Harger Chihuahuas

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Hypothyroidism

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Hypothyroidism

Veterinary & Aquatic Services Department, Drs. Foster & Smith, Inc.

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.

© 2004 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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