Abstract. Currently, the use of thyroid hormones in the setting of heart failure (HF) is still an "open book". There are several unanswered questions: the regimen, doses and schedule of drug use, as well as the consequences of such therapy. Large clinical studies can provide information on the effect of these hormones on the long-term prognosis in patients with heart failure. At the same time, the presence of comorbid thyroid pathology, which requires the prescription of levothyroxine (LT), makes it possible to partially answer these questions. The aim is to study of the dose-dependent effect of LT on the course of HF in patients with autoimmune thyroiditis (AIT). Material and methods. The study included 218 patients with HF on the background of post-infarction cardiosclerosis. 109 (50.0 %) patients with AIT received LT due to hypothyreosis in the past. These patients intake LT during 2 years before included in the study and have euthyreosis. Whether the levels of thyroid stimulating hormone (TSH), free triiodthyronine (FT3) and free thyroxine (FT4) were determined. Results. Patients who used LT, comparing with patients without this drug, had smaller end-diastolic diametr (EDD) and end-systolic diametr (ESD) and end-diastolic volum (EDV) and end-systolic volum (ESV) of left ventricle (LV) and 22.9 % greater LV ejection fraction (EF) (+ 22.9 %, p = 0.0001), as well as higher low serum N-terminal prohormone of brain natriuretic peptide (NT-proBNP) level (- 26.3 %, p = 0.009). In the subgroup of patients taking LT at a dose of 0.1 to 0.69 μg/kg, ejection fraction of left ventricle (LVEF) did not differ from patients without this tritment. At a dose of 0.7-1.19 μg/kg, LVEF is higher compared with that of patients who did not take LT (+ 37.9 %, p = 0.0001) and patients who took LT at a dose 0.1 - 0.33 μg/kg (+ 36.9 %, p = 0.0001). LVEF was the highest in patients taking LT at a dose of > 1.20 μg/kg. The use of LT for 2 years reduces the risk of re-hospitalization (RH) due to decompensation of heart failure (Odds ratio = 0.490 (0.281-0.857), p = 0.018) and a tendentious decrease in the risk of combined endpiont achieving (- 27.9 %, p = 0.074). The ROC analysis showed that the risk of RH in patients with heart failure due to decompensation of the disease decreases with the use of LT at a dose of > 0.53 μg/kg (sensitivity – 56.62 %, specificity – 60.98 %, p = 0.016). Conclusions. The use of LT in patients has a dose-dependent positive effect on LVEF. The largest LVEF is observed in patients taking the drug at a dose of > 1.2 μg/kg. The use of an LT dose of > 0.53 μg/kg leads to a significant decrease in the frequency of re-hospitalization due to decompensation of heart failure during 2 years. Keywords: heart failure, autoimmune thyroiditis, levothyroxine, left ventricular, ejection fraction.