Recently, a very large population survey of canine serum samples submitted for diagnosis of suspected hypothyroidism found thyroid hormone autoantibodies in 6.3% (MSU Animal Health Diagnostic Laboratory; 18,135 of 287,948 samples). From this data base, the 10 breeds with highest prevalence were: Pointer, English Setter, English Pointer, Skye Terrier, German Wirehaired Pointer, Old English Sheepdog, Boxer, Maltese, Kuvasz, and Petit Basset Griffon Vendeen. Prevalence was associated with body weight and was highest in dogs 2-4 years old. Females were significantly more likely to have thyroid autoantibodies than males.

In a parallel study of some 2000 cases at Antech Diagnostics, the most commonly affected dog breeds included Golden Retriever, Shetland Sheepdog, American Cocker Spaniel, English Setter, Boxer, Doberman Pinscher, Labrador Retriever, German Shepherd Dog, Akita, Irish Setter, Old English Sheepdog, and Collie, but basically all breeds, and mixed breeds are affected to some extent.

Laboratory and pedigree analyses of affected families show a progressive earlier age of onset of thyroiditis and/or clinical signs of thyroid dysfunction, along with an increased proportion of affected versus normal offspring in successive litters. This situation has been seen previously with respect to autoimmune hemolytic anemia and thrombocytopenia, and may also apply to Addison's disease in affected dog breeds.

Genetic counseling for this heritable disorder has increased awareness of the need to screen canine breeding stock on a regular (annual) basis, starting at puberty. Consequently, more animals are diagnosed in the early stages of this autoimmune process, before they begin to show typical signs of thyroid dysfunction. Most experts agree that these dogs should be removed from the breeding program.

A question remains about whether to begin supplementation with thyroid hormone immediately, conceptually to reverse the production of thyroid autoantibody and the destruction of thyroid tissue, or wait until clinical signs develop. The recent report from MSU's one year followup of 173 thyroglobulin autoantibody positive dogs, indicated that 20% had converted to clinical hypothyroidism in that time period. Presumably, many of the remaining dogs will eventually become hypothyroid.

Given this expectation, there is rationale for beginning thyroid replacement once the diagnosis has been established.