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?
| A hypothyroid dog will need to be on
thyroxine for the rest of his life. |
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.
© 2006 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
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The Chihuahua Club of America Thanks Drs. Foster &
Smith for use of their articles.
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