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What is Hypothyroidism?
The thyroid glands are two small glands located in the neck, near the windpipe. They produce a hormone called thyroxine, which plays an important role in the metabolism of every cell in the body. Hypothyroidism occurs when the thyroid gland stops producing adequate amounts of thyroxine. It affects people and dogs, but only very rarely cats (cats do, however, acquire hyperthyroidism more frequently - there is a separate information sheet about this disease).
Hypothyroidism in dogs usually occurs when the immune system mistakenly attacks the thyroid ('immune-mediated thyroiditis'), destroying it and thus making it impossible to produce enough thyroxine; or when the thyroid becomes damaged for no known reason ('idiopathic atrophy'). There is sometimes a genetic tendency for hypothyroidism, just like there is in people, and the disease is more frequently seen in certain breeds of dogs - Golden Retrievers, Giant Schnauzers, Pointers, Boxers and Old English Sheepdogs to name but a few.
Since thyroxine is involved in metabolism all over the body, a lack of it can cause a wide range of clinical signs. Dogs may be quite dull, lethargic, not want to exercise, and put on weight even without eating any more than usual. There are sometimes changes in the hair coat with areas of hair loss, dandruff, greasy skin, skin infections and recurrent ear problems. Unspayed bitches may have irregular oestrus cycles, there may be fatty deposits on the cornea of the eyes, and dogs may become weak on their legs. There can also be behavioural changes. It is variable which of these many clinical signs occur and depends on the individual dog.
Diagnosis
If your vet suspects your dog may have hypothyroidism, the first step is to run a routine blood test. This often shows changes in the levels of certain substances in the blood, raising suspicion for the diagnosis. Some blood can then be sent to an external laboratory to test for 'T4:TSH' (the chemical equivalents of thyroxine that circulate in the blood). In an otherwise healthy dog, the result from this test should give a diagnosis for hypothyroidism if it exists. However, blood tests do not always give clear-cut yes/no answers: there is some overlap between 'normal' and 'low' thyroxine levels; there is often a daily fluctuation in the level of thyroxine in the blood; and the T4:TSH test is influenced by whether the dog is suffering from any other illness and by some medications. Therefore, a diagnosis of hypothyroidism is made by a combination of the clinical picture and the different blood results. If medication is used to treat hypothyroidism in a dog that doesn't actually have the disorder, the normal thyroid gland can become damaged. Therefore, if your vet isn't happy that your dog definitely does have hypothyroidism, it is normal to redo the blood tests in 2-4 months to see if the results are more conclusive.
Treatment
Hypothyroidism is effectively managed by oral tablets, usually given twice a day. They contain a synthetic version of thyroxine and is well-tolerated by most dogs. The necessary dosage can vary significantly between different dogs, as different individuals absorb it to different extents. Therefore regular blood tests are needed when starting treatment to ensure the correct dosage is used. Your vet will want to take a blood sample for a T4:TSH test at 4-8 weeks after starting treatment, and probably at 2-4 weeks after any adjustment in the dosage. Once the thyroxine level appears stable, blood tests may be done less frequently. Dogs which were lethargic tend to show improved activity after a week on treatment, but the hair coat can take months to become totally normal again. Treatment must continue every day for the rest of your dog's life but with the correct dosage, the clinical signs can all be reversed and your dog will have a normal life expectancy.
Without any treatment, the high metabolic rate puts a lot of strain on your cat’s heart and can lead to heart failure, as well as your cat being unable to put on weight and feeling anxious. With your vet’s help, the appropriate treatment option for you and your cat can reverse the signs of hyperthyroidism, so your cat can continue to lead a normal and healthy life.
