Interferon-alpha (IFN␣) is a major treatment modality for several malignant and nonmalignant diseases, especially hepatitis C. Prospective studies have shown that up to 15% of patients with hepatitis C receiving IFN␣ develop clinical thyroid disease, and up to 40% were reported to develop thyroid antibodies. Some of these complications may result in discontinuation of interferon therapy. Thus, interferon induced thyroiditis (IIT) is a major clinical problem for patients receiving interferon therapy. IIT can be classified as autoimmune type and non-autoimmune type. I nterferons were discovered in the 1950Јs and were immediately recognized for their ability to render cells resistant to virus infection. 1 Interferons have been classified into two major groups, type I interferons and type II interferons, based on their capacity to bind to common receptor types. Type I interferons bind to a type I interferon receptor and include interferon-␣, interferon-␤, interferon-, and interferon-. 2 Type II interferons bind to a distinct type II receptor and include interferon-␥. 2 In addition to their antiviral properties, interferons have immunomodulatory, antiangiogenic, antiproliferative and antitumor activity, and act as important regulators of growth and differentiation. 3 Interferons work by directly binding to either type I or type II receptors. The cytoplasmic domains of the transmembrane glycoproteins are associated with members of the JAK family of kinases. They activate various signaling pathways, including the JAK-STAT pathway, the Crk-pathway, the IRS signaling pathway, and the MAP kinase pathway, which leads to signal transduction and subsequent activation or repression of specific genes. 3,4 More than twenty interferon-induced proteins have been identified. 5 Interferon-alpha (IFN␣) was the first cytokine to be reproduced by recombinant DNA technology and has emerged as a major therapeutic modality for several malignant and nonmalignant diseases. Among the diseases treated by IFN␣ are melanoma, renal cell carcinoma, hairy cell leukemia, Kaposi's sarcoma, and hepatitis B and C. 5 However, the most common prescription for IFN␣