Hypothyroidism
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. Hypothyroidism 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?
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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:
Clinical Symptoms
Percentage of Dogs Showing Symptoms
Lethargy/mental dullness
Hair loss
Weight gain/obesity
Dry hair coat/excessive shedding
Hyperpigmentation of the skin
High blood cholesterol
Anemia
How is Hypothyroidism Diagnosed?
There are several different tests 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 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:
TSH 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 his life. Usually after the treatment is
started, the majority of the symptoms resolve.
Reference for Further Reading
Bonagura, J. Kirks Current Veterinary Therapy XII. W.B. Saunders Co.
Philadelphia, PA; 2000.
Bonagura, J. Kirk's Current Veterinary Therapy XIII. W.B. Saunders Co.
Philadelphia, PA; 1995.
Ettinger, S. Textbook of Veterinary Internal Medicine. W.B. Saunders Co.
Philadelphia, PA; 1989.
© 2004 Drs. Foster & Smith, Inc.
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