Ginger is a great example of the benefit to having our senior pets thoroughly examined every year. Ginger is a 13 year old female Domestic Shorthair cat who has been a regular visitor to our clinic for her yearly check ups. Over the last 4 years she has gradually been losing weight and this year her owners reported that she has started to become disoriented around the house and vocalising more than she used to. She is ravenously hungry and occasionally vomits. Despite eating everything in sight she has now lost 25% of her bodyweight. Blood tests done at her most recent visit confirmed that she had the disease Hyperthyroidism. She will now start on oral tablets and be reassessed in 3 weeks time to check her response to therapy.

Hyperthyroidism is a common hormonal disease of older cats. It occurs when the thyroid glands (in the neck) produce too much thyroid hormone. Thyroid hormone controls the metabolic rate of the body, so cats with this condition burn up energy very quickly, typically losing weight despite eating large amounts. This disease is common in cats over 8 years of age and can occur in any breed, any sex and in – de-sexed or entire cats.

Clinical Signs

Clinical signs of hyperthyroidism often develop slowly so that owners initially dismiss them as normal aging. They then become quite dramatic and cats can become very sick with this disease.  The indicators of possible hyperthyroidism are:

1. Weight loss
2. Increased appetite
3. Increased thirst
4. Irritability
5. Restlessness

6. Vomiting
7. Diarrhoea
8. Weakness
9. Rapid heartrate (>200)


In most cases the increased thyroid hormone is due to a non-cancerous (benign) change in one or both thyroid glands. This means that the cats usually respond very well to treatment. Untreated cats with hyperthyroidism are very likely to develop “hypertrophic cardiomyopathy” (a heart problem) and high blood pressure which will in turn damage other organs. Blindness due to retinal haemorrhages is often a result of prolonged hypertension in cats.Hyperthyroidism does not cause kidney failure but often the two conditions occur together and treating one can impact on the other so the kidney function should be monitored closely.


The definitive test for hyperthyroidism is a blood test measuring T4 (thyroid hormone) levels. At the same time we check renal and liver enzymes and measure blood pressure. On rare occasions the T4 level may be normal in a hyperthyroid cat and if we suspect this we will repeat the test a few weeks later.


There are several treatment options available. Medical treatment is the cheapest in the short-term but if the cat lives for several years more it can become more expensive over that lifetime than the surgical or referral options.


The drugs carbimazole/methimazole work to reduce the circulating thyroid hormone and usually blood T4 levels are normal within 3 weeks. This treatment is then continued for life with annual blood tests to monitor for side effects and T4 levels. Side effects are uncommon but in some animals there may be a few weeks during which appetite is depressed and there may be occasional vomiting. It is very rare for more severe side effects to occur. If your cat is difficult to tablet there is a transdermal gel which can be specially ordered. This gel is rubbed into the ear flap, it is then absorbed and acts in the same way as the oral medication


Surgical removal of the glands is possible and is usually curative so after surgery no further medications are needed.Patients are generally treated with anti-thyroid drugs for a month prior to surgery to stabilise them and to reduce the anaesthetic risks. Also the two glands are often done at separate times to prevent the risk of problems with the parathyroid gland. Occasionally the hyperthyroidism can recur after surgery.

Radioactive iodine

It is possible to use treatment with radioactive iodine which concentrates within the cells of the thyroid gland and destroys them. This is another option that cures the cat, it has minimal side effects, a low reoccurrence and does not require an anaesthetic. It is also suitable for the treatment of those very rare malignant cancers of the thyroid gland. 95% of cats are cured after one treatment and the other 5% are usually cured by the second treatment. Due to government regulations the cat needs to be hospitalised for a week.


There is now a prescription diet available that inhibits the production of thyroid hormone. The diet doesn’t contain iodine, one of the precursors for thyroid hormone. This treatment is a good option in single cat households only as healthy cats need iodine in their diet. Blood tests are used to monitor progress.