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Hypothyroidism in Dogs

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The recommended blood test for screening purposes is the total T4. This test is indicated for dogs who have findings suggestive of hypothyroidism on physical examination. A normal T4 is fairly conclusive evidence that the dog does not have hypothyroidism. However, a low-normal or below-normal level does not mean the dog is hypothyroid, because concentrations below normal are common for many reasons other than hypothyroidism.

To avoid overdiagnosing and overtreating the disease, it is important to confirm the significance of a low T4 using a more accurate thyroid function test, such as the FT4 by equilibrium dialysis. Other blood tests are also available for diagnosing hypothyroidism. One is an assay for thyroglobulin autoantibodies; these autoantibodies are present in about 50 percent of dogs with autoimmune thyroiditis. This test must be sent to a special laboratory for analysis.

Treatment: Hypothyroidism is permanent, but can be effectively treated with daily or twice-daily thyroid hormone replacement using synthetic L-thyroxine (L-T4). The initial dose is based on the dog’s weight. This should be adjusted for individual circumstances. Monitoring is done by physical examination and measuring the total T4. This must be done frequently, particularly early in the course of treatment. Hair loss and other signs of hypothyroidism usually reverse with treatment. A few dogs will need T3 supplementation as well.

The Orthopedic Foundation for Animals (OFA) maintains a hypothyroid registry to identify dogs as normal for breeding (see appendix D). A certificate and breed registry number is issued to all dogs found to be normal at 12 months of age, based on the results of FT4, cTSH, and thyroid autoantibody screening by an OFA-approved laboratory. Screening and registration is of value for dogs at risk of inherited hypothyroidism. If the test is positive, these dogs should not be used for breeding.

Michigan State University also has a thyroid registry and keeps statistics on thyroid function in dog breeds.

WebMD Veterinary Reference from "Dog Owner's Home Veterinary Handbook"

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