Recombinant human interferon alpha (r-IFN-alpha) is often successfully used in the treatment of patients with chronic viral hepatitis. The interferons are a groups of proteins with antiviral, anti-proliferative and immunomodulatory effects. Because of their broad activity, interferons have many side effects. Several investigators have reported the development of autoimmune diseases particularly thyroid disease during interferon therapy. The patient, 39 years old woman was diagnosed of chronic hepatitis C and began to take interferon treatment. On the 6th month of treatment, she was admitted to our clinic because of weakness, lack of appetite palpitation and weight loss for two weeks. The diagnosed of subacute thyroiditis was made by physical and laboratory findings. Interferon therapy was stopped and beta blocker was given. Signs were disappeared within 7 weeks and thyroid hormones became normal. One should keep in mind that the side effects of interferon therapy like weakness, losing weight, palpitation could be the sign and symptoms of a developed hyperthyroidism as well.