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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?
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 |
70
|
| Hair loss |
65
|
| Weight gain/obesity |
60
|
| Dry hair coat/excessive shedding |
60
|
| Hyperpigmentation of the skin |
25
|
| Cold intolerance |
15
|
| Slow heart rate |
10
|
| High blood cholesterol |
80
|
| Anemia |
50
|
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.
References and 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.
© 2001 Drs. Foster & Smith, Inc.
Reprinted as a courtesy and with permission from PetEducation.com
(http://www.PetEducation.com)
On-line store at
http://www.DrsFosterSmith.com
Free pet supply catalog: 1-800-323-4208
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