What is Hyperthyroidism?

The thyroid glands are small organs found under the skin in the neck, either side of the windpipe. These glands produce a hormone that regulates your cat's metabolism. Producing too much of this hormone can cause the cat's metabolism to go into overdrive.

The hormone regulates many of the body's processes, affecting many organs. It can cause the heart rate to increase and the heart muscle to become thick and stiff. This can lead to heart failure and blood clots. Affected cats can also develop high blood pressure, which can cause damage to the eyes, kidneys and brain. 

Causes of Hyperthyroidism in Cats

A benign or non-cancerous tumour on the thyroid gland is usually the cause of hyperthyroidism. It's not known what causes the tumour to form.

However, as a result, the gland produces more of the thyroid hormone than normal. In over 70% of cases, both thyroid glands are affected. Hyperthyroidism can be due to a cancerous tumour, but that's very rare.

What Breeds Often Suffer from Hyperthyroidism?

Any breed of cat can develop hyperthyroidism. Male and female cats are equally affected. However, certain breeds, including the Siamese and Himalayan breeds, appear to have a lesser risk.

Is Hyperthyroidism in Cats Common?

Feline hyperthyroidism is uncommon in young cats, but it's a common condition in middle-aged and older cats of both sexes.

Signs and Symptoms of Hyperthyroidism in Cats

The clinical signs of hyperthyroidism include a scruffy appearance and poor quality or rough coat. They may have muscle wastage and generally appear under the weather. 

Despite an increased appetite (polyphagia), they can lose weight. They can also become more aggressive and restless, drink more (polydipsia), and have vomiting and diarrhoea. Occasionally, cats with hyperthyroidism can go off their food, too.

Your vet will usually recognise the disease from your description, but several tests can confirm it. Your vet will feel if the glands on your cat's neck are enlarged. They'll also want to monitor your cat's heart rate and blood pressure and take blood tests, including those to measure thyroid hormone levels.

Thyroxine hormone (T4) is usually found to be high, but additional thyroid hormone tests may also be measured. X-rays, ultrasounds and ECG (electrocardiogram to monitor heart activity) can also be carried out. Scintigraphic thyroid imaging is a specialist technique that can determine if there's any tissue growing outside the thyroid gland. A technetium scan is available at some specialist centres and is another technique that can be used to confirm the diagnosis and locate any abnormal thyroid tissue.

Hyperthyroidism isn't painful, but early diagnosis is important to prevent further damage to your cat's heart, kidneys and eyes. If hyperthyroidism is uncontrolled or diagnosed at a late stage, the symptoms of weight loss, kidney problems and even loss of vision are much more likely.

What's the Treatment for Hyperthyroidism in Cats?

There are four ways of treating hyperthyroidism in cats - medication, radioiodine treatment, surgery or a special diet. 

Your vet will discuss the advantages and disadvantages of each treatment with you, and what will work best for your cat.

The most appropriate treatment will depend on things like your cat's age, the severity of the condition, any other conditions your cat may have, and the risk of complications. The cost of treatment may also be a factor in deciding which treatment route to go down.

Medication

Hyperthyroidism can be treated with oral anti-thyroid medication for cats – carbimazole or methimazole tablets. These drugs block the production of the thyroid hormone and are given at least once a day (often twice a day). Unfortunately, these drugs don't cure hyperthyroidism, and your cat will have to take the medication for the rest of their life. Some cats (and their owners) can find this stressful. 

But luckily, there's now a medication available as a transdermal gel. As the gel is applied to the inside of the cat's ear, it's much easier to administer, making it much less stressful for the cat and owner.
Your cat will be reassessed after 2 weeks of treatment to monitor kidney function and check that the correct treatment dose is being given. This determines whether the cat will be suitable for further treatment which can cure hyperthyroidism. 

If any cardiac problems have been detected, further medication will be prescribed.

Radioiodine Treatment

If radioiodine treatment is recommended, your cat will be referred to one of the specialist veterinary centres that offers this procedure. It's a safe and effective treatment that has an extremely high success rate (94-98%).
Radioiodine treatment for cats involves the cat being isolated for 4-6 weeks with minimal handling after the injection of radioactive iodine. Unfortunately, it's not suitable for cats that need regular treatment for other conditions, such as insulin injections for diabetes.

The injection destroys the abnormal thyroid tissue, leaving the surrounding healthy tissues unaffected. It doesn't involve a general anaesthetic and has very few side effects. The procedure isn't available at most veterinary practices, and cats must be isolated for long periods. Because of this, treatment is quite expensive.

Surgery

Surgery also avoids the need for regular medication and can be a permanent cure for hyperthyroidism. Surgery to remove thyroid tumours is known as a thyroidectomy or bilateral thyroidectomy if both thyroid glands are removed.

Vets may not remove both thyroid glands simultaneously, so your cat may have to undergo a second surgery later to remove the second thyroid gland. 

Surgery involves a general anaesthetic and carries the risk of damage to the parathyroid glands during the surgery. The parathyroid glands regulate the amount of calcium in your cat's blood and bones. Any abnormal thyroid gland tissue should be identified before surgery to make sure this isn't missed.

After surgery, they will only need the occasional blood test to make sure they stay within the normal thyroid hormone levels for cats and that their parathyroid hormones are working correctly. Surgery isn't advised for cats with severe kidney disease or very old cats.

Special Low-Iodine Diet

Iodine is needed to make thyroid hormone. Another option to manage hyperthyroidism is to feed them a special low-iodine diet for cats. This controls the amount of thyroid hormone produced. 

It's a tricky option as cats need to be fed this diet exclusively – if they hunt or are fed in other homes, the diet won't work! And if they refuse to eat the new diet, they'll become hyperthyroid again...

Regular monitoring of thyroid hormone levels will still be needed, as well as monitoring of kidney and heart function.

Life Expectancy of Cats With Hyperthyroidism

Cats with hyperthyroidism can live for many years after radioiodine treatment or surgery. Those on long-term medication alone generally live for 3-5 years. However, it depends on the cat's age at diagnosis and if they have other medical conditions.

It's important to get treatment as soon as possible before they lose too much weight and develop cat heart disease. If uncontrolled, hyperthyroidism can lead to damage to your cat's eyes, heart and kidneys. However, most cats can be treated successfully and make a good, if not complete, recovery.

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Content provided from Vetstream's Vetlexicon Felis - www.vetlexicon.com/felis

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