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.

Effect of L-thyroxine administration on the incidence of iodine induced and spontaneous lymphocytic thyroiditis in the BB/Wor rat.

Chronic L-thyroxine administration (6 micrograms/100g BW, ip, daily) for 2 or 3 months suppressed serum TSH concentrations and decreased both the incidence of spontaneous lymphocytic thyroiditis (LT) and the serum levels of anti-thyroglobulin (anti-Tg) antibodies in the diabetes prone BB/Wor rat. This suggests that TSH may play a role in the occurrence of LT in this rat model. In contrast to these observations, L-thyroxine administration did not affect the markedly increased incidence of LT or the elevated serum anti-Tg antibodies in iodine supplemented BB/Wor rats, suggesting that TSH stimulation is not necessary for the development of iodine induced LT in this rat model. Other factors, such as the increased antigenicity of highly iodinated Tg, may be more important.