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An owner's guide to hyperthyroidism

Hyperthyroidism is a condition caused by over activity of the thyroid gland. It is the most commonly diagnosed feline endocrine (hormonal) condition. The majority of cases are due to benign (non-malignant) tumours of the thyroid glands, with 70% cases showing involvement of both thyroid lobes (goitre).The thyroid glands are situated at the front of the neck, and if considerably enlarged they can often be palpated (felt) by your veterinary surgeon.

This condition predominantly occurs in middle to old age cats with an average onset of 12-13 years. Many cats will just look like they are getting older and thinner.

Thyroid hormones are involved in the regulation of metabolism and hence can lead to a number of clinical signs. Generally we will see signs relating to an increased metabolism.

Common clinical signs

  • Weight loss
  • Increased heart rate
  • Vomiting and diarrhoea
  • Restlessness
  • Abnormal behaviour
  • Increased urination
  • Increased appetite

The signs described can be symptomatic of many conditions, so it is important that a blood test is performed to ensure that other problems are not overlooked. Sometimes further investigations such as blood pressure, urine tests, and sometimes a heart scan, ultrasound or radiography may be needed. If appropriate, your vet will discuss these with you.
Once a diagnosis of hyperthyroidism has been reached, it is important to stabilise your cat's condition with one of the following treatment options. If left, the heart will be subjected to undue stress that can lead to potential failure, further weight loss will occur and life expectancy will inevitably be shortened.

Treatment 1: Oral Medication

  • This treatment is designed to block the production of the thyroid hormone within the gland.
  • Medication is life long and if stopped your cat will return to the hyperthyroid state.
  • Many owners are happy to administer oral medication, but obviously some cats are less amenable than others!
  • Medication is generally given 1-2 times daily
  • It is necessary to repeat the initial blood test to ensure that the levels of thyroid hormone are reducing back down to normal. Blood testes are generally reuqired at 3, 6 and 10-12 weeks then every 3-6 months
  • Unfortunately approximately 10% of cats will show some adverse reaction to medication including, anorexia (inappetance), vomiting, and lethargy. These effects are usually mild and transient and most occur within the first short period of treatment. Very occasionally blood disorders may develop, but these reactions generally resolve after withdrawal of therapy.
  • Regular check ups will allow your vet to monitor for any problems.

Treatment 2: Radiotherapy

  • This therapy requires a single injection of radioactive iodine
  • This substance concentrates in the thyroid gland and destroys the affected tissue
  • Unfortunately this treatment is only available at a few selected centres including Bristol, Glasgow and London Veterinary Schools, the Animal Health Trust and Barton Vets in Canterbury
  • Radiotherapy treatment can be arranged for your cat via a referral by your vet to one of the specialist facilities
  • This is a very safe and effective form of treatment but means a stay at one of these centres for a period of 2-4 weeks in isolation
  • Radiotherapy can be arranged for your cat via a referral by your vet to one of the specialist facilities

Treatment 3: Surgery

  • This is performed in order to remove the affected thyroid gland(s)
  • It is essential to try to stabilise the condition medically prior to surgery to help reduce the anaesthetic risks. This will generally involve a minimum three-week course of tablets given 2-3 times daily
  • Surgery usually results in a high curative rate but it must be remembered that it is not uncommon for the other gland to become affected at a later date
  • Both thyroid glands can be removed at the same time but this is the decision of the surgeon involved and will depend on the individual patient
  • The risk of post-operative complications is slightly increased with the removal of both glands and will often mean that your cat will stay in the hospital post operatively for between 1 and 4 days to be closely monitored
  • Surgery involving one side only generally only involves day surgery with check ups after 3 and 10 days
  • After surgery there is no need to continue on medication for hyperthyroidism. Any other concurrent medication must be continued as instructed by your vet

Maintenance Therapy

  • Once stabilised it is still important for your cat to have regular health checks
  • Hyperthyroidism causes an increase in blood pressure that drops to normal once the patient is stabilised by one of the treatment options detailed
  • The reduction in blood pressure is vital for the heart but can unmask an underlying kidney disease as the blood supply to these organs normalises
  • Monitoring is achieved by performing regular (every 3-6 months) blood and/or urine tests. These are simple and do not tend to require any sedation to perform
  • Your vet may advise a special 'prescription diet' or senior life-stage diet to help to keep your cat healthy

Ongoing Support

Finally, if you have any questions regarding your cat's condition and/or the information supplied here, then please discuss with one of the Grove Lodge vets who will be pleased to advise you.