Overactive thyroid disease in the cat

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This is a common disease in middle aged and older cats. It is technically known as hyperthyroidism

cat2The two thyroid glands are oval shaped and lie one on each side of the trachea (windpipe) on the underside of the neck. They produce thyroxin (thyroid hormone), which controls the speed of the cat’s metabolism.

In hyperthyroidism the thyroid glands are benignly (not cancerously) enlarged and produce excessive amounts of thyroid hormone. The enlarged glands can often be felt and are called goitres.

Hyperthyroidism is a common condition of middle aged and elderly cats. Due to excessive stimulation of the heart hyperthyroidism can eventually cause heart failure.

  1. Increased thirst
  2. Increased hunger
  3. Weight loss
  4. Poor coat condition
  5. Diarrhoea
  6. Increased yowling and feisty behaviour
  7. Over-activity – an old cat will often seem much more active, stressed and anxious than normal. Occasionally cats will be lethargic and very inactive instead.

Please Note: Not all hyperthyroid cats will show all these signs.

If your cat shows any of the signs mentioned above, a veterinary examination is advised. Most hyperthyroid cats will have very fast heart rates (over 240 beats per minute), often associated with a heart murmur (sound of the blood flowing in a turbulent instead of smooth way through the heart). In order to confirm the diagnosis blood tests will be done:

  • to rule out other common causes of increased thirst and weight loss – kidney failure, diabetes and bowel problems
  • to measure liver enzymes (signs of liver damage) which are often increased in hyperthyroid cats
  • to measure the level of thyroid hormone, which is increased in hyperthyroid cats. Occasionally the level of thyroid hormone will be normal even though your cat is hyperthyroid, so repeat or other tests may be advised if the vet is still suspicious after negative results.

Cats can be referred for special scans to locate abnormal thyroid tissue.

Most hyperthyroid cats are treated with tablets, given once or twice a day for life. The tablets are a type of chemotherapy, which decrease the amount of thyroid hormone produced so that metabolism slows to the correct level. The tablets do not usually have any side effects but occasionally can cause vomiting or itchiness.

The tablets should be given whole (not crushed). Pregnant women should wear gloves to handle the tablets and all women of child bearing age should wear gloves to deal with the faeces or vomit of cats on the medication.

If your cat is put on treatment he or she will need to be re-examined by the vet after 2-3 weeks. If the tablets are working your cat will usually have put on weight, have a slower heart rate, and may also be less active and calmer than before, with a decreased appetite and decreased thirst. Blood tests for the level of thyroid hormone will be repeated. If the level is low enough the dose of tablets may be reduced, but treatment is for life. Once the condition is under control you will be asked to bring your cat back for reassessment by the vet every 3-6 months and you will be able to request repeat medication between appointments.

We usually do blood tests to check for kidney failure as well. Once hyperthyroidism is under control and metabolism and blood flow are back to normal, a hidden kidney problem can be unmasked.

If it is difficult to give your cat tablets or if your cat is relatively young, surgery can be done to remove the enlarged thyroid gland or glands.

Ideally the hyperthyroidism should be under control with medication prior to surgery, to reduce the risk of heart problems under the general anaesthetic. Recovery is usually straightforward.

The tiny glands that produce hormones to control calcium levels are located one at each end of each thyroid gland, so there is a potential risk that these glands could be inadvertently damaged or removed with the thyroids. If both thyroids are removed there is therefore a small risk that your cat’s blood calcium level may fall dangerously low within 4-6 days of the operation. You will be given advice about what to look for and your cat may have monitoring blood tests. If calcium levels do fall they can be restored with medication, which may need to be for life.

Occasionally hyperthyroidism reoccurs in a cat which has had surgery. If one thyroid was removed, it is possible for the other to become affected at a later date. If both thyroids have been removed, some cats have extra thyroid tissue, somewhere else in the body, that becomes affected. If hyperthyroidism reoccurs, depending on the reason, treatment may be tablets, removal of the second thyroid or possibly referral to an outside veterinary institution for treatment with radioactive iodine to destroy extra thyroid tissue.

A special ultralow iodine prescription diet is available and is a potential alternative treatment to tablets or surgery.

Summary

Hyperthyroidism is a common, treatable condition of middle and old aged cats, which is diagnosed by blood tests.