Autoantibodies directed against thyroid proteins and hormones - an additional parameter for diagnosis of canine hypothyriodism

Hypothyroidism is one of the most frequent endocrine disorders in dogs. In 95% of all cases primary hypothyroidism can be diagnosed and most often it is caused by lymphocytic thyroiditis (LT). But diagnosis often turns out to be difficult as the clinical symptoms can be divers and also the laboratory parameters are not always obvious as e.g. the concentration of TT4 (total Thyroxin) can be influenced by many parameters.

Lymphocytic thyroiditis is characterised by diffuse or focal infiltration of mononuclear cells (lymphocytes, macrophages, plasma cells), that can be detected in biopsy or autopsy specimens. In the cause of this inflammatory process thyrocytes are destroyed by cytotoxic T-cells and/or antibody mediated cytotoxicity and thyroid tissue is replaced by connective tissue. As the remaining intact tissue is able to compensate thyroid hormone production the loss of functional thyroid tissue gets clinically apparent not before most of the thyroid gland is destroyed.

 
 

Lymphocytic thyroiditis is caused by an autoimmune process of unknown reason. Several factors are assumed: infectious diseases, environmental toxins, genetic disorders (prevalence in distinct breeds and families).

In the course of this disease also autoantibodies (Aab) are produced and their titre can be estimated in blood serum or plasma. In dogs Aab directed mainly against thyroglobulin (Tg), but also against thyroid peroxidase and the thyroid hormones (TH) are generated. Typically, onset of disease is between 2 to 6 years of age.

In serum or plasma the titre of Aab to Tg and TH can be measured by ELISA. About 50% of hypothyroid dogs have Aab recognizing Tg (there is controversy between different groups). In dogs suspicious of having hypothyroidism, but uncertain TT4 - and cTSH-values the prevalence of Aab-Tg is 25% whereas in healthy dogs or in dogs with other diseases the prevalence is only 5%. Dogs without clinical signs but elevated Aab-titres have a higher risk (20%) of becoming hypothyroid later in life. These animals should be retested after 4 - 6 month to evaluate the progress of the Aab-titre. Mostly it is only temporarily increased due to infection or vaccination.

The frequency of Aab-TH is higher in hypothyroid dogs than in those without a thyroid disorder. Autoantibodies to TH are important as they can interfere with tests measuring the TH concentration leading to falsely elevated or lowered test results depending on the assay design.

The estimation of thyroid specific Aab provides an additional parameter for the conformation of diagnosis of canine hypothyroidism. In subclinical cases an elevated Aab-titre already indicates a thyroid disorder. Furthermore, in breeds and families with recurrent hypothyroidism this test can be used for screening.