Experiments were performed to analyze one mechanism by which elevated levels of dietary iodine may induce thyroglobulin (Tg) autoantibodies. We tested the hypothesis that highly iodinated Tg synthesized by animals fed a high iodine diet is significantly more immunogenic than Tg containing fewer iodine atoms. Cornell strain (CS) chickens, genetically susceptible to iodide-induced thyroiditis, were fed either a high or a low iodine diet. They were killed, and their thyroidal Tg was analyzed for iodine; the high iodine Tg (HI-Tg) had at least 60 and the low iodine Tg (LI-Tg) had less than 13 atoms/molecule of Tg. To determine if the degree of Tg iodination affected its immunogenicity, these Tg preparations were administered iv to normal chickens without adjuvants. Their sera were tested for antibodies by direct binding radioassays and RIAs. HI-Tg stimulated the synthesis of antibodies that reacted well with HI-Tg and the thyroid hormones T3 and T4, but only weakly with LI-Tg. The birds immunized with LI-Tg produced very little antibody to LI-Tg, T3, or T4, but a modest amount to HI-Tg. In other experiments, Tg autoantibodies found in chickens maintained on a high iodine diet similarly demonstrated enhanced binding to HI-Tg. The present studies show that HI-Tg is more immunogenic than LI-Tg and supports the hypothesis that a high iodine diet induces Tg autoantibodies by increasing the immunogenicity of the Tg molecule. In marked contrast with iodide-induced Tg antibodies, the Tg antibodies accompanying the severe and early-onset thyroiditis of obese strain chickens are to a large degree independent of dietary iodine intake.

Dietary iodine has been shown to be important in the induction of thyroiditis in susceptible chicken strains although the underlying mechanism remains unknown. Iodine may exert its effects through the formation of reactive oxidative radicals which would cause thyroidal injury and initiate infiltration. We have tested this hypothesis by examining the ability of butylated hydroxyanisole (BHA), ethoxyquin, and other antioxidants to prevent thyroiditis in Obese strain (OS) chickens, a strain that develops severe disease by 4 weeks of age. BHA, when administered from hatching until death at 5 weeks of age, reduced thyroidal infiltration and serum levels of antibodies binding thyroglobulin, T3, T4. Similar effects were observed with the antioxidant ethoxyquin. Weaker antioxidants such as vitamins C and E and beta-carotene had only slight or negligible effects on these parameters. BHA reduced thyroiditis in OS chicks killed at 3 and 5 weeks of age, but not at 8 weeks. When BHA treatment was initiated after the development of severe disease, it did not reduce thyroglobulin antibody levels. To determine the mechanism by which BHA reduces thyroiditis, studies were performed to assess the effect of BHA on thyroid function and on the immune responses to exogenous antigens. BHA had no effect on thyroid function in normal strain chickens since thyroidal radioiodine uptake and organification and serum T3 and T4 levels were unaffected. BHA did not alter immune responses to exogenous antigens such as sheep red blood cells or Brucella abortus in OS chickens. In summary, potent antioxidant drugs delayed the onset of thyroiditis when treatment was initiated before the onset of disease, suggesting that reactive oxygen intermediates are involved in the early stages of pathogenesis. However, the site of action remains unknown since they had no detectable effects on thyroid function or general immune responses.

Several studies have shown that iodine plays a role in spontaneous autoimmune thyroiditis in man and other animals. In addition, abnormalities of iodine metabolism have been found in patients with Hashimoto's thyroiditis and in chickens of the obese strain (OS), an animal model of spontaneous autoimmune thyroiditis. We have examined several parameters of iodine metabolism before immune damage in this model and in the related Cornell strain (CS), a strain which develops a late-onset mild thyroiditis, to discover a possible causal relationship between altered iodine metabolism and the initiation of autoimmunity. Thyroglobulin was purified from individual chicken thyroid glands and analysed for iodine by the ceric sulphate method. Analogous to the thyroglobulin of Hashimoto's patients, the iodine content of OS thyroglobulin (27 atoms/molecule) was lower than that of normal-strain thyroglobulin (46 atoms/molecule) when the chickens were provided with a normal diet. Also, under conditions of TSH suppression, the iodine content of OS thyroglobulin (18 atoms/molecule) was lower than that of CS thyroglobulin (36 atoms/molecule) and of normal-strain thyroglobulin (32 atoms/molecule). In contrast with Hashimoto's patients, however, the OS and CS chickens had practically no inorganic iodide in their thyroid glands; electrophoretic analysis of thyroid homogenates revealed that essentially all (greater than 99.62%) 125I was organified by 16 h in all strains of birds tested. Despite the relatively poor iodination of thyroglobulin exhibited by OS chickens, they did not iodinate additional 'unique' proteins, when examined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of thyroid proteins labelled with 125I in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)