According to WHO studies, approximately 170 million individuals of the world population are diagnosed to be infected with Hepatitis C virus. Unfortunately around 20 % of these patients experience progressive liver disease leading to cirrhosis or hepatocellular carcinoma over 20 to 40 years. Interferon-alpha and ribavirin combination therapy is the standard treatment for chronic hepatitis C. These drugs may trigger production of different types of non-specific and specific autoantibodies. Thyroid dysfunction, which has been reported to occur from 3.9-33.33%, is the most common autoimmune disorder associated with combination therapy. Therefore, it is necessary that thyroid functions are monitored during therapy and the course of thyroid disease is watched for once autoimmune thyroid disorder develops. This book highlights the effects of Interferon and ribavirin on thyroid functions, its pathogenesis, clinical features and outcome through a cohort study carried out on chronic hepatitis C patients.