Download Canine Hypothyroidism

Canine Hypothyroidism
Thyroid gland function
The thyroid gland is one of the most important glands of the body. It regulates body
chemistry (metabolism). It is controlled by the pituitary gland which is located in the
brain. The thyroid gland is located in the neck near the windpipe (trachea). It is composed
of two lobes, one on either side of the trachea. When the thyroid gland is underactive
metabolism slows down and the clinical signs of hypothyroidism occur.
Causes of hypothyroidism
Hypothyroidism is the most common hormonal disease in the dog but is very rare in cats.
It may be caused by inflammation of the thyroid gland (known as lymphocytic
thyroiditis) or wasting away of the gland (known as idiopathic thyroid gland atrophy).
These are poorly understood problems and the causes are unclear. In both cases normal
thyroid tissue is replaced by non-functional connective tissue and thyroxine (thyroid
hormone) is no longer produced in sufficient quantities.
Together these two problems account for approximately 95% of cases. The other 5% are
due to other problems including cancer of the thyroid gland or pituitary gland.
Clinical signs
Some of the most commonly affected breeds include Golden Retrievers, Labradors,
Dobermanns and Irish Setters. It mainly affects young to middle-aged large breed dogs.
As the circulating thyroid hormone reduces, the rate of metabolism slows down. Virtually
every organ in the body is then affected. Most affected dogs have one or more of the
following signs.
•
Weight gain without an increase in appetite.
•
Lethargy, dullness and exercise intolerance.
•
Cold intolerance (dog will appear to feel the cold easily).
•
Slow heart rate
•
Dry haircoat with excessive shedding.
•
Very thin haircoat to near baldness – flanks, thorax trunk and neck are worst
affected.
•
Increased pigmentation in the skin.
•
Increased susceptibility to skin and ear infections.
•
Failure to re-grow hair after clipping or shaving.
Other less common signs include:
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Thickening of the facial skin resulting in "tragic facial expression syndrome".
•
Severely affected animals may have abnormal nerve function causing non-painful
lameness, dragging of feet, lack of co-ordination, and a head tilt.
•
Loss of libido and infertility in intact males.
•
Lack of heat periods, infertility, and abortion in females.
•
Fat deposits in the cornea (clear part) of the eyes.
•
Keratoconjunctivitis sicca or so-called ‘dry eye’ due to lack of proper tear
production.
Diagnosis
Blood sampling is necessary to make a diagnosis. Initial tests may show high blood
cholesterol, mild anaemia, and mild increases in liver enzyme activity. Definitive
diagnosis depends on a blood test to measure levels of circulating thyroid hormones,
initially T4 (thyroid hormone) and TSH (thyroid stimulating hormone, produced in the
pituitary gland). If the T4 blood level is below normal, the TSH level is high, and correct
clinical signs are present, this confirms. However, not all dogs with hypothyroidism have
high TSH, and dogs that are ill for another reason may have low T4. This can also happen
when certain drugs are being given.
If hypothyroidism is suspected but the results are not definitive, other tests may have to
be performed. These are more expensive so they are not used as first line tests.
Treatment
Depending on the actual cause, hypothyroidism is usually treatable but not curable.
Tablets containing thyroid replacement hormones usually have to be given for the rest of
the dog's life. There is a standard dose that is used initially based on the dog's weight, and
once daily dosing is sufficient for most animals. Improved activity and mental alertness
is usually seen within a couple of weeks of starting therapy, however skin and coat
changes can take up to six months to resolve.
After about one month of treatment, further testing is done to verify that the thyroid
hormone levels are normal. In some dogs, the dose will need to be further adjusted every
6-12 months. Close liaison with your treating vet is necessary in order to ensure that the
dog is neither overdosed nor underdosed.
Signs of hyperthyroidism, a condition which is not uncommon in the cat but is rare in
dogs can be caused if overdose occurs. Signs include hyperactivity, lack of sleep, weight
loss, and an increase in water consumption. If any of these occur, please let us know
without delay.