THYROID EVALUATION
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WHAT DOES THE THYROID DO?
In order to understand thyroid disease, it helps to have an understanding of what the thyroid actually does. The thyroid gland is located in your dog’s neck, where it produces the hormone thyroxine (T4), along with several other important thyroid hormones. These hormones play a large role in your dog’s metabolism and can cause major problems when they are not produced at normal levels. The thyroid is like the thermostat of the body.
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THYROID DISEASE AND AUTOIMMUNE THYROIDITIS
Hypothyroidism is the most common endocrine disorder of dogs, and up to 80% of cases result from an autoimmune disease that progressively destroys the thyroid gland (autoimmune thyroiditis). When more than 75% of the thyroid gland is destroyed by the process of thyroiditis, classical clinical signs of hypothyroidism appear. Because autoimmune thyroiditis is heritable, it has significant implications for breeding stock.
Accurate diagnosis of the early stages of autoimmune thyroiditis offers important genetic and clinical options for prompt intervention and case management. However, it is often difficult to make a definitive diagnosis.
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WHAT IS HYPOTHYROIDISM?
With Hypothyroidism, the thyroid gland is not making enough of a hormone called thyroxine that controls metabolism (the process of turning food into fuel). Hypothyroidism causes a wide variety of symptoms, but is often suspected in dogs that have trouble with weight gain or obesity and suffer from hair loss and skin problems. The good news is this disease isn’t life-threatening, it’s easy to diagnose with a blood test, and it’s fairly easy and inexpensive to treat. Treatment is typically a thyroid supplement taken daily.
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Autoimmune thyroiditis is the most common cause of primary hypothyroidism in dogs. The disease has variable onset, but tends to clinically manifest itself at 2 to 5 years of age. Dogs may be clinically normal for years, only to become hypothyroid at a later date. The marker for autoimmune thyroiditis, thyroglobulin autoantibody formation, usually occurs prior to the occurrence of clinical signs. Therefore, periodic retesting is recommended.
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The majority of dogs that develop autoantibodies have them by 3 to 4 years of age. Development of autoantibodies at any time in the dog’s life is an indication that the dog most likely has the genetic form of the disease. Using today’s technology only a small fraction of false positive tests occur.
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As a result of the variable onset of the presence of autoantibodies, periodic testing will be necessary. Dogs that are negative at 1 year of age may become positive at 6 years of age. Dogs should be tested every year or two in order to be certain they have not developed the condition. Since the majority of affected dogs will have autoantibodies by 4 years of age, annual testing for the first 4 years is recommended. After that, testing every other year should suffice. Unfortunately, a negative at any one time will not guarantee that the dog will not develop thyroiditis.
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The registry data can be used by breeders in determining which dogs are best for their breeding program. Knowing the status of the dog and the status of the dogs lineage, breeders and genetic counselors can decide which matings are most appropriate for reducing the incidence of autoimmune thyroiditis in the offspring.
Dogs should not receive any type of thyroid supplementation for 3 months prior to thyroid testing.
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INDICES OF THYROIDITIS
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Free T4 (FT4) - This procedure is considered the "gold standard" for assessment of the thyroid's production and cellular availabilityof thyroxine. FT4 concentration is expected to be decreased in dogs with thyroid dysfunction due to autoimmune thyroiditis.
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Canine Thyroid Stimulating Hormone (cTSH) - This procedure helps determine the site of the lesion in cases of hypothyroidism. In autoimmune thyroiditis the lesion is at the level of the thyroid and the pituitary gland functions normally. The cTSH concentration is expected to be abnormally elevated in dogs with thyroid atrophy from autoimmune thyroiditis.
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Thyroglobulin Autoantibodies (TgAA) - This procedure is an indication of the presence of the autoimmune procress in the dog's thyroid.
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References: Orthopedic Foundation for Animals website: www.offa.org
American Kennel Club Canine Health Foundation website: www.akcchf.org