Hyperthyroidism in Cats - Factsheet

Cats in older age can develop an overactive thyroid, due to a benign tumour and this leads to excess thyroxine circulating in the body.

Thyroxine is a hormone that controls the rate of metabolism of the whole body. It has far-reaching effects on all the other organs:

The Heart

This can cause heart muscle damage and typically they develop a murmur. Later on, if left untreated you get heart failure, which is quickly fatal.

Burning up calories

The amount of food that is eaten cannot replace the calories burned up in the body. The cat therefore has to dip into reserve supplies, and these come from the fat and muscle. The cat therefore loses weight. It can be quite drastic too with the cat losing up to 1/3rd of its weight. The rapid processing of food leads to diarrhoea.

Organ Damage

The protein and fat that is being broken down has to be processed by the liver. This puts an extra strain on the liver itself, and the breakdown products have to be eliminated by the kidneys. The extra work overwhelms the kidneys, which can go into failure.

Behavioural

The cat becomes 'hyper' – they don't settle and can pace around. Often placid cats become bad tempered.

Increased vocalisation – usually at night, when they meow all the time, which disturbs the owner!

TREATMENT OPTIONS

There are four possible solutions:

1. Support the body

- Give extra food to provide the extra calories needed.
- Stop weight loss by monthly injections of an anabolic steroid as these make the absorption of food more efficient.

This option will only give temporary relief and is not recommended.

2. Medication

- Anti-thyroid tablets are given daily to return the thyroxine level to normal.
- This is a realistic option for many people who do not want surgery, or who attend a veterinary practice that does not have full anaesthetic monitoring facilities.
- However giving two tablets daily for life isn't always easy in cats, as anyone who has tried will know! Also, the drug can damage the liver after a few months, and sickness is a fairly frequent side effect.
- So at best the drug, which is an anti-cancer drug, can keep the thyroxine levels stable but it doesn't actually get rid of the tumour. It is not cheap and over time can cost considerably more than surgery.

3. Surgery

- There are two thyroid glands in the cat, and close to these are the four parathyroid glands which regulate calcium levels in the blood. The cat can live without the thyroids but not the parathyroids, so removing the thyroids isn't a problem. (One or both glands may be affected).
- Surgery calls for a steady hand but the operation is routine and can usually be completed within about 20 minutes. Most patients go home the same day. In rare cases (about 1:50) the blood calcium drops temporarily after surgery but this can be corrected easily and shouldn't have any lasting effects.
- Within two weeks of surgery the patient has usually started to put on weight, be less vocal and the heart settles. Indeed, we have found that any heart murmur disappears. By about three months post-op, the cat looks about five years younger!
- Surgery gets rid of the tumour altogether so is the best option for most patients. The risks are small in experienced hands and where the operating theatre has full monitoring, as it has at this practice.We tend to recommend this option because it is the best for the patient, is not expensive and it works.

4. Radio Therapy

The use of radioactive iodide is commonplace in human patients and can be used in cats too. It is considered the gold standard of treatment because it is so accurate in knocking out and killing only the thyroid tissue. However your cat is radioactive for six weeks so has to be hospitalised at a referral hospital for that length of time and can't be stroked during that time. This makes it an option of last resort. It is also very expensive too.

Note: thyroid operations on cats are now one of our most frequently performed procedures, second only to cat neutering. Our considerable experience in doing this operation, combined with the use of our state-of-the-art anaesthetic facilities, means that our elder feline patients can really expect to reach a good and healthy old age!