Antithyroid drugs ameliorate thymectomy-induced experimental autoimmune thyroiditis

The possible immunosuppressive action of antithyroid drugs in vivo has been assessed using Buffalo (Buf) strain rats with thyroiditis produced by neonatal thymectomy; this model shares many features with spontaneous thyroiditis in man. Both propylthiouracil and methimazole significantly reduced the severity of thyroiditis (P less than 0.01 compared to controls), irrespective of effects on thyroid status. Thyroidal Ox 8 (suppressor/cytotoxic) and W3/25 (helper) T-cells were reduced in number equally in treated animals, and thyroid follicular cell Ia expression was absent in a high proportion of these glands. Neither agent alone altered circulating antithyroglobulin antibody levels but there was a significant fall in antibody levels in animals treated with propylthiouracil and thyroxine. Antithyroid drugs had no effect on circulating T-cell subsets, and there was no direct suppressive action in vitro on Ia expression by the FRTL5 thyroid cell line. These results provide further support for a direct action of antithyroid drugs on autoimmune process in thyroid disease exerted in particular on the thyroid lymphocytic infiltrate.