A breeding line of domestic cats spontaneously developing symptoms of hypothyroidism between the 40th and 60th day of life (fur changes, loss of appetite, growth retardation), elevated levels of antibodies against microsomal structures and thyroglobulin, and lymphocytic thyroid infiltration has been recently established at our facility. Aim of our studies was to examine the effect of high iodine ingestion or prophylactic thyroid hormone therapy on functional and morphological characteristics of this Hashimoto-like thyroiditis in cat. From birth to day 80 of life cats were treated with iodine (n = 9; 0.1 mg/l) or thyroxin (n = 13; 2.0 micrograms/ kg/d) respectively. Untreated animals served as controls (n = 12). Cat-serum was tested for thyroid function (TT3, TT4). After 8 weeks the thyroid tissue was submitted to routine histological processing (H&E) and the inflammatory activity was scored. Additionally immunohistological staining was performed for MIB-1, IgG, IgM and MHCII expression. Both untreated hypothyroid (UHC) as well as iodine-treated (IC) cats revealed a significantly higher degree of thyroid inflammation and higher tissue levels of IgM as the thyroxin-substituted animals (TC). Epithelial proliferation decreased significantly in the IC and TC groups as compared to the untreated controls. No significant differences regarding IgG production and HLAII expression were detectable. Early thyroid hormone therapy significantly decreases both incidence and activity of autoimmune thyroiditis in cats as measured by inflammatory infiltration, IgM production and epithelial proliferation. Animals with excess iodide intake, however, show an aggravation of the autoimmune inflammatory activity.