The thyroid hormone system may be downregulated temporarily in patients who are severely ill. This “euthyroid sick syndrome” may be an adaptive response to conserve energy. However, thyroid hormone also has beneficial effects on the cardiovascular system, such as improving cardiac function, reducing systemic vascular resistance, and lowering serum cholesterol levels. We investigated whether thyroid hormone levels obtained at the time of myocardial infarction are associated with subsequent mortality.

Serum levels of thyroid hormones (triiodothyronine [T₃], reverse T₃, free thyroxine [T₄], and thyroid-stimulating hormone) were measured in 331 consecutive patients with acute myocardial infarction (mean age [± SD], 68 ± 12 years), from samples obtained at the time of admission.

Fifty-three patients (16%) died within 1 year. Ten percent (16 of 165) of patients with reverse T₃ levels (an inactive metabolite) >0.41 nmol/L (the median value) died within the first week after myocardial infarction, compared with none of the 166 patients with lower levels (P <0.0004). After 1 year, the corresponding figures were 24% (40 of 165) versus 7.8% (13 of 166; P <0.0001). Reverse T₃ levels >0.41 nmol/L were associated with an increased risk of 1-year mortality (hazard ratio = 3.0; 95% confidence interval: 1.4 to 6.3; P = 0.005), independent of age, previous myocardial infarction, prior angina, heart failure, serum creatinine level, and peak serum creatine kinase-MB fraction levels.

Determination of reverse T₃ levels may be a valuable and simple aid to improve identification of patients with myocardial infarction who are at high risk of subsequent mortality.