Thyroid neoplasms in a colony of beagle dogs

The histologic, clinicopathologic, and epidemiologic features of spontaneous thyroid neoplasms were evaluated in a control population of Beagle dogs. The mean age of thyroid tumor-bearing dogs (16.2 years) as significantly higher than non-tumor-bearing dogs (13.6 years). Thirteen benign and 18 malignant tumors were identified, with the incidence of both tumors increasing rapidly near the mean age of 16.2 years for tumor-bearing dogs. The age-specific incidence of tumors was 1.1% per year at 8 to 12 years, increasing to 4.0% per year by 12 to 15 years and 67% over 17 years of age. Numbers of malignant tumors were greater than benign tumors at an earlier age. Approximately 44% of the malignant tumors metastasized but only 22% resulted in death of the dog. There was no difference in tumor incidence when compared according to sex, if total tumor numbers were considered or if tumors were separated into benign and malignant categories. The age at death of tumor-bearing dogs was not increased significantly by the surgical resection of the thyroid tumors. Of dogs with thyroid tumors, 15% had clinical diagnoses of hypothyroidism, and no dogs with thyroid tumors had diagnoses of hyperthyroidism.