When Thyroid hormone levels are too low...
What is hypothyroidism?
Hypothyroidism is common endocrine (glandular) disorder in dogs. In
affected dogs, the thyroid gland does not make enough of the thyroid
hormones (T3 and T4). These thyroid hormones are necessary to keep many
body tissues and organs functioning normally. Thyroid hormones regulate
the body's metabolic rate, so hypothyroid dogs are more likely to be
sluggish and overweight. However, some cases come on slowly and the
signs can easily be missed.
Just how common hypothryoidism is in dogs is not known, however,
estimates range from 1 in 150 to 1 in 500. Mid- to large-sized dogs seem
to be affected more commonly than are small dogs.
What causes hypothyroidism in dogs?
Most cases of hypothyroidism in dogs are acquired in adulthood. Rare
cases have been described to occur congenitally. Affected puppies do not
grow normally. There are two causes of aquired hypothyroidism —
lymphocytic thyroiditis and idiopathic atrophy. Lymphocytic thyroiditis
is believed to be immune-mediated, where the dog's own immune system
attacks the gland. After a rather prolonged course (up to 3 - 4 years)
enough of the gland is destroyed such that it no longer works. There
appears to be a genetic predisposition of lymphocytic thyroiditis in
certain breeds including borzoi, beagle, doberman, Great Dane, Irish
setter, and Old English sheepdog. Lymphocytic thyroiditis appears to be
similar to Hashimoto's
thyroiditis in humans, although it's rarely accompanied by other
immune system diseases.
Idiopathic thyroid atrophy is the cause of the other half of the
cases of acquired hypothyroidism in dogs. Atrophy means that the thyroid
cells die and are replaced by fat. Idiopathic means that no one knows
what causes this to happen.
In addition to the breeds listed above as having an increased risk
for lymphocytic thyroiditis, other dog breeds that have an increased
risk of hypothyroidism include: golden retriever, miniature schnauzer,
Airedale terrier, cocker spaniel, and dachsund. German shepherds amd
mixed breed dogs seem to have a lower than average risk for acquiring
the disease.
What are the clinical signs of hypothryoidism in
dogs?
Because thyroid hormones have actions throughout the body, signs of
hypothryoidism can be highly variable. Hypothryoid dogs can have any of
these signs or a combination of signs.
Problems of the skin and haircoat are the most common and include:
hairloss on tail, flanks, or behind the ears; darkening of the skin;
dry, flakey skin; or dry coat. Other common signs are weight gain,
mental dullness, lack of interest in activities, intolerance to cold,
slow heart rate, and infertility.
There are also a number of uncommon but serious signs of
hypothryoidism including: muscle weakness, defects of the nervous system
such as paralysis of the larynx (voice box), low body temperature, and
weakening contraction of the heart muscle.
There are only a few abnormalities on blood tests from dogs with
hypothyroidism. Elevation in cholesterol, however, is very common. [It
is important to remember that cholesterol measurements in dogs must be
taken after a 12-hour fast to be valid]. Some hypothyroid dogs have a
mild anemia