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Health advice from your vets – Cheshire and Manchester

This is a disease of the thyroid gland, which is located in the neck in both cats and dogs. In general, cats suffer from hyperthyroidism (overproduction of thyroid hormones) and dogs suffer from hypothyroidism (underproduction). Thyroid hormones are responsible for regulating many processes within the body and in the case of hyperthyroidism metabolism is increased, leading to severe health problems for affected felines. Most commonly, the change in the thyroid gland is caused by ‘nodular hyperplasia’ which is a non-cancerous cause but similar in behaviour to a benign tumour. In 1-2% of feline cases, the cause will be a thyroid adenocarcinoma which is malignant cancer and can be a much more serious concern, in addition to the metabolic effects it causes.

Typically seen in older cats, hyperthyroidism is rare in those younger than 7 years old but it can have an insidious onset and often is not detected until the disease is in the more advanced stages. Whilst it is still very treatable, even when cats are showing marked signs of disease, far fewer long-term problems are caused if it can be picked up earlier and treated whilst clinical signs are still mild.

Hyperthyroidism in Cats

Classically, hyperthyroid cats present with an increased to ravenous appetite, whilst still losing weight. They have increased thirst and may seem to have behavioural changes at home, becoming irritable and restless. Coat condition deteriorates and their heart rate becomes much faster, which with concurrent weight loss is something owners may notice when they pick their cat up. Vomiting and diarrhoea may be more frequent. Less common signs of hyperthyroidism include a lethargic cat with generalised weakness and inappetence, contrary to the usual presentation.

On examination, the thyroid glands may be palpable if they are enlarged. A definitive diagnosis of hyperthyroidism is made with a blood test of T4 (thyroxine) levels. It is often the only one required, but in equivocal cases, free T4 may be needed as well. It is advisable to run a full biochemistry blood profile at the same time since secondary effects can be seen in the liver and kidneys. Information regarding concurrent disease is useful in determining preferred treatment options and very important if surgery is considered since these patients are often geriatric.

Complications of Hyperthyroidism

Complications of hyperthyroidism include hypertension (high blood pressure) which can cause damage to the eyes, kidneys, heart and brain. Whilst this is not always present if so, tablets to reduce blood pressure will be necessary and this ought to be checked at the time of diagnosis. Heart problems are seen not only as a result of increased heart rate but also through changes to the muscular wall of the heart. If left untreated cats will develop heart failure. Treatment of hyperthyroidism can ‘unmask’ kidney disease and the possible effects on kidney function of any medication or surgical management must be considered.

There are different options for treatment from tablets to dietary changes or more advanced techniques including surgery and radioactive iodine. They can all provide very successful outcomes and a suitable option for each individual cat can be sought to provide minimum stress and best results for both the patient and their owner.

Medical Management of Hyperthyroidism

Medical management involves the use of methimazole and carbimazole to reduce the production and release of thyroid hormones from the thyroid gland. Treatment must be given daily and in some cases twice daily, which can be difficult with some cats, although many will adjust to a new routine and become accommodating over time! Side effects are uncommon but may include vomiting, lethargy and a poor appetite in the first few weeks of treatment. More serious side effects are rarely reported. Drug therapy is often used initially to stabilise patients prior to surgery or radioactive iodine treatment. In particular, it allows kidney function to be assessed once the thyroid levels are controlled.

Surgical Treatment

Surgical treatment is the removal of affected tissues (thyroidectomy) and is usually performed once cats have been successfully stabilised on medication to ensure the general anaesthetic is as safe as possible. Generally, it is a successful cure for the disease, requiring no further treatment. There can be a recurrence of clinical signs if previously unaffected thyroid tissue becomes diseased, or if there is ectopic thyroid tissue (tissue located outside the actual thyroid glands) that cannot be detected and may also be diseased. The biggest risk in surgical patients is damage to the parathyroid glands, which help maintain blood calcium levels, and so can have serious side effects. Most often any effects are temporary and patients are often hospitalised for a couple of days following the procedure to allow calcium levels to be monitored.

Radioactive Iodine Treatment of Hyperthyroidism

Radioactive Iodine treatment is a safe and effective way of destroying abnormal thyroid tissue (within the thyroid glands but also where ectopic tissue may be located). The parathyroid glands and other surrounding tissues remain unaffected as the accumulation of the radioactive iodine only occurs in thyroid tissue. There are no significant side effects from the treatment but cats are temporarily radioactive and so must be hospitalised for a period to ensure urine and other waste products are disposed of safely. The use of radioactive drugs and restricted hospitalisation facilities mean that this treatment is only available at some hospitals. Ongoing treatment, however, is not required and only intermittent checking of thyroid levels are required to ensure that normal levels are maintained in the body. A single injection of radioactive iodine is curative in 95% of cats.

More recently, dietary management has been introduced as a treatment where strictly controlled levels of iodine ensure that the thyroid gland cannot make excessive amounts of thyroid hormone. This is the least invasive course of treatment and some good results have been described using the diet – although this must be done with the exclusion of all other foods and treats. It may not be for every adventurous feline but for older, unfussy house cats, this may be a very successful treatment routine.

If your cat is showing classic signs and symptoms of hyperthyroidism, contact us for an appointment today.

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