Aim-To identify the predictive factors of dysthyroidismduring treatment for chronic viral hepatitis C and to evaluate the longterm outcome of these patients. Methods-Patients treated for chronic viral hepatitis C between 1990 and 2001 were analyzed retrospectively. Patients with dysthyroidism before treatment and patients positive for hepatitis B surface antigen or human immunodeficiency virus antibodies were excluded. Dysthyroidism was defined by an abnormal serum TSH level on two separate occasions. Results-221 consecutive patients were included. Among them, a hundred were treated twice by interferon alpha, 21 had 3 treatments and 3 had 4 treatments. Fifteen of these patients (7%) had dysthyroidism during antiviral therapy. There was no significant difference in the frequency of dysthyroidism during the first and the second treatment [respectively 4.1%(N= 9) and 6%(N= 6)]. Female gender and the presence of antimicrosome or antithyroperoxydase (anti-TPO) antibodies before antiviral treatment were predictive factors of dysthyroidism. Treatment by interferon and ribavirin did not increase the risk of dysthyroidism compared to monotherapy with interferon. Pegylated interferon (N = 49) was not a risk factor compared to standard interferon. Thirteen patients had hypothyroidism (2 of them as a result of biphasic thyroiditis) and 2 had hyperthyroidism. The antiviral treatment was continued in 11 patients. Seven out of 13 patients with hypothyroidism required an indefinite treatment (follow-up: 15 to 90 months). Conclusions-In our series, 7%of patients with chronic viral hepatitis C hada dysthyroidism during antiviral therapy. Predictive factors were female gender and positive antimicrosome or anti-TPO antibodies before treatment. Absence of dystdyroidism during a first antiviral treatment did not preclude from the risk of dysthyroidismduring a second treatment. Aim-To identify the predictive factors of dysthyroidism treatment for chronic viral hepatitis C and to evaluate the longterm outcome of these patients. Methods-Patients treated for chronic viral hepatitis C between 1990 and 2001 were analyzed retrospectively. Patients with dysthyroidism before treatment and patients positive for hepatitis B surface antigen or human immunodeficiency virus antibodies were excluded. Dysthyroidism was defined by an abnormal serum TSH level on two separate occasions. Results-221 consecutive patients were included. Among them, a hundred were treated twice by interferon alpha, 21 had 3 There was no significant difference in the frequency of dysthyroidism during the first and the second treatment [respectively 4.1% (N = 9) and 6% (N = 6)]. Female gender and the presence of antimicrosome or antithyroperoxydase (anti-TPO) antibodies before antiviral tr eatment were predictive factors of dysthyroidism. Treatment by interferon and ribavirin did not increase the risk of dysthyroidism compared to monotherapy with interferon. Pegylated interferon (N = 49) was not a risk factor compared to standard interferon. Thirteen patients had hypothyroidism (2 of them as an outcome of biphasic thyroiditis) and 2 had hyperthyroidism. The antiviral treatment was continued in 11 patients. Seven out of 13 patients with hypothyroidism required an indefinite treatment (follow-up: 15 to 90 months). Conclusions-In our series, 7 % of patients with chronic viral hepatitis C hada dysthyroidism during antiviral therapy. Predictive factors were female gender and positive antimicrosome or anti-TPO antibodies before treatment. Absence of dystdyroidism during a first antiviral treatment did not preclude from the risk of dysthyroidism of a second treatment.