BACKGROUND: The effects of subclinical thyroid dysfunction on cardiac outcome are not well defined. METHODS: To assess the relationship between mild thyroid dysfunction and the incidence of death in cardiac patients, we evaluated 3121 cardiac patients. Cardiac and overall deaths were considered. Four groups were defined: euthyroidism, subclinical hypothyroidism (SCH), subclinical hyperthyroidism (SCT), and low triiodothyronine syndrome (low T3). RESULTS: After mean follow-up of 32 months, there were 65 and 140 cardiac and overall deaths (3.4% and 7.3%), respectively, in euthyroidism, 15 and 27 (7.2% and 13.0%) in SCH, 8 and 9 (8.2% and 9.2%) in SCT, and 59 and 119 (6.5% and 13.1%) in low T3. Survival rates for cardiac death were lower in SCH, SCT, and low T3 than in euthyroidism (log-rank test; chi2 = 19.46; P < .001). Survival rates for overall death were lower in SCH and low T3 than in euthyroidism (log-rank test; chi2 = 26.67; P < .001). After adjustment for several risk factors, hazard ratios (HRs) for cardiac death were higher in SCH (HR, 2.40; 95% confidence interval [CI], 1.36-4.21; P = .02), SCT (HR, 2.32; 95% CI, 1.11-4.85; P = .02), and low T(3) (HR, 1.63; 95% CI, 1.14-2.33; P = .007) than in euthyroidism; HRs for overall death were higher in SCH (HR, 2.01; 95% CI, 1.33-3.04; P < .001) and low T3 (HR, 1.57; 95% CI, 1.22-2.01; P < .001) but not in SCT. CONCLUSION: A mildly altered thyroid status is associated with an increased risk of mortality in patients with cardiac disease.