Feline hyperthyroidism in cats generally affects older cats with the median age for developing the disease being just under 13 years. It is a disease of the endocrine (hormone) system and affects the thyroid gland.

The thyroid gland is located in the neck, with a lobe sitting either side of the windpipe. Hormones from the thyroid help control and regulate metabolism.

Hyperthyroidism is most often caused by a benign (non-cancerous) tumour (called an adenoma) on one or both of the thyroid lobes. This results in the excessive production of thyroid hormones.

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Increased levels of hormones generally cause an increased heart rate, heart murmurs and high blood pressure. If the disease is not treated, the muscles of the heart may become excessively thick (hypertrophic cardiomyopathy) leading to heart failure and death. Even if the cat does not develop hypertrophic cardiomyopathy but is left untreated, the disease can be fatal. This disease may go undetected as the cat may appear bright and alert.

In 90% of cases, there is significant weight loss. There is also increased consumption of food (but loss of weight), vomiting, increased thirst and urination, restlessness, behaviour changes and increased activity, diarrhoea, tremors, weakness and sometimes laboured breathing.

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Hyperthyroidism is not contagious.


In addition to the clinical symptoms already described, the vet will look for an enlarged thyroid gland. The thyroid is normally too small to be palpated but with hyperthyroidism, the gland becomes large enough to feel. It may become so large that it moves down into the chest cavity. If this happens, it will 'disappear' and will not be able to be felt. Another sign will be increased thyroid hormone levels.

Hyperthyroidism can present with similar symptoms to diabetes, kidney failure or heart and/or liver disease. Lab tests which help determine which disease is present and will also point the way to the most appropriate treatment.

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Treatment is usually one of three options. Each option has its pros and cons and veterinarians will choose a treatment depending on the individual animal.

* The anti-thyroid drug, methimazole, may be prescribed. This is usually the first course of action. It can be given in tablet form or as a gel applied to the ear.

Advantages include the fact that the drug is readily available and the treatment can be reversed. There is no need for anaesthesia, surgery or hospitalisation. It is relatively inexpensive in the short-term. Disadvantages are that it is not a cure and the adenoma will continue to grow. The cat will be on the medication for life. The medication may be difficult to administer, it may need to be given more than once a day and it occasionally has side effects. Side effects may include lethargy, vomiting and scratching at the face. Sometimes these side effects can be mitigated by starting with a small dose and gradually increasing the dose.

There are usually 3 to 6 monthly blood tests to keep check on thyroid function.

* Surgery cures the condition unless some of the abnormal tissue is left behind. The cost is equal to several years of methimazole. Hospitalisation is short and there is no daily medication needed. On the down side, the animal must be a good surgical prospect as anaesthesia is required and there could be post-operative complications. Occasionally the surgery will need to be repeated. Surgery is not an option if the thyroid is located within the chest.

* Radioactive iodine treatment needs no anaesthesia, sedation or surgery. All abnormal tissue is treated, there is no daily medication, normal thyroid function is restored and there is no destruction of healthy tissue. Disadvantages are that this option is available only from specialised facilities and it is expensive. Hospitalisation is required and a quarantine period is necessary. The process may need to be repeated.

Chemical ablation of the thyroid may one day be an option but at present it is still under investigation and is not widely available.

Anaesthesia is necessary with both surgery and radioactive treatment. This can be a concern with geriatric animals.

With some cats, hyperthyroidism can be easy to miss. If you think your cat is displaying any of the clinical symptoms of this disease, consult your veterinarian before the condition gets out of hand.

You might like to read my other articles on diseases of cats:
Facts about Feline Immunodeficiency Virus
Facts about Feline Peritonitis
Facts about Feline Calicivirus
Facts about Feline Distemper
Facts about Feline Leukaemia

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