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Hyperthyroidism in cats

Background

  • Hyperthyroidism (overactive thyroid glands) is a very common disorder of older cats. The syndrome is caused by overproduction of thyroid hormones from the thyroid glands, which are situated in the neck. In most cases this is due to a benign (non-cancerous) change in the thyroid glands.
  • Thyroid hormones play an important role in regulating the body’s metabolic rate thus increased thyroid levels typically cause weight loss despite having an increased appetite.
  • Cats generally respond very well to treatment if diagnosed early enough and appropriate treatment is started.

 

Clinical signs

These can be very subtle at first, but become more severe as the disease progresses. The typical clinical signs include one or more of the following:

  • weight loss
  • increased appetite
  • increased thirst
  • increased irritability
  • hyperactivity
  • gastro-intestinal signs (vomiting or diarrhoea)
  • unkempt coat

 

Secondary complications

  • High thyroid levels stimulate the heart to beat faster, and if left untreated can result in heart failure.
  • High blood pressure is another potential complication and can cause organ damage.
  • Once the underlying thyroid disease is treated the heart function and blood pressure should improve.
  • Kidney disease does not occur directly as a result of hyperthyroidism, but the two diseases often occur together because they are common in older cats.
  • The high thyroid levels can have a “protective” effect on kidney function, and in some cases when the thyroid levels are lowered this can result in “unmasking” kidney disease.
  • For this reason it is advisable to start a medical tablet trial initially and monitor the response of the treatment by means of blood tests to check kidney and thyroid function.

 

Diagnosis of hyperthyroidism

  • Your vet would have confirmed the diagnosis by taking a complete history, performing a physical examination and requesting blood tests.
  • Not all cats will have an obvious enlarged thyroid gland present, some cats have overactive thyroid tissue in an unusual (ectopic) site, often within the chest cavity.
  • Thyroxine (t4) levels will usually be increased in affected cats, as well as liver enzymes.
  • Other tests may include urinalysis, blood pressure measurement and cardiac investigations.

 

Treatment options

1)      Medical therapy (drugs)

  • Anti-thyroid drugs are available in tablet form and act by reducing the production and release of thyroid hormone from the thyroid gland.
  • Carbimazole is the drug of choice. Your vet will have determined a starting dose for your cat. Most cats will have thyroid hormone concentrations normalise within 3 weeks, and treatment will be adjusted according to response.  The frequency of revisits will depend on your cat’s clinical response.
  • Carbimazole is a relatively safe drug however side effects (though uncommon) can occur. These include poor appetite, vomiting, lethargy, jaundice and skin itchiness. If these signs occur, please contact your vet as soon as possible.
  • For cats which are impossible to medicate, transdermal methimazole is available from a compounding pharmacist, however, this drug is not as effective as Carbimazole and is more expensive.. Transdermal methimazole paste is applied to the inner ear of your cat.
  • Treatment will be lifelong in all cases, requiring regular check-ups and dose adjustments if required. It is for this reason that radioactive iodine therapy is recommended (see below).

2)      Surgical thyroidectomy

  • Surgery to remove the affected thyroid tissue can produce a cure in some cats; however surgery will not be successful if “ectopic” thyroid tissue is present.
  • Surgical risks include anaesthetic risks and risk of low calcium levels developing after surgery.

3)      Radioactive iodine therapy

  • Radioactive iodine (I131) is the gold standard therapy for hyperthyroid cats and is a once-off treatment.
  • Cats treated with radioactive iodine will have a survival time twice as long as cats treated with medications.
  • It has the advantage of being curative in most cases with no ongoing treatment required.
  • The radioactive iodine is administered by a single injection (or capsule), which then destroys the abnormal thyroid tissue.
  • Because this treatment involves the use of a radioactive substance, it can only be carried out at specially licensed facilities and a treated cat needs to be hospitalised for about one week.
  • Lort Smith Animal Hospital does not have this facility, but we can refer you to licensed facilities in Melbourne.

 

Please contact us to book an appointment with one of our vets if you have any questions. 03 9328 3021

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