Autoimmune thyroid disease, including Hashimoto’s thyroiditis and Graves’ disease, represents the most prevalent autoimmune condition (18), and CTLA-4 blockade has joined other systemic immunomodulators [e.g., interferon-α, -β, interleukin (IL)-2] and leukocyte-target agents (e.g., anti-CD52) in triggering thyroid dysfunction (1, 3, 19, 20). This evidence concerns the gene CTLA4 and Hashimoto thyroiditis.