The most common hormone problem
encountered in dogs is hypothyroidism. It results when the thyroid
gland does not secrete an adequate quantity of thyroid hormone called
thyroxine. Many internal organs are affected, and the resulting
problem depends on which organs are most affected.
Cats do not get this problem, but
get an opposite problem called hyperthyroidism.
Their problem involves excess thyroxine and its effect on the
The thyroid gland is a small gland located at the
throat, near what might be termed in people the "adam's apple". It
has two lobes, and can be felt with careful palpation.
this view of the thyroid gland you can also see the
parathryoid gland at the far left and the lymph node
The role of the thyroid gland is to take iodine
and convert it into the 2 main thyroid hormones; thyroxine (T4) and
triiodothyronine (T3). T4 and T3 then circulate through the
bloodstream and affect the metabolism of every cell in the body.
To control the level of these hormones the
hypothalamus and pituitary secrete compounds called releasing
factors. In the case of the thryoid gland, they secrete a releasing
factor called thyroid stimulating hormone (TSH). It is the amount of
TSH circulating in the blood stream that tells the thryoid gland how
much thyroxine to secrete. In a very refined feedback mechanism
between the hypothalamus, pituitary, and thyroid gland, the cells of
the body get just the right amount of T4 and T3.
Thyroxine circulates throughout the bloodstream
and affects almost all organs. It plays a major role in controlling
metabolism, and is needed for growth.
Primary hypothyroidism accounts
for almost every case. It has 2 main causes:
This cause, also known as
autoimmune thyroiditis, occurs when the body makes antibodies
against the thyroid gland. This effectively destroys part of
it, so it has less thyroxine to secrete into the bloodstream.
It is one of the most common causes of primary
This cause of hypothyroidism
can start early in life. Symptoms will appear when it
progresses to the point that the reserve power of the thryoid
gland is affected.
In this form we do not know
the cause, which is why it is called idiopathic.
accounts for only a small percentage of cases. It arises when
there is a lack of TSH, or secondary to some medications or
There are other causes of
hypothyroidism that are encountered only rarely.
Thyroxine affects many internal organs, so a
deficiency can have various symptoms.
Classic symptoms include
mental dullness, lethargy, obesity, and heat seeking behavior,
although many hypothyroid dogs do not have any of these
Early diagnosis of hypothyroidism
is beneficial because a dog can have this disease and not show any
symptoms for many years. In every disease we treat, the sooner we
start the better-this applies particularly to
This is the most common manifestation of
hypothyroidism. Typical skin symptoms include symmetrical hair loss (alopecia)
along the trunk, although the hair loss is not consistently symmetrical.
The hair coat is thin and dull, the hair easily falls out, it grows back
slowly, and shedding occurs more often. Sometimes the hair coat resembles
that of a puppy. Alopecia, if it occurs, is more common at pressure points
and the tail.
The skin might be cool to the touch and
be darker (hyperpigmentation) than normal. A leathery feel called lichenification
might also exist. Hyperpigmentation and lichenification usually occur when
the problem has been long-standing. Also, the skin might be greasy due to
seborrhea, and inflamed due to secondary bacterial or fungal infections.
These secondary complications might cause excess scratching (pruritis) and
They skin lesions present in
hypothyroidism mimic those in other skin conditions, especially
terrier has hyperpigmentation on its neck. Hypothyroidism
is not the only potential cause of this condition.
The ears can be affected,
resulting in hair loss, inflammation and infections.
Neurologic signs might be seen, and include
dullness, mood swings, muscle wasting on the head, facial paralysis, head
tilt, disorientation, muscle weakness or paralysis, and lameness. On very
rare occasions there will be seizures, and coma. Two specific diseases associated
with hypothyroidism are megaesophagus and laryngeal paralysis. A loss of
smell and taste are also possible.
This is a severe head tilt in a cat. There are numerous other causes to head tilt, most of them are more likely than hypothyroidism.
The cornea might undergo fat (lipid) deposits or become ulcerated. Changes with adequate tear production along with internal structures of the eye could occur.
When a dog does not produce enough tears
to keep the cornea moist it develops a disease called keratitis sicca.
A tenacious discharge adheres to the eye and makes it susceptible to
Diarrhea, constipation and
vomiting, if they occur, could occur in hypothyroid
Abnormalities in heart strength, rate and
rhythm, along with atherosclerosis, could occur.
Arrhythmia's are usually diagnosed with
an electrocardiogram (ECG). This is a lead II ECG on a pet with a heart
rate of 106 beats per minute.
Inadequate thyroxine makes
the immune system less effective at fighting infections,
especially the bacterial skin infections (pyoderma) that occur
secondarily. Suppression of the immune system might even
increase susceptibility to demodex.
Anemia is the most noted symptom. anemia
is not a disease but a sign of disease. It occurs when the red blood cells
are low. There might also be a bleeding tendency, low white blood cells
from bone marrow suppression, and low platelets.
This blood sample shows
three different tests on a CBC that check for anemia.
RBC- red blood cell
HGB- Hemoglobin level
pet is very anemic, much more so than would occur due only
Breeding dogs might have
abnormal heat cycles, infertility, and high puppy mortality.
Testicular atrophy and low sperm, or no sperm.
In addition to low
thyroxine, hypothyroidism is implicated in sugar diabetes
addison's disease (hypoadrenocorticism).
Thyroxine is essential for
the development of bones in young animals.
arrow point to growth plates, areas of bone growth that
allow the bones to grow longer. The top arrow points to
the end of the thigh (femur) bone, the bottom arrow
points to the beginning of the shin (tibia) bone.
Due to the vast number of organs
influenced by thyroxine, and the fact that many skin conditions have
similar symptoms, numerous diseases have to be kept in mind when
making a diagnosis. These include Cushing's
approach is needed for a correct
diagnosis of hypothyroidism. In every disease we encounter we follow the tenet's
of the diagnostic approach to ensure that we make an accurate diagnosis and
that we do not overlook some of the diseases that are also encountered in pets
as they age.
Hypothyroidism can occur at any
age, although it tends to be a problem that affects middle aged
and older dogs, especially the larger breeds.
Several canine breeds are prone to getting
- Great Dane
- Irish wolfhound
- Cocker spaniel
- Golden Retriever
- English bulldog
- German Shepherd
- Doberman Pinscher
- Irish Setter
- Old English Sheepdog
- Miniature Schnauzer
- Airedale terrier
Females and males get it at about the same
frequency, neutered pets might be at higher risk of
Hypothyroidism disease is
suspected in any pet that has some of the symptoms described
above, particularly the skin symptoms. It is important to remember
that some dogs do not show any symptoms early in the course of the
disease. This is another reason for yearly exams and blood sample
with thyroid test in dogs and cats 8 years of age or
Other findings include skin
infections that recur after antibiotic therapy is stopped.
3. Physical Exam
Routine physical exam findings might
- Ear problems
- Slow heart rate or abnormal heart rhythm
- Body temperature might be lower than normal
- Pale mucous membranes due to anemia
- Enlarged lymph nodes due to secondary bacterial infections
- Alopecia that is
- Skin conditions in general
4. Diagnostic Tests
There is no one test that
definitively diagnoses hypothyroidism, save for a thyroid
A CBC (complete blood cell) and biochemistry
panel should be run on every dog 8 years of age or more, especially if they
have any of the symptoms of hypothyroidism.
The CBC might show anemia or
an elevated WBC (white blood cell count). The anemia is due to
thyroxine's direct effect on red blood cell production, the
elevated white blood cell count (leukocytosis) is due to
secondary bacterial infection.
The biochemistry panel might
show an elevated cholesterol. Diet can influence this test,
along with how long after a meal was the blood sample for this
test obtained. To be accurate there should be a 12 hour fast
when assessing cholesterol levels.
tests might also be
elevated, presumably from fatty changes that occur in the liver
due to abnormal metabolism.
panel is very comprehensive. This high cholesterol alerts us
to keep hypothyroidism in our tentative diagnosis list.
Notice the elevated amylase and lipase tests above the
cholesterol test? These are indicative of pancreatitis,
which is exactly what this dog has.
Many factors affect the level of thyroxine
that circulates in the bloodstream, including normal fluctuations. As a
result, there is no blood sample that definitively makes a diagnosis of
hypothyroidism. Over the years many different test have been developed to
help us detect adequate levels of thyroxine in the bloodstream. Our goal
is to diagnose those cases where the problem is not so obvious, and also
not to over diagnose this condition.
Our routine blood sample has
an add on test called a T4 test. If this test is
normal, everything else being equal, a dog probably does not
routine thyroid test by RIa (radioimmunoassay) is at the
high end of normal. This dog most assuredly does not have
If the thyroid test is low
or low normal, then 2 main scenarios are possible:
The first scenario
is called the sick thyroid syndrome or nonthyroidal illness
(NTI). In this situation the thyroid gland is normal, but
there are factors that are suppressing it from secreting a
normal amount of thyroxine into the bloodstream. These
factors include medications like cortisone, valium,
anticonvulsants, and sulfa antimicrobials. Diseases like
addison's disease can also cause NTI. When these factors are
corrected, or these diseases are treated, the apparent
hypothyroid problem corrects itself. No treatment with
supplemental thyroxine is needed.
In the second scenario
the thyroid gland is having a problem secreting adequate
thyroxine due to one of the causes previously mentioned in
section. This is the hypothyroidism we need to treat with
How do we differentiate
between a true hypothyroidism from the sick thyroid syndrome.
We have another blood sample that aids us, called the free
T4 test by equilibrium dialysis. If this is low, and
the signalment, history, and physical exam are consistent with
this disease, then a diagnosis of hypothyroidism is
dogs T4 level by equilibrium dialysis is
low, so it most likely has
Biopsies of the skin can
show changes associated with hypothyroidism. These changes can
also occur with other skin conditions though, especially those
involving the endocrine system.
comments section of this skin biopsy report mentions
endocrinopathies (hormone diseases like hypothyroidism)
and corticosteroids (cortisone) as possible additional
causes of this dogs skin problem.
This is the most reliable
test to confirm a diagnosis of hypothyroidism. It eliminates
some of the variables that suppress thyroxine production by the
thyroid gland. Unfortunately, it is difficult to find TSH of
animal origin. Human recombinant TSH is a possible replacement,
but cost might preclude its use.
An actual biopsy of the thyroid gland can
be taken. This test is rarely utilized since there are many other good tests
that are not so invasive.
Iodine can be used to outline the thyroid. We tend to use
this test much more often in feline
5.Response to Therapy
One of the tenets of the
process is whether
or not a treatment that is instituted actually corrects the
problem. This might apply in hypothyroidism, but it might not.
In some situations we have no choice but to try
supplementation. We reserve this for cases when the thyroid
tests are suspicious (normal but at the low end of the normal
range), we find no evidence of other disease processes, and the
dog has symptoms consistent with hypothyroidism.
This approach has disadvantages though.
Since thyroxine affects metabolism, an increase in metabolic rate due to
supplemental thyroxine might correct some of the symptoms encountered, even
increasing hair growth. This does not necessarily mean that these symptoms
that were consistent with hypothyroidism were actually caused by hypothyroidism.
A delay in the correct diagnosis leads to a delay in proper therapy and
a worsening prognosis.
If a dog has sick thyroid syndrome it is treated
by correcting the underlying problem. This might include
for secondary bacterial infections, or the elimination of drugs like
When hypothyroidism is correctly diagnosed, the
treatment, called levothyroxine (T4), is continued for life.
Levothyroxine has various trade names, including Soloxine and
is the brand we use. It is best to stay away from generic
levothyroxine because it is not absorbed as well as the
name brand version.
Medication is given every 12 hours. A
thyroid level needs to be checked initially at 1 month to make minor adjustments.
The thyroid pill should be give 4-6 hours prior to the recheck blood test. It
is then checked every 6 months in order to refine the dose, because the body
does change in the amount of thyroxine released by the thyroid gland. Also,
as pets age, their cells vary in their need for thyroxine.
In the first week of treatment many dogs will be
more alert and more active. Within one month improvement in problems
related to metabolic changes will be noted, and within 2 months most
skin conditions will be improved. If there is no response to therapy
within 3 months, and the proper dose and type of levothyroxine are
being used, then further diagnostic tests are needed to look for
other diseases. It might take 6 months or more for all changes to
return to normal.
It is possible to overdose your dog with
levothyroxine. Symptoms include excess drinking and urinating,
restlessness, and increased appetite. If you suspect this is
occurring stop medicating and bring your dog in for an exam. Checking
the thyroid level every 6 months will help eliminate this
Pets that have heart disease, diabetes
mellitus, or Cushing's
Disease(hypoadrenocorticism), may need
altered doses of medicine if they occur concurrently with
hypothyroidism. The dose of levothyroxine in these pets, if used at
all, needs to be conservative to prevent other problems.
An additonal treatment modality is called VOM.
It is a non-invasive and non-painful way to stimulate the nervous system to
help the thyroid gland heal on its own.
Since this disease has a strong genetic component
selective breeding can help minimize occurrence. Screening for anti
thyroid antibodies in breeding animals can be utilized once they have
reached puberty. These antibody tests are sent to special labs at
Michigan State University or Cornell University.
The use of VOM
can have substantial positive effects in this disease.
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