Diagnostic and predictive value of autoantibodies to thyroglobulin (TgAA) in dogs

Objective: Autoantibodies to thyroglobulin are widely used as a serum marker for an autoimmune thyroiditis in the diagnosis of canine hypothyroidism. More than 50% of the hypothyroid dogs are TgAA-positive. TgAA are also detected in healthy and sick dogs with no evidence of thyroid dysfunction. The diagnostic and predictive value of TgAA in these animals remains unclear. To answer this question we conducted a prospective study of TgAA-positive dogs. Material and methods: Serum samples of 118 hypothyroid and of 2496 sick dogs were investigated by ELISA. Results: TgAA prevalence was 58.6% and 6.25%, respectively. No gender predisposition was observed. However, certain breeds showed an increased prevalence of TgAA. TgAA positive dogs were significantly younger than antibody negative ones. In a follow-up study of 30 TgAA-positive dogs with hypothyroidism we observed, that all dogs became TgAA-negative within 2.5 year after the initial diagnosis. This indicates that TGAA-negative hypothyroid dogs are most likely in the endstage of an autoimmune thyroiditis. 156 out of 2496 sick dogs were shown to be TgAA positive without any clinical sign of a thyroid dysfunction in the initial examination. 22.5% of these animals developed an overt hypothyroidism within two years. Clinical and scientific relevance: TgAA are a useful tool in the diagnosis of thyroid dysfunction and can be used as a marker to autoimmune thyroiditis. Whether and when these animals will develop an overt hypothyroidism is certainly dependent on genetic, environmental and infectious factors and cannot be foreseen. Conclusion: Our data indicate that TgAA-positive dogs are on a higher risk to be affected by hypothyroidism.