Hypothyroidism in Dogs

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.

Diagnosis
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
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.

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