['Altered thyroid hormone metabolism characterized by a low triiodothyronine (T3), so-called low-T3 syndrome, is a common finding in patients with severe systemic diseases. Additionally, subclinical thyroid dysfunction, defined as abnormal thyroid stimulating hormone (TSH) and normal thyroxine (T4), causes left ventricular dysfunction. Our objective was to identify the prevalence and prognostic impact of low-T3 syndrome and subclinical thyroid dysfunction in patients with acute decompensated heart failure (ADHF).', 'We examined 274 ADHF patients who were not receiving thyroid medication or amiodarone on admission (70\u2009±\u200915\u2009years, 156 male), who underwent thyroid function tests. Euthyroidism was defined as TSH of 0.45 to 4.49\u2009mIU/L; subclinical hypothyroidism as TSH of 4.5 to 19.9\u2009mIU/L; and subclinical hyperthyroidism as TSH\u2009<\u20090.45\u2009mIU/L, with normal free T4 level for the last two. Additionally, low-T3 syndrome was defined as free T3\u2009<\u20094.0\u2009pmol/L among euthyroidism subjects. On admission, 188 patients (69%) showed euthyroidism, 58 (21%) subclinical hypothyroidism, 5 (2%) subclinical hyperthyroidism, and 95 (35%) low-T3 syndrome. Cox proportional hazards models revealed that higher TSH, but not free T3 and free T4, was independently associated with composite cardiovascular events, including cardiac death and re-hospitalization for heart failure. Indeed, subclinical hypothyroidism was an independent predictor (hazard ratio: 2.31; 95% confidence interval: 1.44 to 3.67; '] Is subclinical hypothyroidism an independent predictor of adverse cardiovascular outcomes in patients with acute decompensated heart failure?