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Hypothyroidism in Dogs

By: Dr. Bari Spielman

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Veterinary care includes using diagnostic tests to confirm the presence of hypothyroidism and any secondary changes in the body, as well as instituting appropriate therapeutic and monitoring procedures.

Diagnosis In-depth

Certain diagnostic tests must be performed to make a definitive diagnose of hypothyroidism and exclude other disease processes that may cause similar symptoms. A complete history and a thorough physical examination are essential in reaching the correct diagnosis. The following diagnostic tests are often recommended:

  • A complete blood count (CBC) may reveal a chronic anemia.

  • A biochemical profile may show the presence of high cholesterol and other fats in the blood. Sometimes liver enzyme tests and creatine kinase (CK), a muscle enzyme, are also elevated.

  • A urinalysis is usually within normal limits.

  • Thoracic (chest) and abdominal radiographs (X-rays) may be normal, except for evidence of obesity, but they are important to rule out other causes of similar clinical signs.

  • Thyroid function tests are necessary to confirm the presence of hypothyroidism. There are three common tests used to assess thyroid function in the dog. These include:

    - A baseline total thyroxine (T4) level is commonly measured. A thyroid level in the normal or upper T4 level generally indicates that the dog is not hyp0thyroid. A low T4 level does not always confirm hypothyroidism, however, because other illnesses and certain drugs may affect the thyroid gland and suppress T4 levels. With a low T4 value, further testing may be needed. If clinical symptoms suggest hypothyroidism and the T4 level is normal, additional diagnostic testing is recommended.

    - Serum Free T4 by Equilibrium Dialysis (FT4ED) measures the free available T4 in the blood. With this assay, thyroxine is separated from its carrier proteins, which eliminates the effects of other illnesses and most drugs on the levels of T4 measured. This is the single most accurate test for diagnosing hypothyroidism however it not using a routine screening test because it is more time intensive and expensive to run. A low value on this test is more indicative of hypothyroidism.

    - cTSH - Measurement of canine thyroid-stimulating hormone (cTSH) may also be performed. This test is usually elevated in dogs with hypothyroidism. An elevation occurs in this test because the pituitary gland detects that the levels of thyroxine circulating in the blood are low, and it secretes more TSH that travels to the thyroid gland to stimulate it to make more thyroxine.

    Many veterinarians prefer to measure T4, fT4 and cTSH all at the same time, and then assess how their results compare. A low T4 or fT4 in the presence of an elevated TSH confirms the diagnosis of hypothyroidism in most cases.

  • A positive response to therapy with sodium levothyroxine also helps to confirm the diagnosis.

    Your veterinarian may recommend additional tests to exclude or diagnose other conditions. They are selected on a case-by-case basis:

  • A thyroid gland biopsy may be recommended, especially in those cases where cancer is likely.

  • A radioactive nuclear scan may be used to assess the function of the thyroid gland. Hyperactive glands take up more radioactivity, where as hypothyroid glands take up less.

  • Measurement of antibodies to T4 and thyroglobulin (a thyroxin-binding protein) may be done. Elevations in these antibodies often indicate immune-mediated inflammation of the thyroid gland.

  • Other tests that may be run depend on the clinical signs and what other diseases are being considered as other potential causes of those signs.

    Therapy In-depth

  • Thyroid hormone supplementation is indicated for the treatment of hypothyroidism, and it is administered for the life of the individual.

  • Synthetic (man-made) levothyroxine sodium(synthetic T4) is the drug of choice for treating hypothyroidism. The dosage and frequency of administration are determined by your veterinarian, and are usually based on the dog's body weight. Levothyroxine is usually given twice daily in dogs. A liquid formulation is available in Europe.

  • Dogs with concurrent diseases such as heart disease, kidney failure, liver disease, diabetes, and/or hypoadenocorticism (Addison's disease) should be given a dose reduced by 25 to 50% and gradually increased.

  • There are both brand name and generic levothyroxine products available commercially. Use of a brand name product rather than a generic product is usually preferred. There appears to be great differences in how dogs respond to some generic products, and some dogs do not seem to respond very well to the generic drugs. Once a dog is stabilized on a particular thyroid medication, it is also better if the dog remains on that product consistently, rather than bouncing from one product to another.

  • Liothyronine sodium (synthetic T3) is rarely given and is generally given only when a dog with hypothyroidism has not responded ideally to treatment with levothyroxine sodium (synthetic T4) supplementation. Combining products is not recommended.

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