AIM: To study predictive factors of thyroid dysfunction associated with interferon-alpha (IFNα) therapy in chronic hepatitis C (CHC) and to describe its long-term evolution in a large population without previous thyroid dysfunction.METHODS: We performed a follow-up of thyroid function and detection of thyroid antibodies in 301patients treated for CHC with IFNα from 1999 to 2004.RESULTS: Thyroid disorder developed in 30/301 (10%)patients with a mean delay of 6 ± 3.75 mo: 13 patients had hyperthyroidism, 11 had hypothyroidism, and 6had biphasic evolution. During a mean follow-up of 41.59 ± 15.39 mo, 9 patients with hyperthyroidism,3 with hypothyroidism, and 4 with biphasic evolution normalized thyroid function in 7.88 ± 5.46 mo. Recovery rate of dysthyroidism was not modified by treatment discontinuation, but was better for patients with negative thyroid antibodies before antiviral treatment ( P = 0.02). Women had significantly more dysthyroidism ( P= 0.05). Positive thyroid peroxidase and thyroglobulin antibodies were more frequent before antiviral treatment in patients who developed dysthyroidism ( P < 0.0003 and P = 0.0003, respectively). In a multivariate model, low fibrosis was found to be a predictive factor of dysthyroidism ( P = 0.039).CONCLUSION: In this monocentric population of CHC,dysthyroidism, especially hyperthyroidism, developed in 10% of patients. Low fibrosis was found to be apredictive factor of dysthyroidism. Thyroid disorder recovered in 16/30 patients (53%) and recovery was better in the non-autoimmune form.