Hyperthyroidism in cats

cat hyperthyroidism

Hyperthyroidism is the term for over activity of the thyroid glands. A cat has two thyroid glands which are found in the neck area either side of the windpipe. The glands produce the hormones that control the cat’s metabolic rate. The signs seen in a cat with hyperthyroidism are due to the enlarged gland producing more of these hormones.

Typical signs of hyperthyroidism in cats

Hyperthyroidism most commonly affects middle to old-aged cats and is rare in cats less than seven years old.  Affected cats usually develop a variety of signs which become more severe as the disease progresses.

The classic signs of hyperthyroidism are:

  • weight loss (despite an obvious increase in appetite);
  • increased thirst;
  • increased activity;
  • increased irritability;
  • restlessness;
  • sometimes mild to moderate diarrhoea and/or vomiting; and
  • possibly heat intolerance with your cat seeking out cool places to sit.

When examined by your vet, your cat will have an increased heart rate and commonly a heart murmur where none had been previously diagnosed. There will probably, but not always be, a palpable swelling in the throat area and the enlarged glands can often be felt by your vet as pea-sized, and in some cases marble-sized, lumps. The most noticeable feature though will almost certainly be the amount of weight lost.

Although the clinical signs associated with hyperthyroidism can be quite dramatic, and cats can become seriously ill with this condition, most cases are completely treatable and cats usually make a complete recovery.

So, if all of the above sounds familiar, please bring your cat in for a full check-up. All it may take is a blood test to confirm whether your cat is hyperthyroid or not. You can then get treatment started so you can once again have a happy and healthy cat.

Secondary complications with hyperthyroidism

As stated above, hyperthyroid cats have an increase heart rate due to the excessive circulating thyroid hormones – usually over 200 beats per minute. As the disease progresses this can cause a thickening of the heart muscle wall. In severe cases these heart changes can lead to heart failure.

Another complication seen in hyperthyroid cats is high blood pressure and if present, this will also need to be medicated against. The good news is that once the hyperthyroidism is under control, a lot of the cardiac changes often improve.

Reaching a diagnosis

If you or your vet suspect that you cat is hyperthyroid, the diagnosis can usually be simply confirmed with blood tests to measure the thyroid hormone concentrations.  It is also sensible to check your cat for other diseases at the same time such as kidney disease which may affect treatment and prognosis for recovery.  On very rare occasions blood tests do not give a definitive diagnosis of hyperthyroidism and your cat may need to be referred to a specialist centre for special scans to evaluate the thyroid gland and detect any abnormal tissue.

Treatment of hyperthyroidism

Medical management (drug therapy)

Anti-thyroid tablets (usually given once or twice a day) reduce the production of thyroid hormones. Although they often control the disease successfully, they do not cure it, so life-long therapy is needed.  Occasionally side-effects develop to the drug therapy and intermittent monitoring of the level of thyroid hormones in the blood is recommended.

Surgical treatment – thyroidectomy

Surgical removal of the affected thyroid tissue is a common treatment for many hyperthyroid cats.  Generally, this is a successful procedure although, even after successful surgery, signs of hyperthyroidism occasionally develop later if previously unaffected thyroid tissue becomes diseased.  To minimise surgical risks, cats are usually stabilised by drug therapy before being operated on. One of the possible complications of thyroid surgery is that surgery can sometimes interfere with the parathyroid gland which is very closely associated with the thyroid gland. Cats need to be monitored closely in the 3 days following surgery to ensure that these problems do not develop.

Radioiodine treatment

A third option for treatment involves injecting radioiodine under the skin. This accumulates in the abnormal thyroid tissue and destroys it. This cures the problem and has the added advantages of no serious side effects and an anaesthetic is not required. Unfortunately it is only available at specialist centres, your cat has to be hospitalised for three weeks so you aren’t exposed to the radiation and it is expensive.

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