The inhibitory effect of large doses of methimazole on iodine induced lymphocytic thyroiditis and serum anti-thyroglobulin antibody titers in BB/Wor rats.

The BB/Wor rat spontaneously develops autoimmune insulin dependent diabetes mellitus and lymphocytic thyroiditis (LT). Excess iodine ingestion enhances and low iodine diet decreases the incidence of LT in this rat model but does not affect the incidence of diabetes mellitus. The administration of a low dose of methimazole (MMI; 870 ng/gm bw ip daily) from 30-90 days of age had no significant effect on thyroid function or on the incidence of iodine induced LT and serum anti-thyroglobulin (Tg) antibodies measured by an ELISA assay. A large dose of MMI (0.05% in the drinking water) induced goiter and hypothyroidism. In addition, the incidence of LT was markedly attenuated (76% vs 6%, p less than 0.001) and reduced titers of serum anti-Tg antibodies (0.59 +/- 0.1 OD vs 0.08 +/- 0.01, p less than 0.001) were observed. This inhibitory effect of MMI on the occurrence of iodine induced LT in the BB/Wor rat may be due to the lower antigenicity of the poorly iodinated Tg secondary to MMI therapy and/or to an immunosuppressant effect of MMI itself.