Objectives of the Presentation

 To provide data that shows that canine autoimmune thyroid disease has a genetic component.

 To present evidence that the thyroglobulin autoantibody assay is a valid approach to early detection of the disease.

Overview of the Issue

One of the first studies which looked at the immunologic component of thyroid disease in dogs was by Mizejewski et al., 1971. Using 3 techniques he demonstrated that thyroglobulin autoantibodies were present in beagles with thyroiditis. A follow-up study by Gosselin et al., 1980, showed that 48% of dogs with primary hypothyroidism had positive TgAA titers indicating that autoimmune thyroiditis is a major cause of thyroid disease. The first study which indicated a potential genetic component of the disease was by Musser and Graham in 1968. They had a beagle colony where 42% of males and 32% of females from one male had histologic evidence of thyroiditis.

Additional Detail

A number of studies show that autoimmune thyroiditis is a genetic disorder, but the best study was done in beagles and published by Benjamin et al. in 1997. It was a huge study with 276 normal beagles that lived a normal lifespan under colony conditions. They found that autoimmune thyroiditis was highly heritable. Thyroiditis was present in 26.3% of the beagles and 25% of the sires produced 68.2% of the affected pups.

Another study by Conway, Padget, and Nachreiner with Borzoi looked at breeding a male and female sibling. In that study all of the offspring had histological evidence of thyroiditis. It looked like a simple autosomal recessive inheritance was possible; however, a dominant trait was not ruled out. In a study done in cooperation with Colleen M. O'Keefe DVM, MS, it seemed that a dominant trait was much less likely based upon the breeding of dog 22 with dog 4 in the following pedigree. Also in this pedigree, when positive dogs are known and identified one can "see" the "genes" trickling down through the pedigree and causing thyroiditis and hypothyroidism. See the example from a study of Welsh Springer Spaniels:

Click on the image to see a larger view.

The phenotype change which is used to identify dogs that are positive for autoimmune thyroiditis is via the thyroglobulin autoantibody assay (TgAA). Age is a factor in finding positive dogs with this assay. Very few dogs are affected with this autoimmune disease prior to 1 year of age. At the start of the disease there is a small focal infiltration of lymphocytes beneath the capsule of the thyroid. At this point the dog will have an increase in the amount of TgAA in its serum. We do not understand the inducers of the disease and it appears that there are some factors which induce the onset of lymphocytic infiltration. One animal model (the obese chicken) in studies by Dr. Tom Brown (Wayne State University) has shown that excess iodine in the diet would induce an earlier onset of thyroiditis than the chickens fed normal iodine. Whether iodine is also an inducer in the dog is not known, but the obese chicken model shows that inducers can be an important factor in the onset of the disease. Perhaps that is why we see such a varied age prevalence for the diagnosis of thyroiditis and hypothyroidism. Even though a dog may have the genes for thyroiditis, without the inducer the genes may remain dormant.

The peak prevalence for positive TgAA is in the 3, 4, and 5 year range, but that there are positive dogs at any age. The fact that the prevalence is lower during the early years indicates that dogs may be negative on one test only to become positive later in life. That is an unfortunate fact for breeders since a considerable amount of funding and effort may be put into a dog because it is negative when first tested and then it turns out to be positive at a later age.

We have studied the histological progression of the disease. The first lesion to appear in the thyroid that can be identified is microscopic. A focal infiltration by lymphocytes occurs. At this stage only the TgAA will be positive. There will be no changes in the TSH, T4, or T3. A thyroid with focal thyroiditis would be expected to be classified by the Orthopedic Foundation for Animals (OFA) Thyroid Database as Positive Compensative Autoimmune Thyroiditis though a few may have antibody titers in the equivocal range and be classified as Equivocal. Then the lymphocytic infiltrate becomes more diffuse and starts to interfere with thyroid hormone production. The pituitary compensates with an increase in TSH to stimulate more T4 and T3 production from the remaining normal tissue. This normal tissue will become hypertrophic in and attempt to maintain homeostasis. At this stage, TgAA will be positive and TSH will be elevated, but T4 and T3 will still be normal. This is also classified as Positive Compensative Autoimmune Thyroiditis by OFA.

Click on the image to see a larger view.

Finally, considerable lymphocytic infiltrate occurs and the normal thyroid cells are so few that they cannot compensate. At this stage, TgAA is positive, TSH is elevated and T4 and T3 are low. This would be classified as Positive Autoimmune Thyroiditis by OFA. Note that it is not until this stage that T4 and T3 are low. There are two stages prior to this which have significant changes that the veterinarian and breeder can use to identify the presence of autoimmune thyroid disease. That is why OFA requires TgAA, TSH and Free T4 in their thyroid database for thyroid registry.

At present there is not a gene based test for autoimmune thyroiditis so we must count on a phenotypic test to determine which dogs are affected. The best test for that is the thyroglobulin autoantibody assay (TgAA). This assay has been available for quite some time and in recent years has been improved to reduce the number of false positive and inconclusive tests by adding an assessment of non-specific binding (NSB). One time when NSB can increase is after vaccination. The immune globulins increase because of the vaccine and NSB increases as well. Other factors, such as any wound or disease process which increases IgG probably increases NSB. The modified test with the correction for NSB avoids errors from these problems. It requires an additional ELISA well for each sample. The additional well has all the components of a regular test well, except that no thyroglobulin is present. Hence, any antibodies attaching to the proteins that are present represent non-specific binding rather than true TgAA. The optical density (O.D.) of the NSB well is subtracted from the total O.D. to give the specific TgAA O.D. The NSB modified test procedure gives confidence that dogs that are positive have autoimmune thyroiditis and that they have affected genes which will be passed on to their offspring. I would recommend testing all breeding stock to reduce the amount of thyroid disease. See the following graph which illustrates the breeds which have the highest laboratory prevalence of positive TgAA.

Click on the image to see a larger view.

Summary

There is evidence that autoimmune thyroid disease is genetic in origin and that utilization of an assay for TgAA can identify affected dogs. By testing the majority of dogs in a pedigree, breeders can also identify many of the dogs that are carriers as well.

References

1.  Mizejewski GJ, Baron J, Poissant G. Immunologic investigations of naturally occurring canine thyroiditis. J Immunol. 1971;107:1152-60.

2.  Gosselin SJ, Capen CC, Martin SL, Targowski SP. Biochemical and immunological investigations on hypothyroidism in dogs. Can J Comp Med. 1980;44:158-68.

3.  Musser E, Graham WR. Familial occurrence of thyroiditis in purebred beagles. Lab Anim Care. 1968;18:58-68.

4.  Benjamin SA, Stephens LC, Hamilton BF, Saunders WJ, Lee AC, Angleton GM, Mallinckrodt CH. Associations between lymphocytic thyroiditis, hypothyroidism, and thyroid neoplasia in beagles. Vet Pathol. 1996;33:486-94.

5.  Conaway DH, Padgett GA, Nachreiner RF. The familial occurrence of lymphocytic thyroiditis in borzoi dogs. Am J Med Genet. 1985;22:409-14.

6.  Brown TR, Zhao G, Palmer KC, Sundick RS. Thyroid injury, autoantigen availability, and the initiation of autoimmune thyroiditis. Autoimmunity. 1998;27:1-12.