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Different forms of interferon alpha (IFN-α) have been used for several years in the treatment of chronic viral hepatitis type C (CVHC). Currently, pegylated forms of interferon alpha (PegIFN-α) in combination with ribavirin is the standard treatment. During therapy with IFN-α, side-effects occur, including thyroid diseases. The aim of this study was an evaluation of administered interferon's impact on the frequency of autoimmune thyroiditis (ATI) occurrence among patients with CVH type C treated with INF-α and an assessment as to whether the type of interferon used is significant in ATI development.149 patients aged 18-70 (mean 43.9 ± 2.3 years) with CVH type C participated in the study. The serum concentrations of thyrotrophin (TSH), free tyrosine (FT4), triiodothyronine (FT3), thyreoglobulin (Tg), antithyroid antibodies: antiperoxidase (TPOAb) and antithyreoglobulin (TgAb) were evaluated before, and after six and 12 months of treatment. Additionally, the thyroid echostructure was evaluated with ultrasonography. Sixty out of 149 patients received Peg-INF-α, and 89 patients were treated with recombinant IFN-α.ATI was confirmed in nine patients (6.04%) with CVH type C before the introduction of interferon. Seven of them underwent an exacerbation of hypothyroidsm during therapy with interferon. In 24 patients (17.14%), who did not have the signs of ATI at baseline, an elevated concentration of antithyroid antibodies was detected during therapy with interferon. The mean concentrations of TPOAb before, and after six and 12 months of treatment were, respectively: 12.4; 310.4 and 141.3 IU/ml, and the mean concentrations of TgAb were, respectively: 17.40; 108.0; and 125.6 IU/ml. After six months of treatment in this group of patients, 11 had hypothyroidsm and six had hyperthyroidsm. After 12 months of therapy, four patients had hypertthyroidsm and four showed signs and symptoms of hypothyroidsm; the remaining patients were in a euthyroid state. In ultrasound examination, reduction of echogenicity among patients with ATI before treatment was revealed in 75% of cases at baseline, in 83.3% after six months and in 100% after 12 months of treatment. In the group of patients presenting with ATI during IFN-α therapy, in which no disorders were found in initial examination, after six months of treatment a reduction of echogenicity was found in 69.2%, and after 12 months in 75%, of patients.Among patients treated with interferon due to CVH type C, there is a risk of the development of ATI or the exacerbation of an existing one. There is no significant difference in ATI presentation in relation to the type of IFN-α used for treatment.