = 0.026).
CONCLUSIONS: SCH appears to influence the postoperative outcome for patients by 
increasing the development of postoperative atrial fibrillation. However, it is 
still unproven whether preoperative thyroxine replacement therapy for patients 
with SCH might prevent postoperative atrial fibrillation after CABG.