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CANINE HYPOTHYROIDISM |
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Hypothyroidism in The Dog
Hypothyroidism in Dogs
What is Hypothyroidism
Hypothyroidism in the dog can be due to a number of issues associated with the production of specific thyroid hormones, with the delivery of those hormones to target tissues, with immune mediated interference with proper chemical structure the hormones and with damage to the thyroid gland itself. Hypothyroidism is the most common endocrine gland disorder in the dog. The endocrine glands are listed below. All of these tissues secrete chemical substances called hormones that have a profound effect on certain "target" tissues. A hormone is a chemical that acts on or with tissues throughout the body and is carried away from the gland of production by the blood stream. Saliva, for example, is secreted by various salivary glands but is not considered a hormone because it leaves the glands via a small duct (a small tube) and is not transported by the blood stream and does not have an effect on any body tissue physiology.
| Thyroid Gland (the Master Gland)
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| Hypothalamus |
Pancreas |
Thymus |
Anterior Pituitary |
Pineal Gland |
| Testis |
Ovary |
Parathyroid Glands |
Posterior Pituitary |
Adrenal Glands |
Of all those glands and
hormones, the thyroid gland is considered the Master Gland. If
the thyroid gland is malfunctioning, every cell in the body is affected.
No wonder dogs, properly diagnosed with hypothyroidism and placed on
appropriate thyroid hormone supplements, will look, act and feel so much
better than before treatment.
SIGNS
Most dogs show signs of hypothyroidism between 3 and 5 years
of age but a diagnosis isn't made in some
dogs until they are years older. Males and females
seem to be equally affected. Routine testing of young dogs is
not commonly done unless the veterinarian has a reason to suspect
low thyroid function. Some veterinarians believe that if a dog
has not acquired hypothyroidism by 5 or 6 years of age, the odds are
good that it will never be a problem for that individual. (Cats
rarely acquire hypothyroidism but have more trouble with hyperthyroidism,
especially older cats.)
One of the most commonly seen signs that a dog may have insufficient
secretion of thyroid hormone, or issues with immune mediated thyroid
dysfunction, is weight gain with apparently little food intake.
Any overweight pet should be checked for hypothyroidism. (See this
page for tips on how to get a pet to lose weight.) In
long-standing cases there often is lack of proper hair coat
and even hair loss. This loss generally displays a pattern over
the lumbar area on both sides, sparse hairs on the back of the rear legs
and lack of hair along the abdomen. The coat tends to lack luster
and the finer hairs of the undercoat may be missing entirely. Most
dogs with hypothyroidism lack energy, prefer warm environments and have
poor exercise tolerance. Of great concern to breeders is the fact that
dogs with hypothyroidism may be infertile and many breeders have their
dogs tested for thyroid function prior to breeding; in dogs with a poor
breeding history, hypothyroidism often is the culprit.
| Hypothyroidism is rare in toy and miniature
breeds of dogs. Research definitely indicates that most hypothyroid cases are inherited. In fact it is fairly common in certain breeds such as... |
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| Golden Retrievers |
Doberman pinschers |
Greyhounds... Caution! Updated info on Greyhounds and their apparent normally low thyroid hormone levels can be viewed here |
| Irish Setters |
Dachshunds |
Cocker Spaniels
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| Shetland Sheepdog |
Boxer |
English Setter
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DIAGNOSIS
If the physical exam or history indicates probable hypothyroidism,
the veterinarian will take a blood sample and have one or more tests
run. The most common tests for thyroid function are T4 (the main Thyroid
hormone) and canine TSH (Thyroid Stimulating Hormone from
the Pituitary Gland). If the screening test using the T4 analysis indicates
low a value for T4, further evaluation is very important in determining
the actual origin of the dysfunction causing the signs of hypothyroidism.
Some veterinary laboratories now recommend TgAA (Thyroglobin Auto-antibody)
analysis be done because it identifies thyroiditis much earlier in the
progression of the disease. All of these assays are relatively inexpensive,
and the information they provide is very important for establishing a
diagnosis. University Veterinary Medical Colleges such as the U.
of Illinois and Michigan State University have state of the art diagnostic
instrumentation that have advanced the ability and accuracy of local veterinarians
attempts to establish the presence of hypothyroidism in their patients.
TREATMENT
L-thyroxine (T4) tablets are generally administered twice
a day. When given twice a day, some veterinarians prescribe
0.1 mg/10 pounds twice per day as the initial dosage but each case
is considered individually. Repeat exams and occasional follow-up
blood testing really helps to fine tune the proper amount of medication
needed for each patient.
Is hypothyroidism inherited? Evidence indicates some
familial patterns of inheritance. There are more than half a dozen
studies reporting the familial incidence of autoimmune thyroiditis where
the dog's immune system actually damages its own thyroid gland.
Much of the research on hypothyroidism is dogs has been done at Michigan
State University Veterinary School.
Not all cases of hypothyroidism are due to autoimmune lymphocytic
infiltration of the gland. There can be other "inducers" of
the disease such as consumption of too much Iodine. These inducers
can be very difficult to identify. Humans may acquire what is called
Hashimoto's Disease, a genetically transmitted form of hypothyroidism
but this disease is not the same as autoimmune thyroiditis in dogs.
In Hashimoto's Disease females are five times more likely to get the disease
than males. There are other differences, as well.
The following case represents
a fairly atypical case of hypothyroidism in that most cases are identified
prior to such advanced skin and coat signs. The dog had been
treated for non-specific allergies, and in fact does some allergic problems
and skin scarring due to chronic inflammation of the skin. However,
once the Hypothyroidism was discovered and proper treatment instituted,
the allergic conditions were less severe simply because the skin became
much healthier and more resistant to infections, irritants and allergens.
This is how a dog recently looked when presented...
Hypothyroidism was suspected and the blood values of thyroid hormone
supported the presumptive diagnosis. The dog was placed
on Soloxine in early June, 2001 and the photos in the second row show
the improvement in her after six weeks on medication. You can see what
a change can occur when a diagnosis of Hypothyroidism is made and proper
therapy instituted. The dog's owners report excellent new growth of hair,
renewed vigor and weight loss... all improvements!
| Test |
Results |
Normal Ranges |
| Total Thyroxine (TT4) |
8.0 |
Normal is 15 to 50 |
| Total Triiodothyronine (TT3) |
0.7 |
Normal is 1.0 to 2.5 |
| Free (unbound) T4 |
3.0 |
Normal is 12 to 33 |
| Free (unbound) T3 |
QNS |
(Insufficient quantity to test) |
| T4 Autoantibody |
7.0 |
Normal is less than 20 |
| T3 Autoantibody |
3.0 |
Normal is less than10 |
| TSH (Thyroid Stimulating Hormone) |
55 |
Normal is up to 37mU/L |
| Thyroglobin Autoantibody |
88 |
Normal is less than 200 |
| Click on the image to see a close-up
view. |
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| Above... Before Treatment
Below... After Treatment Click to enlarge the photos |
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click to enlarge photos |
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This dog will need to be on thyroid
supplementation for life but the medication is not expensive and the patient
has resumed a more normal attitude and looks much improved. Be sure
to ask your veterinarian to check for hypothyroidism if your dog displays
a poor coat, weight gain, poor exercise tolerance and seeks warm areas in
which to spend time.
(See this page
for tips on how to get a pet to lose weight.)
The study of Greyhounds
and Thyroid Testing has been competed and the results reported in an
article written by Dr. Marcus Rick and published in the GCA Fall 2005
Newsletter. Of one hundred samples that were collected, 98 were
analyzed statistically to establish a reference range of thyroid test
results in the Greyhound. The majority of the samples were from
AKC registered Greyhounds, seven were from NGA dogs. The results
were then compared graphically with the reference range for the general
dog population used at Michigan State University and the result ranges
from three previous studies by other researchers.
Dr. Rick states “Overall it can be seen that the reference
range of Greyhounds is considerably less than that of the general canine
reference range.” In very short summary he concludes:
1.) Overall, hypothyroidism is very unlikely in the
Greyhound, however well documented cases exist.
2.) If either total T4 of free T4 by dialysis are detectable
with the currently available assays (GT 6nmol/L, GT 2pmol/L, respectively)
it is very likely that your Greyhound is NOT hypothyroid.
3.) Lymphocytic thyroiditis is unlikely as well.
Of 1,409 Greyhound samples received at Michigan State University, 2
% were TgAA positive compared with 10 % of 143,800 dogs of the general
dog population. Of these 1,409, only 43 (3%) had thyroid results
including elevated TSH consistent with hypothyroidism compared to 8 % of
the general 143,800 samples.
4.) Diagnosing hypothyroidism might be difficult due
to decreased reference values that, in many cases, are below the detection
limit of currently available assays.
5.) Greyhound-specific complaints, including bald thigh
syndrome, poor racing performance, and infertility do not have an
association to thyroid hormone concentrations. Supplementing
these animals with thyroxine is not recommended.
Hypothyroidism is the most common
hormone imbalance of dogs. It seems like it would be a straight forward
problem: the body does not produce enough thyroid hormone, problems
result, a test shows thyroid hormone level is low, the hormone replaced
given in pill form, problem solved. Unfortunately, it is not quite so
straight-forward in real life. This article attempts to review the issues
relevant to this condition and the pitfalls that keep it from being
a simple problem.
WHAT IS THE THYROID GLAND AND WHAT DOES IT DO?
The thyroid gland is an H-shape in one's throat. It produces
two forms of thyroid hormone: T3 is the active form of
the hormone, and T4 is the inactive form created to circulate
in the bloodstream. When T4 is absorbed out the bloodstream and into
tissue cells, it is converted into T3. Most of the circulating T4 is
carried by blood proteins and is not available for tissue absorption; the
portion that is not carried by proteins (the so-called "free T4") is the
portion that is able to enter tissues for activation.
Production of T4 is regulated by the pituitary gland at
the base of one's brain. (This gland is called the master gland as
it regulates hormone production in the adrenal system, the thyroid
system, the reproductive system and more). The pituitary produces a
substance called TSH, which stands for thyroid stimulating hormone. When
T4 levels are dropping, the pituitary gland stimulates the thyroid gland
to make and release more T4.
Active thyroid hormone serves as a sort of a volume dial
for metabolism. Since virtually every cell in the body can be affected
by reduced levels of thyroid hormone it is not surprising that reduced
levels of thyroid hormone lead to symptoms in multiple body systems.
WHAT IS HYPOTHYROIDISM?
In short, hypothyroidism is the natural deficiency of thyroid
hormone. This deficiency is produced by immune-mediated destruction
of the thyroid gland, by natural atrophy of the gland, by dietary
iodine deficiency, or as a congenital problem. In the dog, the first
two causes listed account for almost all cases.
Hypothyroidim generally develops in middle aged or elderly
dogs. Breeds with definite predisposition to develop hypothyroidism
include: the Doberman pinscher, the Golden retriever, the Irish Setter,
the Great Dane, the Dachshund, and the Boxer.
MANIFESTATIONS OF HYPOTHYROIDISM
Hypothyroidism is a classical disease with a classical collection
of clinical signs. One particularly well published survey of 162
confirmed hypothyroid dogs showed the following common findings:
88% had some kind of skin abnormality
40% had hair loss (often this starts on the tail leading to a "rat tail" appearance or a bald area around the collar is created)
22% had skin infection (often dogs are scaly and smelly due to an excessively oily coat)
14% had brittle or dry coats (often the outer hairs break off leaving a short, softer under coat, classically described as a "puppy-like coat")
49% were obese
48% were described as lethargic or listless at home
36% were anemic (had a reduced
number of red blood cells due to slowed red blood cell production
in the bone marrow.)
One classical finding in hypothyroid dogs is a thickening
of some tissues, especially of the face and head. The skin in particular
thickens leading to more skin folds and what is classically referred
to as a tragic face. This thickening is called myxedema and can occur
in some other tissues as well (such as facial nerves - see later).
Cardiovascular Signs - Hypothyroidism interferes with
the electrical fibers that more or less provide the wiring for the
heart. The rhythmic contractions of heart muscle as normally stimulated
by these electrochemical fibers. Abnormal rhythms or slow heart rate
occur in as many as 26% of hypothyroid dogs; still, the significance
of this in terms of overall lifestyle is not clear. Many specialists
feel thyroid supplementation should be started at a lower dose for patients
with obvious heart disease.
Neurologic Signs - According to surveys of confirmed
hypothyroid dogs, only about 2% to 4% have nerve problems. There
are several syndromes reported.
Polyneuropathy - In hypothyroidism, nerves simply
do not conduct electrical impulses normally. This may account for some
of the general weakness and listlessness seen in hypothyroidism. Response
to thyroid hormone therapy is rapid (improvement within the first week
of treatment).
Focal Neuropathy - Single nerves can get entrapped
as they exit the skull or spinal cord as they (like other tissues)
swell with myxedema. Pressure on these nerves can lead to paralysis
of the facial muscles and/or head tilt, bizarre eye motions and balance
disruption (vestibular disease).
Central Nervous System - may represent abnormal electrical
conduction within nerves; however, vascular disease has been found
in hypothyroid dogs with central nervous system signs. Clinical signs
have involved ataxia (drunken gait), hemiparesis (weakness in front
and back legs on the same side of the body), hypermetria (inappropriate
measurement of steps), head tilt, circling and cranial nerve abnormalities.
It may take several months of therapy to see a response. Coma from myxedema
in the brain's tissues is a rare possibility but has been known to occur.
Ocular Signs - Ocular changes are not common in hypothyroidism
but the high levels of blood cholesterol and circulating fat can
sometimes lead to eye changes. When these changes are seen, often
thyroid testing is recommended. Corneal dystrophy, an abnormal change
in the clear covering of the eye, is such an eye sign. This finding
is usually represented as a small white spot (sometimes a white circle)
on the eye surface. At this degree it is only a cosmetic problem and does
not interfere with vision. In more severe forms, painful bubbles can
erupt on the corneal surface leading to ulceration. Obviously, this form
would require treatment.
Conditions not proven (but previously suspected) to be associated
with hypothyroidism: megaesophagus, laryngeal paralysis, infertility,
and behavioral aggression.
VON WILLEBRAND'S DISEASE
For a long time, von Willebrand's disease, a hereditary
blood clotting disease, was felt to have an association with hypothyroidism
(when a dog borderline for von Willebrand's factor levels became hypothyroid
later in life, the abnormal clotting ability would become evident). This
theory has largely been abandoned but there does seem to be a positive
effect when a von Willebrand's dog is treated with thyroid hormone.
TESTING FOR HYPOTHYROIDISM
One would think testing for hypothyroidism would be simple:
a blood test of the T3 or T4 level could be checked and if it is
low, the patient is hypothyroid. Unfortunately, the situation is rarely
so simple.
Measurement of T4: The T4 level (also called the
total T4 level) is measured commonly and is included in most routine
blood panels. It would seem that a low T4 would indicate hypothyroidism
and a normal T4 would indicate normal thyroid function. Unfortunately,
it isn't that simple. Dogs on certain drugs (most notably phenobarbital,
prednisoneor other corticosteroids, or trimethoprim sulfaor with illnesses
other than thyroid disease often have depressed T4 secretion. These
dogs will have low T4's but are not hypothyroid. This means a normal
T4 indicates normal thyroid function but a low T4 may or may not indicate
hypothyroidism.
Measurement of T3: If T3 is the active hormone, why
can't we just test blood levels of T3? Due to assorted compensatory
mechanisms, T3 levels often fluctuate into the normal range in even
truly hypothyroid dogs. This means that T3 values are virtually useless
in diagnosing hypothyroidism.
Free T4 - T4 is the precursor hormone that is not
active but is converted by body tissues into T3, which is active. T4
exists in two forms: the form that is carried around bonded to a blood
protein (this is called bound T4) and T4 floating around loose in the
bloodstream (called free T4). Only free T4 can enter cells and be converted
to T3 and the concentration of free T4 corresponds to thyroid hormone
activity where it counts (i.e., at the tissue level). Free T4 levels
are less subject to fluctuate into a falsely low range in response to
non-thyroidal diseases or drugs than is a total T4 level.
Because free T4 levels are typically 1000 times smaller
than total T4 levels and tests for free T4 have to work in such a
way so as not to convert bound T4 into free T4 and thus interfere with
results. It is somewhat difficult to test for free T4 and the only
acceptable way to do this is by a method called Equilibrium dialysis.
At this time most labs offer equilibrium dialysis free T4 but it is
important to realize that a free T4 level on a lab report is not an
equilibrium dialysis free T4 unless it specifically says so. (Equilibrium
dialysis free T4 is sometimes notated fT4(ED)).
TSH Stimulation and Endogenous TSH levels - Before
there was readily available equilibrium dialysis free T4, the test
that was felt to be the gold standard of reliability was called the
TSH stimulation test. In this test, a T4 was measured and then compared
with a value drawn 8 hours after administration of an injection of thyroid
stimulating hormone. Inability to respond to TSH was considered indicative
of hypothyroidism. The problem was expense, the need for 8 hours of
hospitalization, and the fact that injectable TSH was frequently off
the market. This test is rarely performed nowadays.
But TSH can be directly measured in a patient's bloodstream.
The idea is that the pituitary gland should be secreting high levels
of TSH in a futile attempt to stimulate a diseased thyroid gland. Measurement
of TSH level is apparently a very important means of diagnosing hypothyroidism
in humans and a canine version of the test was long sought. Unfortunately,
when it finally became available, it was found that many dogs with true
hypothyroidism did not have elevated TSH levels as one would expect;
still, this test is often helpful in making the diagnosis of hypothyroidism.
Testing with a Trial of Medication - Sometimes the
only way to test for hypothyroidism is to simply administer the medication
for several months and see what happens. Often an improvement in attitude
and energy level is seen within the first week. Hair re-growth takes
substantially longer (typically 4 months minimum) as the follicles must
"reawaken" and then grow a hair long enough to create a visible coat
change.
TREATMENT OF HYPOTHYROIDISM
At least treatment of hypothyroidism is relatively straight-forward.
Hypothyroidism is treated with oral administration of thyroid hormone
(T4). Even hypothyroid dogs are perfectly capable of converting T4
to T3. Pills are given usually twice daily to start but may be dropped
to once a day after good thyroid control has been achieved.
There are many brands of thyroid supplementation available
and prices are somewhat variable depending on the manufacturer.
TREATMENT IS FOR THE LIFE OF THE DOG
Occasionally we are asked if it is reasonable to use dried
or powdered thyroid glands of hogs or cattle as a more natural form
of treatment. The answer is simply no. These products are not produced
with adequate quality control to insure that they contain a reliable amount
of thyroid hormone. Each dose may be completely different when such a
product is used.
Re-testing later on?
Whenever an animal goes on a medication long term, periodic
blood testing is a good idea. In the case of hypothyroidism treatment,
it is important to know if the medication dose is too low or too high.
Thyroxine (T4) is a very safe medication but if it is not given in
adequate doses, obviously the patient will not be adequately treated.
If the dose is too high and given for too long a time, excessive water
consumption, weight loss, and restlessness can result.
Monitoring with T4
The lowest thyroid level of the day is found by taking the
blood sample right before a tablet would normally be given. The highest
level of the day is found by taking the blood sample 4-6 hours after
it is given. Expect one or both such tests to be periodically recommended.
Our hospital recommends some type of blood test monitoring annually.
Monitoring with TSH level
The canine thyroid stimulating hormone level (abbreviated
cTSH) does not seem to require any particular timing issues and can
be run on a blood sample taken any time of the day. The cTSH level
will not indicate whether or not the thyroid dose is too high but is
quite accurate at determining if the level is too low. (Inadequate treatment
is associated with high TSH levels indicating that the pituitary gland
is vainly trying to stimulate the thyroid gland with high levels of
stimulating hormone.) This test may be run instead of or in addition
to a T4 level when it comes time to monitor thyroid hormone treatment.
IS THE DIAGNOSIS CORRECT?
Because of difficulties in diagnostic testing methods (especially
before 1995), many dogs have been erroneously diagnosed with hypothyroidism
and have been on medication for years. If there is any question about
a patient and one wishes to re-test now that newer testing methodologies
are available, thyroid hormone supplementation must be discontinued
at least 2 months for blood testing to be valid. If possible, medications
known to interfere with testing should be discontinued for testing (though
this is obviously not always possible).
OTHER HYPOTHYROIDISM RESOURCES
http://www.soloxine.com
(Soloxine is one of the more prominent brands of canine thyroid
supplements.)
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What is hypothyroidism?http://www.upei.ca/ciddrelated terms: familial thyroiditis, lymphocytic thyroiditis, congenital hypothyroid dwarfism |