The influence of methimazole on thyroglobulin-induced autoimmune thyroiditis in the rat.

Experimental autoimmune thyroid disease was induced in August rats by immunization with rat thyroglobulin in complete Freund's adjuvant. Disease severity, assessed by thyroid histology and circulating levels of anti-TG antibody measured by an enzyme immunoassay, was maximal between 30 and 60 days after the initial immunization and thereafter waned. Thyroid function through the duration of the disease, assessed by measurement of serum TSH levels by RIA, remained normal. Once the natural history of the disease was established, groups of rats received methimazole (MMI) with or without T4 in their drinking water, either before or after disease induction. The animals were bled at regular intervals and killed on day 49 for histological grading of their thyroids. MMI alone (group 3) or with T4 (group 4) before disease induction significantly reduced the severity of the disease, although the effect on circulating antibody levels was less marked in the animals in group 4. In animals given MMI alone (group 5) or with T4 (group 6) after establishment of the disease, MMI again significantly reduced the severity of the established disease, although this effect was less marked in the T4 supplemented animals. MMI significantly impaired the induction and reduced the severity of experimental autoimmune thyroid disease in August rats. The ability of MMI to influence the autoimmune process may have important implications for the use of this and other agents that act on the immune system in the management of human autoimmune disease.