The canine thyroglobulin autoantibody assay is to be used for the diagnosis of autoimmune thyroid disease in dogs. Antithyroglobulin antibodies are present in about 50 percent of hypothyroid dogs. In this report, the sera of the five canine patients ($P1{sim}P5$) referred to the Veterinary Medical Teaching Hospital in Seoul National University were assessed by T4, TSH and thyroglobulin autoantibody (TgAA). P1 was diagnosed as severe hypothyroidism since he showed very high TSH levels with low T4 values. P2 and P3 born with a genetic predisposition were assessed as normal in thyroid function. P4 was normal, but needed follow-up examination for TgAA assay. P5 showed the positive result in TgAA assay, so it was diagnosed as autoimmune thyroiditis. As the cases above show, not only T4 and TSH, but also TgAA assay to be considered for more accurate assessment of the status of the thyroid.