Famished felines - could your cat have an overactive thyroid gland?

Has your cat started eating like a horse, yet become thin as a rake? If your answer to this question is 'yes', then he/she may be suffering from hyperthyroidism. This hormonal condition is very common in middle-aged to older cats (typically 8 years of age and over), and if left untreated can be extremely detrimental to your pet's health. However, the good news is that with prompt diagnosis and treatment there is a very good chance that your cat will be able to live a normal life.

What is the thyroid gland?

The thyroid gland is a small, double-lobed organ located in the neck. It is responsible for controlling the body's metabolic rate - in other words, how quickly energy is used up. It produces thyroid hormones which regulate the rate of metabolism and affect the growth and rate of function of many body systems. Iodine from the diet is extracted from the blood and concentrated within the thyroid gland, and is essential for thyroid hormone production.

What causes hyperthyroidism in cats?

Hyperthyroidism in cats is usually caused by a benign (non-cancerous) enlargement of the thyroid gland. We do not know what causes this change but several risk factors have been identified, including indoor living and eating canned food. Male and female cats are affected equally. There are no known breed predispositions but the disease appears to be less common in Siamese cats.In rare cases, hyperthyroidism can be caused by a cancerous growth called a thyroid adenocarcinoma, which unfortunately makes treatment much more difficult. These cases may benefit from being seen by a specialist referral vet. Fortunately, however, these only account for 1 to 2% of cases.

The disease process

The enlarged thyroid gland produces higher than normal levels of thyroid hormone which causes the cat's metabolic rate to increase, thus promoting a greater appetite and activity level. However, fat is burned at a higher rate, so the cat tends to lose weight in spite of eating more.Hypertension (high blood pressure) often develops as a result of hyperthyroidism, and left untreated this can lead to serious complications such as blindness. The increased level of thyroid hormones promotes faster and stronger beating of the heart. Over time, secondary changes in the structure of the heart and rhythm abnormalities may occur. These can compromise heart function and eventually result in heart failure.

Symptoms of hyperthyroidism

In the early stages of hyperthyroidism the symptoms can be quite subtle, but may be picked up by observant owners. Cats start to lose weight and often drink and urinate more. They may also develop intermittent vomiting and diarrhoea. (These symptoms can also be caused by other illnesses, such as diabetes or kidney disease, so it is recommended that they be fully investigated by your vet). As the disease progresses, typically over the course of several months, cats usually develop a ravenous appetite yet become emaciated. Their coat quality may deteriorate, and they often show behavioural changes such as restlessness, wandering and irritability. Some cats are unable to tolerate heat and may pant at times.

How is hyperthyroidism diagnosed?

If you think that your cat may have hyperthyroidism then it is best to take him/her to your vet for a full check-up at the earliest opportunity. If the history and clinical examination are consistent with hyperthyroidism then your vet will usually recommend a simple blood test which measures the level of thyroid hormone within the blood to determine whether this is excessive. Your vet is also likely to advise other investigations (usually further blood and urine tests) to check that there are no problems occurring with other internal organs such as the kidneys. Note that an overactive thyroid sometimes 'masks' evidence of concurrent kidney disease (another common condition in older cats), so the kidneys may need to be rechecked after starting treatment for hyperthyroidism. If your vet detects any heart abnormalities, such as an abnormal rate or rhythm, they may recommend further investigations to assess heart function - for example, an ultrasound or ECG. A blood pressure check is advisable in any cat with suspected or confirmed hyperthyroidism.Blood test results are typically available within a few days and, assuming there are no other complications, treatment can be started once the diagnosis has been confirmed. It is important to be aware that the thyroid test can occasionally give false negative results - for example, in the very early stages of disease or where there is a concurrent illness. Where there is a strong suspicion of hyperthyroidism, it may therefore be necessary to repeat the test after a few weeks or use an alternative type of blood test.

Treatment options

There are currently four main methods of treating hyperthyroidism available in the UK. These are summarised here. Your vet will be able to discuss these treatments with you in more detail, including their pros and cons and typical costs, in order to help you determine the best choice for you and your cat. Of course, if there are any concurrent conditions or secondary complications then these will also need to be managed.

Tablets

These lower thyroid hormone synthesis, thus bringing the hormones back to a normal level and alleviating the clinical signs. Depending on how your cat responds to the medication, he/she will need to take tablets either once or twice daily. Unless otherwise advised by your vet, this must be continued for the rest of your cat's life, even once he/she is apparently back to normal - otherwise the problem will recur. Your cat will require regular check-ups and blood tests to make sure that he/she is continuing to respond appropriately to the medication and is not developing any other problems. As with all medications, there is always a small risk of side effects.

Surgery

If long-term medication is not for you or your cat, then surgery may be performed to remove either the whole thyroid gland (bilateral thyroidectomy), or just the one lobe, if only one appears to be affected (unilateral thyroidectomy). As with all surgeries, there is a small risk of complications. Hyperthyroid cats do represent a substantially higher general anaesthetic risk than young, healthy patients, for the reasons already outlined. Therefore it is preferable to medicate them for a few weeks prior to the planned surgery in order to stabilise their condition and allow them to be as fit as possible for the anaesthetic. Surgery does have the potential to be a permanent cure, but occasionally the condition recurs, even when the whole gland has been removed. This is due to so-called 'ectopic' thyroid tissue, which is located in another area of the body, typically the chest.

Radioactive iodine treatment

This treatment can only be performed at one of a handful of small animal hospitals in the UK with specialist facilities. An injection of radioactive iodine is administered under the skin. This is absorbed by the abnormal, overactive thyroid tissue, which then gets destroyed. Your cat will need to be hospitalised for several weeks until the radiation has fallen to a low enough level to be safe to you. The procedure has an excellent success rate, with 95% of cats being cured with a single injection.

Prescription diet

Very recently a promising new prescription diet created for hyperthyroid cats (Hill's y/d) has become available. This contains a limited iodine content which promotes normalisation of thyroid hormone levels within just a few weeks. It is very important that your cat only eats this diet (no treats!) as other foods typically contain higher levels of iodine. Your cat will need to stay on this diet for the rest of his/her life.

In summary, hyperthyroidism in cats is a serious disorder which can have a knock-on effect on other body organs, but with prompt recognition and management the prognosis is usually good. There are a variety of ways of managing this condition - hopefully one of these will be suitable for you and your cat.