This is one of the most commonly diagnosed endocrine (hormonal) diseases in cats.
Many older cats suffer from a variety of symptoms which might just be put down to ageing, or might previously have been attributed to kidney disease, but many of these will actually be in the early stages of hyperthyroidism. Over the last 20-30 years a great deal of research has been done on this disease, and treatment has improved as a result.
The thyroid glands lie in the neck, one either side of the windpipe, with occasional extra smaller glands present in some cats. The glands produce thyroid hormones which are involved in regulating metabolism, so they have an effect on most systems of the body. The glands can become enlarged and overactive, producing too much thyroid hormone. This is usually because of a benign (non-cancerous) change in the thyroid gland, but more rarely it can be caused by a tumour called a thyroid carcinoma.
The typical cat with hyperthyroidism will be an older cat with some or all of the following symptoms:
- loss of weight
- increased appetite
- increased thirst
- increased heart rate
- heart murmur
- restlessness or hyperactivity
- digestive upset
- an unkempt coat
- swelling of the thyroid glands in the neck
These sorts of symptoms will arouse suspicion that hyperthyroidism is the cause, but it can only be confirmed with a blood test to measure the levels of thyroid hormones. This would usually be combined with other tests to check kidney and liver function and to check for diabetes, as all these can have similar symptoms and of course there might sometimes be more than one problem going on.
If hyperthyroidism is diagnosed, treatment usually begins with tablets. The drugs reduce the production of thyroid hormone. The dose and frequency will depend on which drug is used and on how high the thyroid hormone levels were on the blood test. After 2 or 3 weeks a second blood test will show whether the levels are becoming closer to normal, at which time the dosage may be changed. If this treatment suits the patient, it can be continued long term with regular monitoring by blood tests. However, some cats are harder to give tablets to than others, and a few will suffer from side effects.
Another treatment option is surgical removal of the thyroid glands, which is usually very successful and offers a more permanent solution. The operation does involve some risks, particularly the risk of damaging other small structures next to the thyroids, like the parathyroid glands. (These are important in regulating the levels of calcium in the blood, and if damaged during surgery supplementation with calcium could be needed.) A cat with heart problems may be a poor risk for surgery, but often tablets can be used first to improve health so surgery is a better option, and additional drugs to control any heart problems may be given. In most cases, cats which have had their thyroids removed will not need to take tablets, but sometimes the problem can still return later, if for example the cat has some smaller gland tissue which was not removed with the main glands. This extra thyroid tissue, known as ectopic thyroid tissue, can be located anywhere in the neck or even within the chest. In some cats only one thyroid is affected at first so only one is removed, then some years later the same condition could occur on the other side.
The other main treatment available is with radioactive iodine, which is a specialist treatment only available at some centres in the UK. Radioactive iodine is given to the cat by injection and it becomes concentrated in the thyroid gland, where the radioactivity destroys the damaged tissue. One of the disadvantages of this treatment is that the cat has to be hospitalised for several weeks because of safety issues surrounding the radioactive material used. It is not dangerous to the cat itself but has to be handled safely to protect people working with it.
Decisions on which treatment would be best for an individual cat are best made in conjunction with the vet who knows all the details of the case. Where complicating factors like heart disease or kidney disease are present, these need to be treated as well. Once diagnosed, the outlook for a cat with hyperthyroidism is usually very good. Whichever treatment is used, it is likely to prolong life and improve the quality of life.
If you are worried that your cat is showing any of the symptoms listed, talk to your vet or use our interactive Cat Symptom Guide to help you decide what to do next.