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Feline Hyperthyroidism

Cats can live to a ripe old age. In our clinic, the oldest feline patient was 24 years old, and there are plenty in the 16-20 year-old age group.

Generally speaking, when cats reach old age they tend to be less heavy than when they were younger, just as people are in their 90’s. Old cats become ‘wasted’- there is less muscle and they are less strong. It often pays to check these cats for arthritis. The bones of old cats are thinner, lighter and more fragile, thus your favourite puss is seen wobbling down the garden path, spending more time sitting in the sun and no longer leaping in the kitchen window. The cat is quite happy nevertheless, just unable to do the things he or she used to do, and perhaps feeling the cold a little more.

However some cats look old before their time. They lose condition and look skinny when they are only 12 or 13 years old. They may also be glutinous eaters and will demand attention every time you go near the fridge or pantry. Despite all this, they lose weight. There can be many reasons for this, such as poor digestion or problems with worms. If the cat also drinks a lot of water there may be kidney problems or sugar diabetes. Sadly in some cases there may be growths in the body.

Nowadays veterinarians have been finding that many of these cats have in fact developed an overactive thyroid gland. This disease is known as hyperthyroidism.

Thyoidglandimage Hyperthyroidism (an overactive thyroid gland) is very common in middle-aged and old cats. It is always a treatable disease. The thyroid glands are situated in the neck. One or both of these glands enlarge when diseased and sometimes become palpable (the vet can sometimes feel the enlarged thyroid gland, about the size of a small pea, beneath the skin of the cat ‘s neck). Over 98% of enlarged thyroid glands are benign (non-cancerous). Hyperthyroid cats may have the problem in both thyroid glands.

The thyroid gland is involved in regulating the cat’s rate of metabolism.When overactive, the thyroid gland produces more thyroid hormone than usual, often 2-3 times as much. This hormone circulates in the bloodstream and causes an increase in the rate of metabolism. This usually becomes apparent in the patient as weight loss, sometimes increased activity and increased appetite, an increased heart rate and often a poor coat. Often the blood pressure is raised, sometimes leading to kidney problems and changes in the eyes. The heart is often affected and enlarges, leading to heart disease. Many of the changes are reversible with treatment of the underlying thyroid disease. Less common signs of hyperthyroidism include skin changes; drinking and urinating excessively; vomiting; diarrhoea; aggressive behaviour; lethargy; weakness; anorexia (decreased appetite).

When vets suspect thyroid problems, we diagnose the problem using a blood test. Some cats with early thyroid disease have normal thyroid hormone levels in the blood; it is important that these cats are re-tested in 4-6 weeks, and sometimes a modified blood test is required.
HyperT4DiagramWhen diagnosing thyroid disease, it is important that we make sure that the cat has no other problems which may make treatment difficult, unrewarding or dangerous. For example, many older cats have kidney or liver problems. Sometimes we may need to take an x-ray to identify enlargement of the heart or problems in the chest or abdomen. We also like to test a urine sample to help us rule out other problems. Once we have a diagnosis of hyperthyroidism, there are several treatment options:

1)Injection with radioactive iodine

This is a very effective, non-invasive treatment. It is the treatment of choice in both cats and human beings. It is a simple, single injection. Cats generally do really well after this treatment.

Because they excrete radioactive material in urine and faeces after this injection, the cats must be kept in a special facility for 7-10 days until no longer excreting dangerous levels of radioactivity. This procedure is only performed at licensed clinics and costs about $750 depending on which clinic used.

2)Administration of Carbimazole tablets (Vidalta) to interfere with the production of thyroid hormone.

VidaltaFor cats that accept tablets readily (once daily), Carbimazole is an easy and effective form of treatment. Most cats will take these tablets readily if crushed and mixed with their food. Regular monitoring of blood thyroid levels (and kidney function) is important with this method of treatment. The tablets must be administered indefinitely. Stabilistion to normal thyroid hormone levels is usually achieved in two weeks. It has been widely used for years as a treatment for hyperthyroidism in cats and has few side effects.

3)Dermal Gel Methimazole.MethSpotOn

Methamizole is the active form of carbimazole and is available as a gel that can be rubbed onto the inside surface of the ear. This makes treatment much easier for most people because no pills are involved. Using a special application method, you just apply a bit of gel to the inside of the ear.

4)Surgical removal of the affected thyroid gland.

This method can be effective, especially with very enlarged glands, and means that the tablets or gel only have to be given to achieve initial stabilisation (2-3 weeks). The surgery involves a general anaesthetic. Cost is usually about $1200.00 including blood sampling.

5)Hills YD

Hills pet food makers have manufactured a new prescription diet for hyperthyroid cats. It is severely restricted in its iodine content, and trials have shown it to reduce thyroid hormone levels back to normal. This means no pills are required- nor any other treatment. However, (there is always a “but” isn’t there?) the cat has to eat this food exclusively-it is not allowed any treats or to steal the food from the neighbour next door. They are not supposed to eat game either but you have little control over this of course. Interestingly, rodents may not have high iodine levels in their carcases so their consumption might be acceptable if they do go hunting. No one has measured the iodine content of birds. This food comes in a wet and dry form. So if you can guarantee your cat won’t eat anything else, it may a be simple approach to take.


The best approach these days is to initially start these cats on the tablets or skin gel and administer these for a month or so. If the cats do really well, it is advisable to then do the radioactive iodine treatment so that neither owner nor cat has to endure persistent lifelong treatment.

The price of the carbimazole tablets and dermal gels now means that the radioactive iodine treatment is becoming more cost effective. It is also the most convenient and easiest treatment for both cat and owner and overall these cats seem to do better than those maintained forever on pills or gels.

The good news is that the disease can be successfully treated in almost all cases.

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