Canine Hypothyroid Disease
By Geneva Coats
There are many different glands
in the body. A gland is a type of tissue which produces and
secretes a specific product. There are two main types of glands. There are endocrine glands, which secrete their product into
the bloodstream to eventually reach a target tissue. Examples of endocrine glands are the adrenal glands, the sex organs,
the pituitary and they thyroid gland. There are also glands known as exocrine glands which secrete their products via a duct,
or tube. Examples of exocrine glands are the glands which produce sweat, tears. saliva, and digestive enzymes, and the mammary
glands which produce milk.
The thyroid gland is
a butterfly–shaped endocrine gland located in the front
of the neck, just below the larynx (voice box). There are two lobes of the thyroid, one on each side of the trachea.
The thyroid gland produces a hormone called thyroxine. Thyroxine regulates
the body's growth, metabolism, digestion, body temperature and heartbeat. A deficiency of thyroid hormone in the body is known
as hypothyroidism, while an excess
of thyroid hormone circulating in the body is known as hyperthyroidism.
Hypothyroidism is one of the
most common medical disorders affecting dogs. It is estimated that one out of ten dogs will develop hypothyroidism in its
lifetime. Certain breeds are highly disposed to develop hypothyroidism. Middle aged dogs are at increased risk. In one study,
mean age at diagnosis was 7.2 years, Spayed females and neutered males are at increased risk over intact animals for developing
hypothyroidism.
Hypothyroidism usually comes on gradually, over several months or years. Because the thyroid hormone supports the metabolism of all body systems, the
symptoms can be numerous and varied. The early clues may
be scarcely noticeable or may be attributed to other causes. Symptoms may include low energy, weight gain, depression, intolerance
to cold, infertility, dry skin, brittle/thinning coat, frequent infections, (especially of skin, ears and eyes). Your veterinarian
may notice enlargement of the gland, a slowed heart rate, anemia and elevated blood cholesterol. An ultrasound of the enlarging
thyroid may be useful for early diagnosis of thyroid problems.
Under normal conditions, the
brain senses and detects drops in thyroid hormoneblood levels. When thyroid hormone levels in the body start to drop, the
hypothalamus portion of the brain nudges the pituitary gland to send out a hormone known as TSH, or Thyroid Stimulating Hormone.
TSH stimulates the thyroid gland to step up production of the thyroid hormone known as T4. T4 is subsequently converted by
the body to T3, which is the active form used by the cells of the body. When the brain detects a surplus of thyroid hormone
in the bloodstream, it drops the production of TSH, and then with lower TSH levels, the thyroid gland in turn slows production.
When thyroid hormone levels again drop a bit, TSH levels rise to stimulate the thyroid gland to once again step up production.
This relationship between TSH and thyroid hormone is the ongoing natural feedback process by which thyroid hormone level remains
optimal for proper body metabolism.
Sometimes there is the temptation
to supplement your dog with thyroid hormone in the belief that it will boost metabolism and promote weight loss or enhance
fertility. This is not a good idea in a dog with normal thyroid function, because in so doing you depress the production of
TSH by the brain, and upset the body’s delicate natural feedback mechanism which is necessary for proper thyroid balance
and function. When the supplement is withdrawn, it is difficult for the body to regain balance of these hormones again. Also,
diagnostic thyroid testing will be inaccurate if the dog is receiving a thyroid supplement.
When your vet does a blood
test for thyroid function, he will usually check the T4 level and the TSH level.
If the T4 is low and the TSH is high, a diagnosis of hypothyroidism can be presumed. The brain is trying to stimulate the
thyroid gland, but the gland is not able to respond, hence the high TSH and low T4.
T4 testing is a simple technique that is relatively inexpensive; however, it is not always reliable as the results can
appear falsely low due to the effects of medication or other medical conditions. As a result, another technique known as Free
T4 or FT4 testing may be required. This test is much more reliable and is far more reflective of the thyroid's true state
and function. This method of testing is more expensive than basic T4 testing, but it may be required in order to make a confident
diagnosis. T3 levels fluctuate widely, and are not generally useful in diagnosis.
According
to a recent MSU study, the majority of canine hypothyroidism is the result of an autoimmune process known as autoimmune thyroiditis.
The body's immune system develops antibodies against its
own thyroid gland cells. As the thyroid gland cells are attacked and destroyed, the remaining cells work harder to compensate.
Finally, when the gland is about 75% destroyed, the remaining cells are unable to produce enough thyroid hormone, and the
dog begins to display symptoms of deficiency.
Autoimmune thyroiditis is felt to have an inherited basis. This makes testing
for the disease marker in breeding dogs important for reducing its incidence.
The
best current marker for this disease is a positive TgAA test result. TGAA is a test specifically for thyroid
autoantibodies, and is usually diagnostic of autoimmune thyroiditis. This is a useful test because it can predict development of thyroid dysfunction many years before the dog actually
develops any symptoms.
These
TgAA autoantibodies are present when there is active thyroid disease or inflammation. Assuming a single gene disorder and
recessive trait, TgAA will be positive only in dogs having both genes for thyroiditis. TgAA will not be positive in the carriers
(having only one gene for the trait) and may not be positive early in the life of some dogs that become affected later.
Studies in a laboratory suggest
that vaccination may also cause stimulation of the production of anti-thyroglobulin antibodies. Whether other variables such
as viral infections can also cause transient thyroiditis is unknown.
The OFA established a registry for canine hypothyroidism. The purpose is to assist breeders in selecting sound dogs
for breeding purposes and to develop and maintain a database for the study of hypothyroidism in dogs. The dog’s owner
takes the dog to their regular veterinarian who draws blood for submission to a certified laboratory. The laboratory performs
the tests and report to the owner, his veterinarian and the OFA. At the time of submission, the owner has the option of placing
results in an open or closed registry. A certificate and breed registry number will be issued to all dogs found to be normal
at 12 months of age.
While a blood test can detect the disease before the appearance
of clinical signs, a single negative blood test will not guarantee that a dog will never develop thyroid disease. A dog that
is negative at 2 years old can still become positive at 6 years old. It is recommended
that re-examination occur at ages 2, 3, 4, 6, and 8 years.
From studies done on all breeds of dogs at Michigan State University (MSU)
it was concluded that:
- Very few dogs developed TGAA antibodies under the
age of 1 year, so testing should be started after 1 year.
- The majority of dogs who would go on to develop
thyroiditis tested positive for high TGAA between the ages of 2 and 6 years.
- After the age of 6 years, dogs with immune
mediated thyroid disease had enough damage to the gland that there was not enough tissue present to stimulate the production
of TGAA, which would cause measured levels of TGAA to fall within the normal range.
Recommendations for breeders:
- All breeding stock can be tested for early autoimmune
thyroid disease by measuring only TGAA between the ages of 12 and 30 months.
- Between 30 months and 6 years a TGAA along with
a T4 and TSH levels should be measured to rule out immune mediated thyroid disease and early hypothyroidism.
- If the dog is negative for TGAA after 6 years and
there is no abnormality of the T4 and TSH, the dog will probably not develop autoimmune thyroid disease.
- Dogs positive for TGAA should have a T4 and TSH
run annually to monitor thyroid function. When clinical signs develop the dog should be put on thyroid supplementation.
- Carefully consider whether the dog should be used
in a breeding program. If dogs with high TGAA are bred it should be to dogs that are negative for TGAA and/or tested clear
(normal T4 and TSH levels) for thyroid disease as older dogs.
Once a diagnosis of hypothyroidism
is made, the standard treatment is oral thyroid hormone supplement for life. In most cases this means a T4 supplement. A periodic
check of thyroid levels will assist with determining the optimal dose. Several medications available are used as a replacement for T3, T4, or their combination.
Levothroid, Levoxyl, Synthroid are brand names of the chemical compound levothyroxine
sodium (T4). Cytomel is a brand name of the chemical liothyronine sodium (T3). Euthroid and Thyrolar are names for the chemical
liotrix (a combination of T3 and T4).
Armour Thyroid is the
brand name of a desiccated thyroid which is not normally recommended for replacement therapy, since it contain significant
amounts T3, the active form of the hormone. Synthetic T4 is preferred by veterinarians.
Trying
to regulate doses when using the active form, T3, is almost impossible because T3 is rapidly metabolized and depleted from
the body. T4 blood levels remain more stable and are more useful in regulating treatment. (Remember, T4 is converted by the
body to the active form, T3, as needed). Supplementing with T3 instead of T4 can result in overstimulation of the hypothalamus-pituitary-thyroid
axis, due to lack of T4 in the feedback chain. This can provoke a thyroid storm or hyperthyroidism (see below) if there is much functioning thyroid tissue remaining.
In some rare cases your vet may need to prescribe T3 or a T3-T4 preparation if T4 alone is ineffective.
Other conditions are associated
with hypothyroidism, such as Cushing’s disease and diabetes. High levels of steroids in Cushing’s disease can
cause low levels of circulating thyroid hormone. Administration of steroids such as prednisone or cortisone over a prolonged
period of time can affect thyroid hormone levels.
Many drugs, medication,
toxins and preservatives can adversely affect thyroid function. There is also a condition known as euthyroid sick syndrome. Thyroid function is depressed temporarily due to another
illness. Once the underlying illness is resolved, thyroid function will usually also recover.
A little information about canine hyperthyroidism.
Hyperthyroidism occurs when there is excess circulating thyroxin. This condition is not common and usually occurs
in dogs with tumors of the thyroid gland. The T4 will be above normal and the TSH is below normal in this situation. A symptom of canine hyperthyroidism is weight loss despite an unusually ravenous appetite.
Sometimes the dog will eat so quickly that he ends up vomiting. Some dogs may also experience diarrhea. Also, excessive thirst
may accompany the characteristic voracious appetite.
Other very common warning signs
of this disease include hyperactivity and nervousness. Your dog may become more active and restless than normal and may also
show signs of irritability. These symptoms are often accompanied by an elevated heart rate. In addition to beating faster,
an affected dog's heart may also exhibit stronger beats, which you may be able to feel by placing your hand against your dog's
chest. Some dogs that are suffering from hyperthyroidism also tend to breathe more rapidly than usual. These general symptoms
are due to an increase in the metabolism of the body
A dog with hyperthyroidism may also experience changes in its coat. The coat may become dull and appear unusually unkempt,
and the dog can eventually develop bald patches.
Referenes:
“Hypothyroidism” by Susan Thorpe-Vargas,
PhD, and John Cargill, MA, MBA, MS “Diagnosis of Canine Hypothyroidism and Thyroiditis”, by J. Catharine Scott-Moncrieff,
MA, MS, Vet MB, Dipl. ACVIM (internal medicine) Dipl. ECVIM-CA, Purdue University, West Lafayette, In, 47907
“Canine Hypothyroidism; An Overview”,
Roe Freeman, DVM
“Thyroid Study in Welsh Springer Spaniels”,
Colleen O’Keefe, DVM, MS; 2001
Websites with useful information:
THYROID TESTS FOR DOGS
|
TEST |
ADVANTAGES |
LIMITATIONS |
|
TT3 |
Simple,
Inexpensive |
Does
not reflect thyroid function |
|
TT4 |
Simple,
Inexpensive |
Can
be falsely low due to drugs or non-thyroid illness |
|
FREE
T4 |
Better
reflects true thyroid function |
Relatively
expensive |
|
TSH
STIMULATION |
Currently
the best indicator of thyroid function because it measures thyroid gland response to stimulation |
Cost
& Availability |
|
TRH
STIMULATION |
Also
measures thyroid gland response to stimulation but harder to interpret |
Cost
& Availability
Side Effects |
|
TSH
CONCENTRATION |
Potentially
may be best indicator of thyroid gland function when used with Free T4 |
Limited
use so far; moderately expensive |
|
THYROGLOBULIN
ANTIBODIES |
Early
detection; Specific for immune-mediated thyroid involvement |
Can
be positive in normal dogs; Currently available at Southwest Veterinary Diagnostics, Inc., Phoenix, Arizona |
|
THYROID
HORMONE ANTIBODIES |
Early
detection; Specific for immune-mediated thyroid involvement |
Can
be positive in normal dogs; Available at Michigan State University |
|
THYROID
BIOPSY |
Demonstrates
specific thyroid involvement; distinguishes primary from secondary hypothyroidism |
Cost,
surgery, anesthesia; May not reflect degree of thyroid dysfunction |
Karen J. Wolfsheimer, D.V.M., Ph.D. Diplomate,
American College of Veterinary Internal Medicine Associate Professor Department of Physiology, Pharmacology & Toxicology
and Colleen Brady, B.A. Veterinary Student School of Veterinary Medicine Louisiana State University Baton Rouge, LA 70803
