Hypothyroidism in the Dog

Hypothyroidism in the Dog

 

The Thyroid gland is located in the neck and has two lobes either side of the wind pipe.  It is a very important gland and produces the hormone thyroxine in two forms: T3 and T4.  This hormone is responsible for controlling our metabolic rate  (or speed our body runs at, rather like a car’s idle speed).  It is also responsible for the growth and development of the body and helps regulate many body functions.  It is ultimately controlled by the pituitary gland in the base of the brain that produces the hormone TSH (thyroid stimulating hormone).   Conditions that affect the thyroid gland can either cause it to produce too much thyroxine hormone, which is called ‘hyperthyroidism’ or too little thyroxine which is called ‘hypothyroidism’.  Cats are commonly affected by a thyroid tumour which causes hyperthyroidism (too much), but in contrast dogs are usually affected by conditions that cause hypothyroidism (too little hormone).

 

What causes Hypothyroidism in Dogs?

Hypothyroidism in dogs is usually caused by destruction of the thyroid gland – either from a condition called lymphocytic thyroiditis where the dogs own immune system attacks the gland, or idiopathic atrophy of the gland which is an unknown condition where the normal tissue is replaced by fat.   Cancer is a very rare cause of hypothyroidism in dogs.

 

Which dog breeds are predisposed to this condition?

Middle aged large breed dogs seem to be more predisposed to developing hypothyroidism but some breeds that are more susceptible include:

Golden retrievers, Boxers, Dachshunds, Doberman Pinscher, Airedale Terrier and Irish Setter.

 

What clinical signs can we see?

Because the thyroid hormones affect many body systems, it means the clinical signs of decreased production can be vague and cause many varied symptoms.   The more common symptoms however include:

Weight gain (without an increase in appetite)

Lethargy or lack of energy

Hair loss – mainly around the body along the back which can often be symmetrical.  It does not usually cause redness or itching.

A dull/brittle hair coat

Scaling dry skin sometimes with increased pigment – skin getting darker

Cold intolerance – seeking heat

A ‘tragic’ facial expression caused by excessive accumulation of a substance caused mucopolysaccharides in the face causing it to droop

An increased number of infections e.g. ear infections or skin infections

 

How can we diagnose hypothyroidism?

 

Apparently, hypothyroidism in dogs is one of the most over diagnosed conditions in veterinary medicine.  This is because many diseases can mimic the symptoms of hypothyroidism and can also cause a reduction in the level of total T4 (one the hormones we measure) – as can many drugs, for example antibiotics and anti-inflammatory medications.   In addition, the T4 level can vary throughout the day making a single measurement of T4 an unreliable test.  In NZ we have a limited number of thyroid tests available to us compared with other countries, which can perform several tests to increase the certainty of diagnosis of this condition.  In NZ however, we can also measure TSH produced by the pituitary gland.  Your veterinarian will also do other blood tests as a low red blood cell count (anaemia) and an elevated cholesterol can also be seen in hypothyroidism – in addition it can help to rule out other diseases.   In summary your veterinarian will use both medical tests and clinical signs to determine if hypothyroidism is likely and if a treatment trial is warranted.

 

How do we treat Hypothyroidism?

 

Treatment of hypothyroidism requires lifelong medication with synthetic thyroid replacement hormone.  This is initially given orally twice daily, but some dogs can be maintained on daily medication.   It can take several weeks for some clinical signs to improve – often energy levels will improve first, but with hair loss and poor coat condition, improvement may not be noticed for a number of weeks.  You will need to bring your dog in for occasional blood tests to check that the medication dose is correct and that the thyroid level is not too high or too low.  Maintained on medication your dog can live a long and healthy life.