PTPN22 and Hashimoto thyroiditis: The predominant cause of hypothyroidism in the United States is an autoimmune disorder called—Hashimoto thyroiditis—where several candidate-gene analysis and linkage studies suggest that loci contributing to the pathogenesis of hypothyroidism include CTLA4 (Cytotoxic T-Lymphocyte Antigen 4), PTPN22 (Protein tyrosine phosphatase, non-receptor type 22) and TG (thyroglobulin) genes[20-22].