Hypothyroidism is a common problem in dogs but rarely occurs in cats. The thyroid gland has a number of different functions but it is most well known for its role in regulating metabolism. Hypothyroidism is the condition that occurs when not enough thyroid hormone is produced. Hypothroidism 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. Hypothyroidism is easy to diagnose with a blood test that checks the level of various thyroid hormones including T3 and T4. Most hypothyroid dogs respond readily to treatment with synthetic thyroid medication such as Soloxine. Many dogs suffer from a low thyroid hormone level for years without treatment. If your dog has chronic recurrent skin problems, she may be suffering from hypothyroidism.
What causes hypothyroidism?
Hypothyroidism results from the impaired production and secretion of thyroid hormone. The production of thyroid hormone is influenced by the pituitary gland, the hypothalamus and the thyroid gland. Although dysfunction anywhere in the complicated hypothalamic-pituitary-thyroid pathway can result in hypothyroidism more than 95% of all cases occur as a result of destruction of the thyroid gland. About half of the causes of thyroid gland destruction are suspected to be caused by the dog's own immune system killing the cells of the thyroid gland. The other half is caused by atrophy of the thyroid tissue and resultant infiltration of the tissue by fat. The cause for this form of the disease is unknown.
Who gets hypothyroidism?
Although the onset of clinical signs is variable, hypothyroidism most commonly develops in middle-aged dogs between the ages of 4 to 10 years. The disorder usually affects mid to large size breeds of dogs, and is rare in toy and miniature breeds of dogs. Breeds that appear to be predisposed to developing the condition include the golden retriever, Doberman pinscher, Irish setter, miniature schnauzer, dachshund, cocker spaniel, and Airedale terrier. German shepherds and mixed breeds appear to be at a reduced risk of contracting the disease. There does not appear to be a sex predilection but spayed females appear to develop it more often than intact females.
What are the symptoms?
Thyroid hormone is needed for normal cellular metabolic function. A deficiency of thyroid hormone affects the metabolic function of all organ systems. As a result the symptoms are usually variable and non-specific. There is not a specific symptom that is diagnostic for hypothyroidism. There are however several symptoms that when combined together make the veterinarian more suspicious of the likelihood of the animal having the disease. A study on hypothyroid dogs revealed the following information on the variety and frequency of symptoms seen with the disease:
How is hypothyroidism diagnosed?
There are several different test used to diagnose hypothyroidism in the dog. The test chosen will depend on the symptoms and the availability of different tests to your veterinarian.
Baseline T4 Test: The most common test run is the baseline T4 test. A blood sample is drawn and tested by radioimmunoassay to determine the level of T4 thyroid hormone in the bloodstream. The T4 hormone is produced only in the thyroid gland and dogs with a failure of the thyroid gland will have a lowered level of this hormone. However there are other conditions that can cause a lowering of T4 so if this screening test is positive for hypothyroidism another more specific test is often done to confirm the diagnosis.
Baseline T3 Test: Another screening test that can be run is the baseline T3 test. T3 is another form of thyroid hormone found in the bloodstream. This test can be used as a screening test instead of T4. The T3 test is not as accurate in early cases of hypothyroidism and occasionally will be normal when the T4 level is reduced. For these reasons this test is often used in combination with the T4 test or with the TSH stimulation test.
TSH Stimulation Test: The TSH stimulation test is the most definitive blood test available for diagnosing hypothyroidism in the dog. If a dog has a low T4 or T3 level this test may be performed to confirm a diagnosis of hypothyroidism. A small amount of Thyroid Stimulating Hormone (TSH) is injected into the vein. After 6 hours a blood sample is drawn and the T4 level is checked. A dog without thyroid disease that may have other conditions causing a low T4 will have a high T4 level after the TSH injection. A dog with true hypothyroidism will not have an increase in T4 after the injection.
As I mentioned earlier, 95 % of thyroid disease is caused by destruction or loss of the thyroid gland function. If hypothyroidism is suspected but not confirmed by these three described tests, then it is possible that the condition may be caused by one of the other 5% of conditions that cause hypothyroidism. To diagnose those problems, one or several of the following tests may be used: TRH stimulation test, serum total reverse T3 concentration (a radioimmunoassay), serum free T4, and serum free T3 concentration.
How is hypothyroidism treated?
One of the nicest things about this disease is that it is easily treated. Treatment consists of putting the dog on a daily dose of synthetic thyroid hormone called thyroxine (levothyroxine). There are numerous brand names of this drug. The dose and frequency of administration of this drug varies depending on the severity of the disease and the individual response of the animal to the drug. A dog is usually placed on a standard dose for his weight and then blood samples are drawn periodically to check his response and then the dose is adjusted accordingly. Once therapy is started the dog will need to be on treatment for the rest of its life. Usually after the treatment is started the majority of the symptoms resolve.
Joe Bodewes, DVM