Canine Hypothyroidism


The thyroid gland is located at the base of the neck, just under the larynx and consists of two distinct lobes. It is responsible for secreting two different thyroid hormones, T3 (liothyronine) and T4 (levothyroxine), which are required by the body to regulate the metabolism. The level of these hormones is carefully regulated by a feedback mechanism involving the pituitary gland, which produces another hormone called thyroid stimulating hormone or TSH. The function of TSH is to stimulate the production of T3 and T4 from the thyroid gland.

Under-activity of this gland (hypothyroidism) is a common problem in humans and is now commonly diagnosed in dogs, in fact it is the most common endocrine disorder seen in dogs today. The basis of the disease in around 90-95% of cases is an autoimmune thyroiditis, where the animal’s own immune system gradually destroys the gland leading to a reduction in the output of T3 and T4. This type of hypothyroidism (referred to as primary hypothyroidism) is genetic in origin and is more prevalent amongst certain breeds of dogs, particularly Golden retrievers, Boxers, Dobermans, Greyhounds, Irish setters, Dachshunds, Cocker spaniels and Airedale terriers. It is quite rare in toy and miniature breeds. Dogs between the ages of 4 and 10 are most often affected with a slightly increased risk in spayed bitches. Symptoms do not usually appear until over 70% of the gland has been destroyed.

The remaining 5-10% of (secondary) hypothyroidism cases are due to dietary issues (lack of iodine), cancer of the thyroid gland and a variety of developmental disorders.

The role of thyroxine

The essential function of thyroxine is to govern or regulate the metabolism. Too much thyroid hormone (as in hyperthyroidism in cats) speeds up the metabolism with consequent effects on other organ systems in contrast to hypothyroidism, where too little slows the metabolism right down with subsequent effects on behaviour, weight and the dog’s skin.

Signs of hyperthyroidism

The majority of dogs affected show clinical signs suggestive of the illness, but which can also be easy confused with other conditions. In addition, an increasing number show few or no symptoms at all, making initial diagnosis quite difficult. The most common symptoms include:

  • Lack of energy and unwillingness to exercise
  • Generalized weakness
  • Laziness
  • Depression or mental dullness
  • Weight gain with no increase in appetite
  • Alopecia (hair loss) without skin irritation, which is usually bilaterally symmetrical
  • Excessive moulting
  • Poor hair growth
  • Dry and dull coat
  • Excessive scaling
  • Increase in the pigmentation (blackening) of the skin in some areas
  • Thickening of the skin (myxoedema) especially around parts of the face and the eyes
  • Recurrent  skin infections (pyoderma)
  • Intolerance to cold

 Less frequently observed symptoms include:

  • Tilting of head to one side
  • Epilepsy/fits/seizures
  • Infertility in both dogs and bitches, failure to cycle (anoestrus) or irregular cycles in bitches and poor libido in males


Accurate diagnosis is essential before treatment is started, and is normally based on both clinical symptoms and the results of blood tests. However the level of thyroid hormones in the blood can be influenced by other factors, especially drugs such as steroids, sulphonamides, anticonvulsants (such as phenobarbitone) and non-steroidal anti-inflammatory drugs (NSAID’s). These need to be considered in determining the final diagnosis as they can adversely affect the blood test results.

Measurements taken include:

  • Baseline Total T4. This is the most common test. Hypothyroid dogs will have a lowered level of the T4 hormone. However, there are other conditions that can cause the T4 to decrease, so if this test comes back positive for hypothyroidism your vet may recommend additional blood tests, such as the T3 Test or the Baseline TSH test
  • Baseline TSH Test. This measures the level of Thyroid Stimulating Hormone. In combination with the T4 or T3 test, this  provides a more accurate view of the situation
  • Free T4 by radio immunoassay (RIA). This test does not appear to be either more or less accurate than the T4 test
  • Free T4 by equilibrium dialysis (ED). This test can provide more accurate information on the level of T4 hormone
  • Baseline T3 Test. In combination with the T4 or TSH test, these blood tests can give a clearer picture of the hormone levels found in the bloodstream. This test is not reliable when used alone
  • TSH Response Test. In this test, your vet takes an initial T4 measurement and then injects Thyroid Stimulating Hormone (TSH) intravenously. After 6 hours a blood sample is taken and the level of T4 is measured. If your dog is hypothyroid, the level of T4 will not increase at all in response to the TSH injected

 Treating hypothyroidism problems conventionally

Once your vet has confirmed the diagnosis, treatment is normally started on synthetic L-thyroxine (Levothyroxine, trade name Soloxine). This is normally given twice daily with the dose based on your dogs bodyweight. It is normal to repeat some of the blood tests (T3/T4 and TSH levels) after a few weeks to check that things are returning to normal. Gradually the symptoms linked with hypothyroidism should ease. Your dog should become more alert, energetic and active, the condition of the skin and associated problems should improve and little by little any weight gained as a consequence of the disease, should disappear.

Support from Denes

We have several products that can be used in conjunction with treatment from your vet to ease some of the symptoms linked with hypothyroidism.

Complete Healthcare (All in One+) Powder

This popular nutritional supplement includes seaweed which contains significant amounts of iodine, which is essential for normal thyroid function. Seaweed will help improve both hair growth and pigmentation.

Denes Homeopathic Hepar sulph 30c drops

This homeopathic remedy will help clear some of the skin infections that occur as a result of an underactive thyroid gland.

Denes Arsenicum album 30c drops

You should consider using this remedy where the skin is excessively dry and scaly as it will help clear away the dandruff.

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