The thyroid gland is one of the most important glands in the body.
It is located in the neck near the windpipe and is composed of two
lobes, one on each side of the windpipe. This gland is controlled by
the body's master gland, the pituitary gland, located at the base of
the brain.
The thyroid gland regulates the rate of metabolism (body chemistry).
If it works more than normal, metabolism speeds up. If it is less
functional than normal, metabolism slows down. The latter is the
basis for the clinical signs of hypothyroidism. Excess thyroid
function is extremely rare, or does not occur, in dogs.
Why does it happen?
Hypothyroidism is almost always caused by one of two diseases:
the gland gets "invaded" by a certain
type of white blood cells (lymphocytic thyroiditis), or
it shrinks and stops working, but the
cause of this is unknown (idiopathic thyroid gland
atrophy).
The first disease is the
most common cause of hypothyroidism and is thought to be an
immune-mediated disease. This means that the immune system decides
that the thyroid is abnormal or foreign and attacks it. It is not
known why the immune system does this. The second disease is also
poorly understood. Normal thyroid tissue is replaced by fat tissue
in what is considered a "degenerative disease".
These two causes of hypothyroidism account for more than 95% of the
cases. The other five percent are due to uncommon diseases,
including cancer of the thyroid gland.
How does this disease manifest itself?
When the rate of metabolism slows down, virtually every organ in the
body is affected in some manner. Most affected dogs have one or more
of several "typical" physical and/or chemical abnormalities. These
include:
a. Weight gain without an
increase in appetite, or inability to lose weight despite
dieting b. Lethargy and lack
of desire to exercise c. Cold intolerance
(gets cold easily) d. Dry haircoat with
excessive shedding e. Very thin haircoat
to near baldness f. Increased
"blackness" (pigmentation) in the skin g. Failure to re-grow
hair after clipping or shaving h. High blood
cholesterol, and fatty blood i. High blood muscle
enzyme levels j. Anaemia (lack of
red blood cels) k. Seizures (rare)
Some dogs also have other
abnormalities that are not the typical findings. These include:
a. Thickening of the skin
around the face so they have a "tragic look on their face" b. Abnormal function
of nerves causing non-painful lameness, dragging of feet, lack
of co-ordination, and a head tilt c. Loss of libido and
infertility in intact males d. Lack of heat
periods, infertility, and abortion in females e. Fat deposits in
the surface (corneas) of the eyes f. So-called "dry
eye" (Keratoconjunctivitis sicca) due to lack of proper tear
production.
How is it diagnosed?
The most common test is for the T4 level (T4 is a hormone produced
by the thyroid gland). This is a measurement of the main thyroid
hormone in a blood sample. However, testing for the T4 level can be
misleading because some dogs that are not hypothyroid may have
levels of T4 lower than normal. This happens when another disease is
present or when certain drugs are given. The test may need to be
performed a number of times to diagnose the disease.
Can it be treated?
Hypothyroidism is treatable but can't be cured for good. It is
treated with oral administration of a thyroid replacement hormone.
This drug (hormone) must be given for the rest of the dog's life.
The level of medication will initially need to be monitored by blood
test to determine the correct dose.
What happens if the medication is
overdosed?
Although overdosing is difficult, excess medication may produce
signs identical to that of excessive thyroid gland function. These
include hyperactivity, lack of sleep, weight loss, and an increase
drinking. If any of these occur, please let us know immediately.
