Background The function of deiodinases – selenoproteins converting thyroid hormones may be disturbed by oxidative stress accompanying heart failure. Selenium (Se) may be used by glutathione peroxidase, leading to a lack of deiodinase and triiodothyronine (T3). The aim of the study was the evaluation of the prevalence and clinical significance of low T3 syndrome in heart failure and the assessment of the association of low fT3 and Se deficiency. Methods The study group consisted of 59 consecutive patients hospitalized due to decompensated HFrEF NYHA III or IV. Exclusion criteria were: thyroid dysfunction, severe systemic disease, treatment with amiodarone, steroids or propranolol. Group A included 9 patients with low free T3 (fT3) concentration below 3.1 pmol/L. Group B consisted of the remaining 50 patients with normal fT3 levels. Results The prevalence of low T3 syndrome was 15.3%. The prevalence of Se deficiency was 74.6%. We demonstrated correlations between fT3 and main clinical variables (i.e. NT-proBNP, LVEF, hsCRP), but we did not find correlation between fT3 and the Se level. Kaplan-Meier survival analysis showed lower survival probability in patients with low fT3 ( p < 0.001). Conclusions Low T3 syndrome is frequently found in patients with HFrEF and is associated with a poor outcome. We did not identify any significant correlation between Se and fT3 level.

may benefit from Se supplementation. Dietary Se intake varies worldwide from 7 μg to 4990 μg per day. The mean value in Europe is 40 μg per day and in the United States, 93 μg per day (in women) to 134 μg per day (in men). Nutritional products that are rich in Se are Brazil nuts, offal, seafood, cereals, and grains. [1][2][3][4] Oxidative stress caused by the insufficient antioxidant dietary intake and improper balance INTRODUCTION Selenium (Se) is a micronutrient important for human health. It is incorporated as selenocysteine at the active site of 25 selenoproteins, for example, glutathione peroxidase (GPX), iodothyronine deiodinases, selenoprotein P, thioredoxin reductase, selenoprotein S, and other. Due to the U -shaped dose -response relationship, toxic concentrations of Se may be harmful, which is why only people with low Se levels