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Printed with Permission ~ 29 December 1999 That is a question that many of us have been asking lately. The following is a list of clinical signs of hypothyroidism compiled by W. Jean Dodds, DVM. It originally appeared in ROTT N CHATTER, October 1992, V.1, N.3, and November 1992, V.1, N.4 (may be reproduced). Once you realize how broad the problems can be, you may want to learn more about this little gland! Clinical
Signs of Hypothyroidism
Now that we have your attention…would you like to learn more? Please read on. Introduction The thyroid gland is located on the dogs trachea, just below the larynx. The thyroid gland makes two hormones, L-thyroxine (T4) and triiodothyronine (T3). These two hormones regulate the body’s basic metabolism - including control of growth and development and maintenance of protein, carbohydrate, and lipid metabolism. More T4 is produced, however, T3 is the more active form and most of T4 is converted into T3 in the tissues. The majority of the hormones travel in the blood bound to protein, but a small proportion is unbound (free). Only the unbound or free hormones are available for physiologic action. Hypothyroidism is the most common endocrine disease in dogs. The majority of cases are due to autoimmune thyroiditis. Idiopathic thyroid atrophy is characterized by loss of thyroid tissue, which is replaced by fat and connective tissue. This differs from lymphocytic thyroiditis, in which inflammatory cells invade and eliminate the thyroid tissue. Clinical signs of hypothyroidism become apparent as the thyroid gland is progressively destroyed. This is an inherited disease (genetic) in dogs and effects both males and females equally. The familial tendency has been demonstrated in Great Danes, Beagles and Borzois. More recently, many other breeds have been added to this list. History For many years the only test for thyroid function was the total T4 (TT4) test, which measured all the T4, bound and free. This was an insensitive measure of thyroid function as it usually does not go low until at least two-thirds of the functional thyroid tissue has been lost. Total T4 (TT4) and total T3 (TT3) levels may also be suppressed by liver and kidney disease, tumors and other physical diseases, as well as by various drugs, including steroids, Phenobarbital, anti-inflammatory drugs, and certain antibiotics. Free T3 and T4 (FT3 and FT4) levels are less likely to be affected by disease or drugs. In the past, thyroid stimulating hormone (TSH) was injected in order to measure the hormonal response. This thyroid stimulation test is rarely done now as TSH is expensive and the dog is required to remain at the vets to have 2 blood samples drawn throughout the day. This test was not very accurate and was influenced by many factors including stress and reproductive status. Later test for endogenous thyrotropin (eTSH) were incorporated in most thyroid screening panels. However, there were a significant number of cases were the T4 was normal and the eTSH was elevated or T4 depressed and eTSH normal. This led to considerable diagnostic problems. In August of1996, the American Kennel Club Canine Health Foundation hosted an international symposium on canine hypothyroidism at the University of California at Davis. A group of approximately 100 people gathered at the university for the symposium. Presenters included leading researchers in canine endocrinology. Craig Sparkes, an English Setter Fancier, spoke about issues of hypothyroidism that affect breeders. Representatives from the AKC and the AKC Canine Health Foundation (AKC/CHF), which helped sponsor the event, included Dr. Sheldon Adler, chairman of the Delegates Committee on Canine Health Research and Education; Robert Kelly, a Board Member of the AKC/CHF, Ed Gilbert, an AKC Delegate; and Deborah Lynch, executive director of the AKC/CHF. These experts in their fields, got together to discuss the problems associated with testing and diagnosing thyroid disorders. The proceedings of the conference were published in Canine Practice, Vol. 22 No. 1, January/February 1997. The AKC Gazette also published an article summarizing the findings of the conference in February 1997. The AKC Canine Health Foundation decided to provide a summary directed to dog breeders. Elizabeth Bodner, DVM, reviewed all of the material from the conference proceedings and produced the first "white paper" for the Foundation. It summarizes the proceedings in lay language and captures the primary principles expressed at the conference in a shortened format. Findings
The most significant outcome was the establishment of a practical approach to diagnosing and monitoring hypothyroidism. Out of this was born the OFA thyroid screening panel and certification registry. Recommendations For all dogs, the initial work-up begins with a complete blood count (CBC), blood chemistry panel, urinalysis, case history and physical examination. After this initial database is developed, testing depends on whether the dog is a pet, performance or breeding dog.
Current Much research has been conducted since the publication of the white paper in March 1997. Large studies have been conducted at Michigan State University supporting the familial incidence of autoimmune thyroiditis. One of the most significant findings to date has been the hypothesis that idiopathic hypothyroidism may be the end stage of autoimmune thyroiditis. Previously these were considered to be two separate diseases. Recent data from Michigan State University shows that 50% of dogs shown to have idiopathic primary hypothyroidism may actually have had autoimmune thyroiditis as the original problem. 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. This reinforces the justification for routine annual screening of all breeding stock. The development of the TgAA test offers breeders an accurate, reliable, and cost effective option for routine testing. TgAA is the FIRST indicator of autoimmune thyroiditis and can be positive long before clinical signs of the disease occur. Hypothyroidism usually manifests itself between the ages of three and five years, however, the TgAA test can detect problems in dogs as young as one year. . TgAA is a very stable gamma globulin and therefore is not effected by changes is temperature like fT4d. The TgAA assay has been found to be 96% accurate. Statistics
Current Testing Recommendations OFA thyroid panel: this includes fT4d, TSH, and TgAA. The minimum testing requirement for all dogs is TgAA. The blood sample should be taken when the dog is otherwise healthy, is not approaching or in a heat cycle, and is not taking pharmaceuticals such as steroids, non-steroidal anti-inflammatories, or anti-seizure drugs. Dogs currently on thyroid replacement therapy can not be tested and should be free of drug for at least 3 months prior to testing. Dogs should be tested at 1, 2, 3, 4, 5, 7 and 9 years of age. Since this is an autoimmune disease, dogs can test negative at one age and develop the disease at a later time. If a dog does not acquire this disease by 5 years of age, odds are very good that it will remain negative. Breeders have the option of having the data from positive TgAA (thyroiditis) dogs reported in the open OFA registry or not. Practically, most breeders chose not to report this data. This defeats the purpose of an "open" registry. The registry can help identify dogs that are phenotypically normal for breeding programs and help gather data on the prevalence of the disease within our breed. Testing of all dogs is just the first step. Sharing of this information is what is necessary in order to eliminate this disease from our breed. Teresa La
Brie References Graham, P.A., Nachreiner, R.F., Refsal, K.R,, Hauptman, J., and Watson, G.L. Thyroglobulin autoantibody is an effective Marker for thyroiditis in dogs. Am J Vet Res (submitted). Nachreiner, R.F., Refsal, K.R., Graham, P.A., et al. Prevalence of autoantibodies to thyroglobulin in dogs with non-thyroidal illness. AM J Vet Res, Vol. 59, No. 9, August 1998. Thacker, E.L., Refsal, K.R., and Bull, R.W. Prevalence of autoantibodies to thyroglobulin, thyroxine or triiodothyronine and and relationship of autoantibodies and serum concentrations of iodothyronines in dogs. AM J Vet Res, Vol. 53, No. 4, 449-453, 1992. Nelson, R .W., Et al: Serum free thyroxine concentration in healthy dogs, dogs with hypothyroidism and euthyroid dogs with concurrent illness. J AM Vet Med Assoc. Vol. 198, 1401-1407, 1991. Additional information was obtained from the following: Thyroid
Testing in Dogs: A Reference for Dog Breeders and Owners Colleen
Brady, B.A., Veterinary Student Orthopedic
Foundation for Animals Oxford
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