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What is Hyperthyroidism?
The thyroid glands are two small glands located in the neck, near the windpipe. They produce a hormone called thyroxine, which plays an important role in the metabolism of every cell in the body. Hyperthyroidism is a relatively common condition which occurs when the thyroid becomes overactive, producing more thyroxine than is normal. This excess thyroxine circulates in the bloodstream and affects the body in a variety of ways.
Older cats are most commonly affected by hyperthyroidism; it is extremely rare in dogs (canine thyroid disorders tend to be underactive – for more information please refer to the separate information sheet on hypothyroidism).
A high level of thyroxine in the body causes a very fast metabolism, so the cat uses up energy much faster than is normal. This often makes your cat very hungry, whilst losing weight. Cats with hyperthyroidism can also seem very restless (sometimes even aggressive), may drink more than normal (and so urinate lots too), may have a poor haircoat and have gut upsets.
Diagnosis
There are many possible reasons for an older cat to lose weight, so your vet will need to give your cat a full clinical examination. Sometimes it is possible to feel the enlarged thyroid gland in your cat’s neck and hyperthyroid cats have a fast heart rate, sometimes with a heart murmur that your vet can hear with a stethoscope. Obviously a healthy cat which is scared will also have a fast heart rate though!
If your vet suspects your cat may have hyperthyroidism, routine blood tests will be done and we will also send a blood sample to an external laboratory for a specific thyroxine test. In a few cases, this test is not conclusive and further tests may need to be done.
Treatment
Hyperthyroidism can be treated in one of three possible ways. It is often treated using an oral tablet, usually given twice a day (there is a new once-a day tablet now available too). After about a fortnight of tablets, another blood test will be done to monitor your cat’s thyroxine level, and decide whether the tablet dosage needs altering. It is also important to have a blood test to look at your cat’s kidney function – some cats have damaged kidneys as well as being hyperthyroid (both can be quite common in the older cat), and treating the hyperthyroidism can actually worsen the kidney function. In these cases it is a balancing act to optimise the health of both the thyroid and the kidneys, hence regular blood tests are essential. Once your cat’s thyroxine level becomes stable, blood tests may only need to be repeated every 6 months or
even less frequently.
Whilst it is always best to stabilise your cat on tablets, there are two other options for long-term treatment. Surgery to remove the overactive part of the gland is a common treatment option, especially if your cat doesn’t like taking tablets. There are always risks associated with any surgery, and your vet will discuss these with you.
Radioactive iodine therapy is the final option, using radiation to destroy the overactive thyroid gland. This treatment can only be done in a few places in the UK and requires your cat staying away from home for a couple of weeks, but again it may be a valid option if your cat dislikes tablets.
Without any treatment, the high metabolic rate puts a lot of strain on your cat’s heart and can lead to heart failure, as well as your cat being unable to put on weight and feeling anxious. With your vet’s help, the appropriate treatment option for you and your cat can reverse the signs of hyperthyroidism, so your cat can continue to lead a normal and healthy life.
