Associations between lymphocytic thyroiditis, hypothyroidism, and thyroid neoplasia in beagles.

The thyroids were evaluated in 276 control Beagles that were allowed to live out their full life span (mean = 12 years) in a closed breeding colony. Lymphocytic thyroiditis was found in 26.3% of the dogs. This lesion was characterized by lymphoplasmacytic inflammation accompanied by follicular destruction. The thyroiditis was progressive, resulting in severe atrophy of follicular tissue, and 44 dogs (15.9%) were diagnosed as hypothyroid at the time of death. In accordance with the experimental protocol, hypothyroid dogs were not given thyroxine replacement therapy. There was a high degree of heritability for the hypothyroidism. Hypothyroid dogs had an increased risk for thyroid follicular epithelial neoplasia and, in particular, for follicular adenocarcinomas. Twenty-four of the 44 hypothyroid dogs (54.5%) had one or more follicular thyroid neoplasms, whereas only 53 of the 232 (22.8%) clinically euthyroid dogs had similar tumors. Multiple thyroid tumors were present in 14 of the 44 (31.8%) hypothyroid dogs but in only 12 of the 232 (5.2%) euthyroid dogs. One or more follicular adenocarcinomas were present in 15 of the 44 (34.1%) hypothyroid dogs but in only 16 of the 232 (6.9%) euthyroid dogs. There was no difference in prevalence of hypothyroidism or tumors between the sexes. The strong association between progressive lymphocytic thyroiditis, hypothyroidism, and thyroid follicular neoplasia in these Beagles probably relates to promotion of residual follicular epithelium by chronic excess thyrotropin stimulation.