Autoimmune Thyroiditis

The thyroid gland sits on the throat below the larynx. Its function is to produce thyroxine. a hormone that controls metabolism. Autoimmune thyroiditis is an immune-mediated disease characterized by inflammation of the thyroid gland which is inflitrated by lymphocytes that destroys the thyroid gland. The disease very often leads to hypothyroidism.

Breeds at Risk

Autoimmune thyroiditis has now been identified in more than 50 breeds. The condition is heritable in those breeds that have been studied, and progression to hypothyroidism, if it occurs, can be slow. The disease is particularly often seen in Doberman Pinscher, Beagle, Golden Retriever, Borzoi, Great Dane, Newfoundland, Rottweiler, Chow Chow, Boxer, German Shepherd Dog, Old English Sheepdog (Bobtail), Maltese, Cocker Spaniel, Airedale Terrier, Skye Terrier , Poodles, Shetland Sheepdog and Akita.

Autoimmune thyroiditis, also called lymphocytic thyroiditis, has also been reported with increased prevalence in Irish Setters and Dachshunds which are a genetically susceptible breeds. Research suggests that the onset of this disease occurs in Irish setters before the onset of hypothyroidism.

This condition may occur as part of other systemic diseases such as lupus, rheumatoid arthritis or idiopathic arthritis.

Types of Hypothyroid Diseases

There are four hypothyroid syndromes. The two primary forms are lymphocytic thyroiditis and atrophic hypothyroidism, and the two secondary forms are pituitary-dependent hypothyroidism and euthyroid sick syndrome.

Lymphocytic thyroiditis resembles Hashimoto's thyroiditis in man. Lymphocytic thyroiditis is a common canine condition which is associated with more than 50% of cases of canine hypothyroidism.

Dogs are affected between ages 1 and 3 years. Less than half of the affected dogs are obese. Most individuals experience lack of energy and recurrent infections. A large number of dog skin disorders are caused by thyroid disease.

Lymphocytic thyroiditis may be initiated by medications, vaccinations, viral infections, and heavy metal toxicity from water and food supplies.

Dogs can live for years with thyroiditis without there being any signs. The presence of anti-thyroid antibodies in the dog's blood or tissues marks the disease, during which the dog becomes susceptible to immune-mediated or other diseases. The primary indicator on the thyroid profile is elevation of the T and/or T4 antibodies. Factors that influence the progression from subclinical thyroiditis to hypothyroidism in dogs are still to be identified.

The Orthopedic Foundation For Animals (OFA) and The American Spaniel Club maintain a registry for Cockers and other dogs that have been tested for autoimmune thyroiditis. Based on their statistics, Cockers are about the sixteenth most affected breed. According to D. Caroline Coile, author of "The Cocker Spaniel Book," screening should begin at 2 years of age and continue every year or other year through 8 years of age. Thyroglobulin autoantibody (TGAA) is measured to detect lymphocytic thyroiditis in dogs, although positive results may also suggest other diseases, such as hypoadrenocorticism and diabetes mellitus.⁵

The Importance of Early Testing

An autosomal recessive mode of inheritance has been suggested. Breeders should test thyroid levels in their dogs upon the onset of puberty, and then annually thereafter. Because thyroid hormones play a role in the metabolic function of all cells, the symptoms affect all systems, blood, neuromuscular, skin, circulatory, gastrointestinal, reproductive system and so forth. Early in the disease, the disorder can be identified through blood tests; the symptoms do not appear until the thyroid gland loses its reserves and can no longer compensate for the throid antibodies. This disorder can be treated with thyroid hormones.

References:
1. Irish Setter. Margaret Williams
2. Lymphocytic thyroiditis. Graham PA, Nachreiner RF, Refsal KR, Provencher-Bolliger AL. Vet Clin North Am Small Anim Pract. 2001 Sep;31(5):915-33
3. Holistic Guide for a Healthy Dog. Wendy Volhard, Kerry Brown
4. Genetics: An Introduction for Dog Breeders. Jackie Isabell
5. Steven L. Stockham, Michael Alan Scott. Fundamentals of veterinary clinical pathology.

Thyroiditis makes dogs highly susceptible to anesthesia, especially if the disease has destroyed a large part of the thyroid gland. Apparently, deficiency of the thyroid hormone renders dogs highly ensitive to anesthesia.



A complete baseline thyroid profile should be measured and typically includes total T4, free T4, thyroid autoantibodies, and may also include total T3, free T3 and cTSH. If included in thyroid profiles, the T3 and freeT3 assays usually reflect thyroiditis when both are spuriously elevated due to presence of T3 autoantibody. The autoantibody (AA) assays

(T3AA, T4AA, TgAA) are especially important in screening breeding stock for heritable autoimmune thyroid disease. The normal reference ranges for thyroid analytes of healthy adult animals tend to be similar for most breeds of companion animals. Exceptions are the sighthound and giant breeds of dogs which have lower basal levels. Typical thyroid levels for

healthy sighthounds, such as retired racing greyhounds, are at or just below the established laboratory reference ranges, whereas healthy giant breeds have optimal levels between the lower end and midpoint of these ranges.



Further, because young animals are still growing and adolescents are maturing, optimal thyroid levels are expected to be in the upper half of the references ranges. For geriatric animals, basal metabolism is usually slowing down, and so optimal thyroid levels are likely to be closer to

midrange or even slightly lower. All animals are not the same

• Puppies have higher basal thyroid levels than adults



• Geriatrics have lower basal thyroid levels than adults



• Large/giant breeds have lower basal thyroid levels



• Sighthounds have much lower basal thyroid levels



Do not breed dogs with autoimmune thyroiditis

• Heritable trait, regardless of clinical status

• Screen relatives annually from puberty

• Consider for breeding, if negative, after age three

Posted by Guest_3499 on Tuesday, April 22, 2008

Lymphocytic thyroiditis is the underlying cause in many cases of primary hypothyroidism in

dogs and the predisposition to its development is believed to be highly heritable. It is an immune mediated disorder characterized histologically by a diffuse infiltration of lymphocytes, plasma cells, and macrophages in the thyroid gland. Antibodies interact with the follicular cell, colloid, or thyroglobulin antigens and activate the complement cascade and cell-mediated cytotoxicity. The progressive destruction of follicles and secondary fibrosis eventually leads to a failure of thyroid hormone production. More than 60 or 70% of the thyroid tissue needs to be destroyed before we see changes in laboratory measures of thyroid function. This process can take months or years to cause classic hypothyroidism. In some animals it may not progress. Anti-thyroglobulin antibodies are released into the circulation in animals with this condition. A test for these antibodies in serum is included in the Michigan State University Canine Thyroid Diagnostic profile.



In addition to testing for serum levels of total thyroxine (TT4), total tri-iodothyronine (TT3), free thyroxine (FT4), and free tri-iodothyronine (FT3), this profile tests for T4 antibodies (T4AA), T3 antibodies (T3AA), canine thyrotropin (cTSH; thyroid stimulating hormone), and thyroglobulin antibodies (TgAA). The cTSH test provides much needed information in any attempt to diagnose hypothyroidism. Many non-thyroidal factors can cause decreases of TT4, TT3, FT4, and FT3 into the hypothyroid range in a dog with normal thyroid function making it difficult to

differentiate sick-but-euthyroid animals from those with hypothyroidism.



Some researchers have looked for and failed to find such an association of lymphocytic thyroiditis with vaccination.



(This information was extracted from Michigan State University Diagnostic Center for Population and Animal Health 517-353-1683 www.animalhealth.msu.edu

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Posted by Guest_3497 on Tuesday, April 22, 2008

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