Evidence has been presented to support the idea that iodine plays an important role in autoimmune thyroiditis. Excessive amounts induce thyroiditis in genetically susceptible animal strains, while intrathyroidal depletion of iodine prevents disease in strains susceptible to severe thyroiditis. While the mechanisms by which iodine promotes thyroiditis is unknown, several hypotheses have been proposed. (1) T and/or B cells may react specifically to iodinated portions of thyroglobulin (Tg) so that severe iodine depletion renders Tg non-immunogenic. (2) A defect in the iodine processing machinery in thyroid epithelial cells of a susceptible person or animal may, in the presence of iodine, result in elevated levels of oxygen or iodine radicals, which could damage membrane lipids or proteins. (3) Defective iodine processing may result in the iodination of lipid or proteins (other than Tg) which could act either as immunogens or polyclonal activators.