Introduction
Canine Autoimmune Thyroiditis is a genetic disease which is the cause of over half the cases of hypothyroidism diagnosed in dogs. While just one individual symptom may not alert a veterinarian to this significant genetic problem, a combination of the symptoms should signal the need to test for autoimmune thyroiditis and thyroid function. Hypothyroidism is not considered to be a life-threatening disease. However, the quality of life of an animal suffering from hypo-thyroidism is substandard. The most common clinical symptoms of canine autoimmune thyroiditis are; sluggishness, obesity, skin disease, cold intolerance, hair loss, weakness, poor coat and infertility.

The incidence of autoimmune thyroiditis in males and females is similar. Dogs with autoimmune thyroiditis produce antibodies against the storage form of thyroid hormone. These antibodies are present in the dog's bloodstream, so a simple blood test is all that is needed for accurate diagnosis.

Detection
The best available technology for early detection of autoimmune thyroiditis in dogs is a sensitive assay for THYROGLOBULIN AUTOANTIBODY (TgAA). A commercial kit for TgAA analysis is manufactured by Oxford Laboratories (Product Number: VT 10), and is available to veterinary laboratories worldwide. The TgAA assay is recommended by the Orthopedic Foundation for Animals (OFA) for its canine thyroid registry. In the United States and Canada, veterinarians can have blood tested by an approved veterinary laboratory. This OFA registry records and stores data concerning the incidence of autoimmune thyroiditis. This enables more complete genetic counseling and provides data for research purposes. This test is recommended for all dogs that will be used in breeding programs or those with a history of thyroid disease.

Genetic Transmission
Canine Autoimmune Thyroditis is genetically transmitted. The most conclusive study was done at Michigan State University by Conaway, DH, Padgett, GA, and Nachreiner, RF: The Familial Occurrence of Lymphocytic Thyroiditis in Borzoi Dogs. Am J Med Genetics 1985;22:409-414. Many other studies report the familial patterns of inheritance of the disease as well. One recent report of a large study in beagles was reported by Benjamin SA, Stephens LC, Hamilton BF, et al. Associations between lymphocytic thyroiditis, hypothyroidism, and thyroid neoplasia in beagles. Vet Path 1996;33:486-494.

Is canine autoimmune thyroiditis the same as Hashimoto's disease in humans?
Technically, no. Though the human disease is also genetically transmitted, Hashimoto's disease is 5 times more common in females than males. This sex difference is not seen in canine autoimmune thyroiditis. In addition, both thyroglobulin autoantibody and thyroperoxidase autoantibody are found in humans while Thacker reported that only thyroglobulin autoantibody is found in dogs. However, the histopathology is very similar.

Autoimmune thyroiditis is reported to cause about 50% of the primary hypothyroidism in dogs. What causes the other 50%?
Recent data from Michigan State University shows that the 50% of dogs shown to have idiopathic primary hypothyroidism may actually have had autoimmune thyroiditis as the original problem. However, as the thyroid is destroyed by the lymphocytes, thyroglobulin is no longer around to stimulate the autoantibody production. Hence the dogs become TgAA negative and are placed under the classification of idiopathic hypothyroids. It appears that the end stage of autoimmune thyroiditis is "idiopathic" hypothyroidism. One can conclude from this that the majority of dogs with primary hypothyroidism have the disease because of autoimmune thyroiditis.

Is autoimmune thyroiditis (positive TgAA) the same as hypothyroidism?
No, autoimmune thyroiditis is the major cause of the disease, but many dogs may harbor the thyroiditis for years before showing clinical signs of the disease. Studies (T. Brown et al., Wayne State University) done with and animal models have shown that there are some conditions which promote the disease. One of the inducers is an increase of iodine in the diet. It appears that the thyroglobulin becomes more immunogenetic as the iodine changes the steroconfiguration of the molecule. Hence, diet and other factors can promote thyroiditis and hypothyroidism in animals that are genetically prone to the condition.

Should TgAA be tested in routine thyroid testing protocols?
A positive TgAA titer is very helpful in diagnosing primary hypothyroidism. The commercial assay available from Oxford Laboratories is quite sensitive and specific for autoimmune thyroiditis and can identify dogs with the presence of lymphocytic thyroiditis. Since this is the major cause of hypothyroidism, it is a valuable addition to the diagnostic profile.

Progressive histological alterations that occur in primary canine autoimmune thyroiditis:

Normal canine thyroid

Initial stage of canine autoimmune thyroiditis

Progressive autoimmune thyroiditis

End stage disease