HYPOTHYROIDISM IN THE DOG
Canine hypothyroidism seems to be a topic of conversation
within all kinds of dog owning groups. It is no more
prevalent in show dogs, agility dogs, hunting dogs or house
pets than any other demographic.
Situated on both sides of the first five to eight tracheal rings,
the thyroid gland secretes into the blood stream two major
types of thyroid hormones. These active hormones are called T3 and
T4. Think of the thyroid gland as the body's thermostat . . . if it
is set too high, too much T3 and T4 are secreted and the dog's
metabolism burns too hotly. The dog may become hyperexciteable and
restless, pant and lose weight. Hyperthyroidism in dogs is
rare; in older cats it is fairly common.
If the thermostat is set too low, subnormal amounts of T3 and T4 are secreted
into the blood stream and the dog's metabolic fire is too cool. You
can see that if a dog is hypothyroid the signs will be lethargy,
weight gain, poor skin and coat condition, shivering and poor
stamina. Many hypothyroid dogs become "watchers"
rather than "doers"... they have little interest in engaging
in common daily activities.Not every dog will show ALL the
signs, of course. Some will display only one or two of these signs
and even then, in borderline cases, some dogs show only very subtle
changes from normal. Hypothyroidism is very rare in cats
but can occur if treatment for hyperthyroidism to reduce
thyroid function isn't monitored closely.
Control of Thyroid Function - The thyroid gland's activity is
directed by outside factors such as the pituitary gland located at
the base of the brain and the hypothalamus, another area deep in the
brain. As the dog's metabolism changes due to changes in
activity, nutrition, environmental temperature, stress, etc., the
need for more or less T3 and T4 is detected by the hypothalamus and
pituitary gland - they then send signals to the thyroid to "hurry
up" or "slow down". The brain signals to
the thyroid gland via production of TSH (Thyroid Stimulating Hormone) in the pituitary gland
at the base of the brain. The thyroid secretes more or
less hormones depending on the signals from TSH.
Now, for the abnormal.
Only ten percent of hypothyroidism in dogs is due
to any interruption of this supply and demand relationship between
the brain-pituitary gland and the thyroid gland. For example,
if the pituitary gland fails due to a tumor or other disease of if
the hypothalamus is diseased, the thyroid won't "get the message" to
"hurry up". So the thyroid gland, even though it can function
normally, doesn't receive the order to do so. T3 and T4 production
from the thyroid gland slows down and so does the hypothyroid dog!
Ninety percent of canine hypothyroid cases are due to what is called
primary hypothyroidism. The thyroid gland itself is not functioning
up to par even when told to do so by the pituitary gland and
hypothalamus. The thyroid gland is being told to "hurry up" but
simply can't respond.
Causes of Primary Hypothyroidism
1. Lymphocytic thyroiditis- Abnormal infiltration of the thyroid
gland with white
blood cells. This is likely an autoimmune type of disorder.
2. Idiopathic atrophy - This just means that for unknown reasons the
gland degenerates and shrinks in size or becomes infiltrated with
fat cells.
Secondary Hypothyroidism
Cancer - Occasionally tumors will occur in and damage the thyroid gland.
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Breeds Most Likely to Develop Hypothryoidism |
| Golden Retriever |
Miniature Schnauzer |
Boxer |
| Irish Setter
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Doberman Pinscher |
Chow |
| Poodle |
Dachshund |
Malamute |
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View Michigan State University's
veterinary diagnostic lab ranking of 140 breeds with
autoimmune thyroiditis
here. |
Hypothyroidism is disease
that does not seem to be due to environmental factors, vaccinations, nutrition
(unless a diet deficient in iodine is fed) or other
management practices. It is
seen equally in males and females.
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Crusty, circular patches that spread
are due to a
bacterial infection. |
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Blood tests are
essential for making an accurate assessment of the
patient. |
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Images of a
dog before and after treatment for
hypothyroidism |
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An overweight and nearly
hairless dog
due to hypothyroidism |
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Two months after starting
thyroid hormone replacement therapy |
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Your veterinarian may do skin
scrapings to be certain the hair loss isn't due to
skin mites such as these demodex mites seen under
the microscope. |
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Hypothyroidism has inhibited new hair
growth after the dog shed some hair. These
areas of scant hair regrowth have the same pattern
on the left and right side of the dog and is called
bilateral, symmetric alopecia. |
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Follicular dermatitis with hair loss
is more common in dogs with low levels of thyroid
hormone. |
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Most common signs:
With varying degrees of intensity the most common signs seen are
a thinning of the skin, hair loss over the back and sides without new hair production, and
weight gain in the presence of normal caloric intake.
Diagnosis:
Proof of hypothyroidism is gained via blood analysis for
the hormones called T3 and T4 and several other forms of
thyroid hormones, if needed. (See this site for full
information). Anemia and high blood cholesterol levels are common.
The veterinarian may make a diagnosis of hypothyroidism based upon
the patient's history, physical exam, and laboratory data.
Treatment: Correction of hypothyroidism
is aimed at increasing the blood levels of T3 (triiodothyronine) and T4
(tetraiodothyronine). An animal origin thyroid product ("natural
thyroid") has no advantage over synthetic
medications and, in fact, can be unreliable in some cases.
Synthetic T4, called sodium levothyroxine (or just L-thyroxine) is the best choice for
nearly all hypothyroid patients. Giving this medication in
tablet form allows normal levels of both T3 and T4 in about
90% of cases. The dose is usually given twice a day but the
amount used varies with each patient.
Summary:
If your dog is hypothyroid and is getting sodium levothyroxine, be sure
you are giving it twice a day at the proper dose. If the
treatment is working, the dog will "perk up" within a week
or two, although skin and coat improvement may take six
weeks or longer. Reproductive status may take six to
nine months to stabilize. Follow up blood tests are a good
idea to establish the correct dose needed.
As with all hormone related disorders, hypothyroidism can have all
sorts of clinical signs, some subtle, some obvious. However, it can
be a rewarding and inexpensive disorder to treat. Set that
"thermostat" right and the patient often returns to an
active... and interactive... happy, healthy dog!
Note: Many dog owners have had a dog "blow their
coat". This excessive shedding over a short period of time is
not
due to hypothyroidism; using hormone replacement to prevent this
natural occurrence is unfair to the dog.
SIGNS OF CANINE HYPOTHYROIDISM
GENERAL
Weight gain
Lethargy
Cold intolerant
Exercise Intolerance
Becoming a "watcher" rather than a "doer"
Sparse coat over dorsum on both sides
DERMATOLOGIC
Dry, scaly skin
Course, dull coat
Bilaterally symmetric alopecia
"Rat-Tail" (sparse, bristly hair on tail)
"Puppy coat" in adult dog
Hyperpigmentation (dark pigment infiltrating the skin)
Seborrhea dermatitis (oily, greasy skin)
Thin skin when pinched
Pyoderma (skin infections
REPRODUCTIVE ALTERATIONS
Female infertility
Male lack of libido
Failure to cycle
Prolonged interestrus interval
Weak, dying or stillborn puppies
Silent heat cycles
Prolonged estrual bleeding
Testicular Atrophy
Hypospermia
CARDIAC
Bradycardia
(slowing heart rate)
Cardiac
arrythmias
Cardiomyopathy
OCULAR
Corneal lipid deposits
Uveitis
Keratoconjunctivitis
sicca ("dry eye")
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These focal areas
of hair loss are not typical of hypothyroidism and
are due to a fungal (dermatophyte) infection. |
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Read about tests done to evaluate
