Canine thyroid disease can be tough to diagnose.The symptoms can be legion and sometimes contradictory: lethargy, mental lassitude, weight gain, dull coat, skin infections, constipation, diarrhea, cold intolerance, skin odor, hair loss, greasy skin, dry skin, reproductive problems, aggression, and more.
The associated diseases or conditions can be serious: megaesophagus, ruptured knee ligaments, testicular atrophy, cardiomyopathy, excessive bleeding, and corneal ulcers.
The disease can be inherited or of unknown or uncertain origin. The diagnosis can be complex; the treatment as simple as supplementing a basic essential hormone.
This is the description of canine hypothyroidism, the absence of sufficient thyroid hormone to maintain healthy body functions.
The mammalian body has 10 systems — skeletal, muscle, digestive, circulatory, excretory, integumentary (skin), respiratory, nervous, endocrine (ductless glands), and reproductive — that must be working properly for the animal to stay in good health.
Endocrine glands secrete hormones that help manage the body’s processes. The thyroid gland lies on the dog’s trachea, just below the larynx; triiodothyronine and levothyroxine, the hormones produced by the thyroid, govern the body’s basic metabolism — including control of growth and development and maintenance of protein, carbohydrate, and lipid metabolism — throughout life. Failure of the thyroid gland means trouble of one sort or another for the body.
The clinical signs can mimic other diseases. Weight gain, lethargy, mental dullness, skin abnormalities, weakness, and a decrease in tolerance for exercise are most often seen, along with behavioral changes that owners may not attribute to physical causes often occur: the sweet dog can become aggressive and the steady dog may become flighty or fearful.
Thyroid disease is considered the most common endocrine disease of dogs. Because susceptibility to one form of the disease may be inherited, it is of great concern to breeders. However, in spite of the attention the disease has received from researchers and the development of more precise diagnostic tests, hypothyroidism is not easy to identify. Part of the problem is that chronic or temporary illness, reproductive hormones, drugs, obesity, and exposure of the dog to temperature extremes can affect the test. Sometimes the only sure test is to supplement with thyroid hormone; if symptoms subside, the diagnosis was accurate.
Veterinarians may suggest a thyroid test if a pet has gained weight or is having chronic skin infections or if a breeding dog is experiencing reproductive difficulties, especially if the animal lacks energy and has a scruffy or dull coat. The veterinarian draws the blood and sends it to one of several laboratories with the equipment for conducting the test. 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. The latest tests include measurement of two forms of the thyroid hormones T3 (triiodothyronine) and T4 (levothyroxine) and a search for antibodies that could indicate autoimmune thyroiditis, the genetic form of the disease. Interpretation of the numbers recorded is as important as the numbers themselves, for the relationship between the hormones is complex. In addition, normal ranges of hormone vary somewhat with the breed or mix.
Treatment consists of two daily doses of levothyroxine, the hormone identified in the test as T4. Levothyroxine is converted to triiodothyronine by the body; dogs that cannot make this conversion will need both levothyroxine and triiodothyronine. The dosage is based on body weight; thyroid hormones are quickly metabolized and excreted from the body, so splitting the dose is most effective.
One to two months after starting the dog on thyroid therapy, the veterinarian will probably want to redo the tests to make sure the levels are within the normal range. Blood should be drawn four-to-six hours after the morning dose. Dogs on long-term thyroid therapy should have a complete panel of tests every six to 12 months.
Some dogs have a genetic susceptibility to diseases that attack their own immune system. Researchers suspect that these immune-mediated diseases may be triggered by environmental chemicals, viruses, repeated inoculation with multi-valent modified live vaccines, and other immune system challengers. The presence of autoantibodies in the thyroid test is considered by some researchers and breeders to be a forecaster of autoimmune lymphocytic thyroiditis — the inherited form of the disease — but other researchers consider the data base of information to be too small to make that call. However, breeders should test their dogs and bitches, keep good records, and be wary of using animals with the potential to further spread the disease.
Studies indicate that the breeds most commonly affected by autoimmune lymphocytic thyroiditis include Golden Retriever, Great Dane, Beagle, Borzoi, Shetland Sheepdog, American Cocker Spaniel, Labrador Retriever, Rottweiler, Boxer, Doberman Pinscher, German Shepherd, Akita, Old English Sheepdog, and Irish Setter. Symptoms usually appear between one and five years of age, but blood tests can indicate the potential for disease before clinical signs appear. Unfortunately, a clean thyroid test at one year of age does not mean the dog will remain free of disease throughout its life.
In August 1996, the American Kennel Club Canine Health Foundation hosted an international symposium on canine hypothy-roidism at the University of California at Davis. Here the world’s experts on the disease shared findings, asked and answered questions, and suggested avenues for further study to increase understanding of the disease, improve diagnostic tests, and identify a genetic marker for the inherited form of the disease. Until more is known, however, dog owners can watch their pets for the classic signs of thyroid disease manifestation as outlined above and potential dog owners can ask breeders if the sire and dam of that wonderful litter have been screened for thyroid disease or are taking thyroid medication. Even though the tests are not perfect, the answer will indicate a breeder's commitment to ridding his dogs of thyroid disorder.
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