The thyroid glands produce two hormones, thyroxine which controls metabolic and activity levels and calcitonin which regulates calcium metabolism. Hypothyroidism is commonly caused by an auto-immune response; that is, the body creates antibodies directed against the thyroid glands causing their destruction. This self-destruction is referred to as autoimmune thyroiditis or lymphocytic thyroiditis. As this disease develops, antibodies recognize the thyroid tissue as being foreign and lymphocytes, a form of white blood cell, are signaled to cause the actual destruction. Although the exact incidence of this disease among dogs is not known, it is equally distributed between males and females in the canine population. Of the 1,991 Flat-Coated Retrievers involved in the 1997 Health Survey, 3.9% of the males and 6% of the females were reported with hypothyroidism.
Scaly, thickened and/or darkened skin. Hair that is coarse, brittle, fine, matted or lighter than normal. Loss of hair especially in areas of wear. Poor re-growth of hair. Poor wound healing. Skin infections and blackheads (comedones). Obesity. Lethargy. Slowed heart rate or abnormal heart rhythm. Reproductive dysfunction (infertility). Cold intolerance. Clinical signs usually appear between 2 and 6 years of age, but may occur very young (congenital hypothyroidism).
The following two statements are crucial to the accurate diagnosis and reporting of hypothyroidism. Do not contribute to the misinformation problem by ignoring them:
- Despite recommendations to the contrary, many dogs are diagnosed with hypothyroidism and placed on thyroid supplement without the benefit of a full, diagnostic work-up.
- A definitive diagnosis can be made only after a dog's history, clinical signs and lab results are all considered.
There are a number of blood tests available to help diagnose hypothyroidism. The following table is provided courtesy of Ray Nachreiner D.V.M., Ph.D. of Michigan State University. He is presently recommending that a TgAA (thyroglobulin autoantibody) assay be performed as it will detect the disease earlier than the other tests. Please note that you can expect a negative TgAA once the thyroid gland has been totally destroyed and auto-antibodies to the gland no longer circulate in the bloodstream. Free T4 by dialysis is the preferred test for T4.
|CONDITION||TgAA||TSH||FREE T4||TOTAL T4|
In addition to the thyroid tests, a complete blood count, a chemistry panel and a urinalysis should be performed in order to rule out presence of other disease processes.
Daily lifelong administration of an oral synthetic thyroid hormone. Prognosis for disappearance of clinical signs with treatment is excellent.
MODE OF INHERITANCE:
Hypothyroidism is a genetic disease and is thought to be an autosomal recessive disorder. Because clinical signs may not appear until well after a dog is used in a breeding program, it is essential to test for hypothyroidism before a dog is bred. The following guidelines are recommended after a complete diagnostic work-up is performed: IF:
- No clinical signs are apparent and test results are normal, may proceed with breeding plans.
- Clinical signs are present, but test results are normal, wait to breed and repeat tests in 2-6 months.
- No clinical signs are apparent, but test results are abnormal, wait to breed and repeat tests in 2-6 months.
- Clinical signs are present and test results are abnormal, begin treatment and exclude the dog from your breeding program.
The future holds the promise of a blood test which could detect the gene responsible for hypothyroidism A genetic marker test could be easily performed to allow breeders to know if their dogs are normal, affected or carriers. Breeders could then eliminate affected dogs from their breeding program and avoid breeding carriers to carriers. These efforts could significantly decrease the incidence of disease in a breed and over time eradicate it. At present, the OFA (Orthopedic Foundation for Animals) provides a Canine Thyroid Registry using specific veterinary laboratories. One of these is: Animal Health Diagnostic Lab at Michigan State University. Contact the OFA for further information.
OTHER HYPOTHYROIDISM FACTS:
- The most common biochemical abnormality noted in conjunction with hypothyroidism is hyperlipidemia.
- It is possible that thyroid hormone levels may decrease as a normal response to the process of aging.
- The estrus cycle with its fluctuating estrogen levels can affect certain thyroid hormone test results.
- Other illnesses that can lower thyroid hormone levels are diabetes, chronic renal failure, liver disease and hypo/hyperadrenocorticism.
- Drugs that can lower thyroid hormone levels are: general anesthesia, glucocorticoids, potentiated sulfas, iodine, radio-contrast dyes, phenobarbital, furosemide, aspirin and phenlybutazone.
To order a copy of the 'AKC Canine Health Foundation White Paper#97-1 on Canine Hypothyroidism", please send $5.00 (check or money order payable to AKC Canine Health Foundation) along with your name and address to: AKC Canine Health Foundation, 251 W. Garfield Road, Suite 160, Aurora, Ohio 44202.
Bodner, E. AKC canine health foundation white paper #97-1. A summary of the International Symposium on Canine Hypothyroidism. Davis, CA: University of California: 1997.
Foster, R., Smith, M. What's the diagnosis?: understanding your dog's health problems. New York: Howell Book House, 1995; 148-150.
Nachreiner, R. Familial autoimmune thyroiditis in purebred dogs. Paper presented at the AKC/CHF Canine Health Conference. St. Louis, MO., 1997.
Nachreiner, R. Genetic autoimmune thyroiditis in dogs. Paper presented at the AKC/CHF Canine Health Conference, St. Louis, MO., 1997.
Nelson, R.W., Feldman, E. C. The endocrine system and metabolic disorders. In: Siegal, M., ed. UC Davis school of veterinary medicine books of dogs: a complete medical reference for dogs and puppies. New York: HarperCollins, 1995; 313-314.