Hypothyroidism in Dogs

Hypothyroidism is a common hormonal disorder in dogs.  It is caused by the Thyroid gland under-producing the hormone thyroid.  The symptoms of the disease can be very variable and in some cases it can be challenging to diagnose.  However, it is easily treated and the long term prognosis tends to be very good.  

What causes Hypothyroidism?

  • 95% of causes of canine Hypothyroidism are caused by the Thyroid gland stopping functioning properly.
  • In about half of the affected dogs the gland is destroyed by 'autoimmune attack', which means for some reason the body destroys the thyroid tissue.  The other 50% are caused by 'Idiopathic Atrophy' of the gland.  In both types the the underlying cause of the destruction is usually unknown.
  • The remaining 5% are caused when the hormone signals released by the brain, which turn the Thyroid gland on, malfunction.

Symptoms of Hypothyroidism

  • The symptoms of Hypothyroidism are very variable and, as the thyroid hormone is required by all the body's systems, non-specific.
  • So, rather than there being one or two symptoms which are associated with the disease, it is more likely to be a combination of several abnormalities which will make your vet suspicious of the disease.
  • Hypothyroidism is most commonly seen in young to middle aged dogs, typically between the ages of 3 and 6 years and some pedigree breed are more prone to the problem than others.  Irish Setters, Golden Retrievers and Dobermans are all vulnerable to the disease.
  • Common symptoms associated with Hypothyroidism include;

    ▪ A dry and flaky coat with brittle fur.  This is often one of the first signs which occur.
    ▪ Hair loss, this occurs gradually and the dogs coat tends to thin out and then go bald. Thyroid hormone is needed for the hairs growth cycle and low levels mean new hair fails to grow to replace the old when it falls out.  The changes are most obvious in areas of high friction, for example around the neck where the collar goes, or on the tail.  In fact, a balding or thinning tail (commonly called a 'rat tail') is one of the more common symptoms of the disease.  In some cases the hair loss is bilaterally symmetrical, which again is typical of hormonal disorder hair loss.
    ▪ Recurrent skin infections
    ▪ Lethargy and dullness.  Affected dogs often become quite depressed and seem sluggish. Weight gain is common and they can often put on significant amounts in short periods of time.
    ▪ More unusual symptoms are neurological changes such as head tilts, facial paralysis and nystagmus (eyes flicking from side to side).

Diagnosing Hypothyroidism

  • Hypothyroidism is primarily diagnosed using blood tests, testing both the thyroid itself and the health of the dog in general.
  • Health screening blood tests are useful so your vet can rule out problems with the rest of the body and look for results which would be suspicious of Hypothyroidism such as high cholesterol and mild anaemia (low red blood cell levels)
  • There are different tests which can be used to look at the thyroid levels in the body and your vet will chose the one which gives them the best chance of diagnosing (or ruling out) the disease.

    ◦ Total T4 – This measures the total amount of thyroid hormone in the body.  A normal results means it is highly unlikely that a dog is Hypothyroid but a low result is not conclusive for the disease.  The thyroid levels in the body can fall for lots of reasons, especially if the dog is poorly with another illness.
    ◦ TSH – This stands for Thyroid Stimulating Hormone.  It is released from the brain and causes the thyroid hormone to be released.  In cases of Hypothyroidism it tends to be very high as the brain is ordering the thyroid gland to produce the hormone and when it detects the levels are low, it raises the TSH further.  Measuring the T4 and TSH together is a good way of diagnosing Hypothyroidism as the T4 should be low and the TSH high.
    ◦ Free T4 – This is a more specialist test but the Free T4 is useful as it is less affected by other illnesses than Total T4.  

Treating Hypothyroidism

  • Once it has been diagnosed, Hypothyroidism is reasonably easily treated with a daily tablet which replaces the thyroid hormone in the body.

Monitoring Hypothyroidism

  • It is very important that Hypothyroid dogs are regularly monitored with blood tests to ensure the levels of thyroid hormone in the system are correct.
  • The blood tests are usually taken 4-6 hours after the pill was given, so the most sensible time to medicate the dog is in the morning.
  • During the early stages of diagnosis and treatment your vet will need to take blood every few weeks but once the correct levels of thyroid in the system have been reached and stabilised, most clinics will only test the dogs every 3-6 months.

Prognosis

  • The long term prognosis for Hypothyroid dogs is very good.
  • They need to take their medication for life and be regularly checked by the vet but otherwise they can lead a long and normal life.  

Please note, this is a advice only article.  If you have any concerns about this, or any other pet related subject, you must speak to your vet.

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