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Uncovering the causes of

Hypothyroidism in Dogs

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Find out what are the causes of hypothyroidism in dogs and
then what you can do about helping hypothyroidism in your dog.

Canine Hypothyroidism: get the facts here.
By: Tippy

Canine Hypothyroidism is classified in three different ways;
primary, secondary and tertiary. How it is determined that a
thyroid case is primary, secondary or tertiary depends on
where the problem is located - in the thyroid gland itself,
the pituitary gland or the hypothalamus in the dog.

The hypothalamus and the pituitary gland control the level
of hormones from the thyroid by secreting TSH (thyrotropin-
stimulating-hormone) and TRH (thyrotropin-releasing-

There is a very rare fourth case of congenital
hypothyroidism in canines, meaning that the puppy is born
with a defect that affects the thyroid synthesis, transport
to tissue or iodine metabolism. This is also known as
Cretinism and puppies with this condition have developmental
defects such as secondary hypothyroidism, mental stunting
and over time physical malformation.

There are also cases where animals later in life manifest
deficiencies in many endocrine glands like the pancreatic
islet, parathyroid, reproductive organs and adrenal. Failure
of these glands is also known as polyendocrineopathy or
autoimmune polyglandular syndrome or a pituitary gland
insufficiency also known as secondary hypothyroidism.

Listed below are the most common to least common cases of
hypothyroidism. The most common cases are lymphocytic
thyroiditis and idiopathic atrophy and these two conditions
account for almost all causes of primary hypothyroidism.

Primary Hypothyroidism:

Primary hypothyroidism accounts for ninety-five percent of
the cases of hypothyroidism. Below are the two main causes
of primary hypothyroidism:

Lymphocytic Thyroiditis:

It is believed that this type of hypothyroidism may be
genetically related. There are other suspected causes but
this disease is such that if there are any other related
causes to hypothyroidism they are undiscovered at this time.
It has been suspected that it may be a part of the
autoimmune polyglandular syndrome that occurs in some

This particular disorder is a dysfunction of the immune
system in which there are increased antibodies to many
thyroid hormones, mostly to the thyroglobulin (thyroid
hormone precursor molecule). Other cells can attack the
thyroid as a part of this disorder.

Once the immune system produces auto-antibodies they attack
the thyroid gland and related tissues, inflaming the cells,
and ultimately disrupt the thyroid gland from producing
needed hormones. The result is an inflammation of the gland
and the tissues of the thyroid which creates a series of
biochemical events that over several years will ultimately
destroy the thyroid gland.

Idiopathic Atrophy:

This particular disorder has no known cause but is
speculated that this represents the last stages of
lymphocytic thyroiditis.

Follicular Cell Hyperplasia (+ dyshormonogenesis) happens
before this particular disease. Follicular Cell Hyperplasia
happens when small thyroid follicles where the hormones are
synthesized in the thyroid gland increase.

This happens when there is an overabundance of TSH and is a
result of very little of the hormones from the thyroid being
produced and little or even no feedback inhibition of the
resulting hormones from the thyroid.

Essentially what happens is that the thyroid fails to
produce the hormones that are needed and the thyroid tissue
is then replaced by fat.


This is a rarer version of primary hypothyroidism and is
when thyroid cancer infiltrates and destroys the thyroid
tissue. As the thyroid tissue is destroyed by the cancer the
thyroid produces less and less hormone.

Thyroid cancer tumors have been known in rare cases to
actually produce the thyroid hormones but it is not
controlled by circulating TSH levels. Animals with this
problem later develop hyperthyroidism which is too much
hormone that isn't controlled from the thyroid.

Secondary Hypothyroidism:

Secondary hypothyroidism is when the pituitary fails to
output normal levels of TSH which results in the death of
the thyroid. Only five percent of the cases of
hypothyroidism fall within secondary hypothyroidism.

The pituitary regulates many of the glands such as the
thyroid, adrenal and reproductive glands and when secondary
hypothyroidism is diagnosed many of the glands can show
abnormalities along with the thyroid.

Also if the pituitary starts failing because it is so close
to the brain it can cause problems with the brain itself,
especially where a space occupying mass such as cancer is

The causes of secondary hypothyroidism where the pituitary
starts failing to output normal levels of TSH can include
birth defect, euplastic invasion or destruction, a disease
like Cushing's Disease or Hyperadrenocorticism, or
malnutrition. Certain drugs may also cause this to happen,
such as overuse of corticosteriods.

Tertiary Hypothyroidism

As of right now there is no way to test and see if your dog
has tertiary hypothyroidism. This is because there isn't a
test available to measure the TSH and TRH levels in the
canine body. But this may change in the future as even now
they are trying to find a way to test these levels in the
dog's body.

For right now the only way this can be determined is through
clinical signs, symptoms and previous history of the

Congenital Hypothyroidism in Toy Fox Terriers:

It has been found that hypothyroidism is a common birth
defect in Toy Fox Terriers. It is estimated that about
thirty-one percent of toy fox terriers carry the gene that
causes the defect and about one and four members of a litter
will be affected when the mother and father carry the gene

Before you breed your toy fox terrier you can have a DNA
test to be sure whether the sire or the potential mother
carry the gene responsible for this defect. Hypothyroidism
has been known to cause physical and mental defects and
stunts the dog's development. Many affected dogs die or are
euthanized because of hypothyroidism.

Clinical Signs of Canine Hypothyroidism:

The hormone produced by the thyroid effects almost every
other organ system in the body. There are many signs and
symptoms if there is no case of multiple glandular
involvements, such as:

- A "sad" Facial Expression
- Altered Pigmentation
- Behavioral Changes
- Bleeding Problems (Rare)
- Cold Intolerance (Seeking Of Warm Places)
- Constipation

- Cranial Nerve Deficits (Blind, And/or Droopy Eyelids...
Which May Also Occur With Myxedema
- Decreased Tear Production Causing Eye Surfaces to Dry
and Be Irritated
- Diarrhea
- Dry Irritated/Red Eyes Due To Paralysis of Eyelids (No Blinking)
- Dull or Brittle Hair

- Including Hair Loss or Abnormal Hair Turnover
- Increased Appetite
- Itchy Skin Because Of Secondary Bacterial or Fungal
- Lethargy
- Mental Dullness

- Obesity
- Odiferous Skin
- Oily or Dry Skin
- Poor Skin and Hair Coat
- Poor Wound Healing

- Proprioceptive Deficits (Dragging of Feet..."knuckling")
- Seizures
- Skin Parasite Invasions
- Swallowing Problems (Rare), General Weakness.
- Thickened Skin ("myxedema")
- Vestibular (Equilibrium) Deficits (Acts "drunk")
- Weight Gain

Other signs of this disease in a dog may be found upon a
physical exam and lab work. Such signs may include: Anemia,
Slow Heart Rate, Arrhythmia, High serum cholesterol.

Myxedema Coma:

With severe hypothyroidism a dog may experience a rare
condition known as Myxedema Coma. This causes low body
temperature, weakness, lethargy, diminished consciousness,
myxedema a non pitting and swelling of the skin because of
an abnormal turnover of ground substance, very slow heart
rate, low blood pressure, inadequate breathing. This is a
very serious condition that requires immediate medical
attention. This condition is fatal to many dogs so affected.


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