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Hyperthyroidism In The Cat

Hyperthyroidism is an endocrine disorder which results from an overactive thyroid gland, where there is excessive production and secretion of the hormone thyroxin due to disease within the thyroid gland. It is generally a disease of older cats and the number of cases that we diagnose has been on the increase over the last ten years making it one of the more common conditions of the older cat.

Clinical signs

The symptoms that we see reflect an increase in metabolism so that initially the cat will lose weight despite an increasing appetite. You may notice your cat drinking more and behaving in a hyperactive way. Occasionally we also see a more atypical presentation of dullness, anorexia and vomiting. On clinical examination we usually find an increased heart rate, often with a murmur present -both of these are due to a thyroid induced cardiomyopathy. Often the liver is slightly enlarged and we may be able to palpate an enlarged thyroid gland (goitre).


To confirm hyperthyroidism a blood sample needs to be obtained which measures the amount of thyroxin in the bloodstream. We will also need to check kidney and liver parameters as this may influence the treatment and the longterm prognosis.


  1. Oral medication – this involves either once daily or twice daily dosing and an initial recheck of thyroxin and renal levels in 3 weeks with adjustments as necessary and repeat blood samples until stable. Once your cat is relatively stable we will aim to review control every 2-4 months but this will vary from case to case. Treatment is lifelong.
  2. Surgery -this involves a general anaesthetic and removal of the thyroid glands. This would be an option once medication has stabilised your cat and once surgery has been performed there is usually no need for further medication. The risks involved with the surgery would be fully discussed by the vet.
  3. Radioactive Iodine – this is done at a specialised referral centre and involves radiotherapy of the thyroid glands. Your cat will have to remain hospitalised for approximately 4 weeks post treatment. Pre treatment with anti thyroid medication is required as well as full urinalysis, blood pressure measurements and full blood work prior to referral.
  4. Prescription diet – this is an iodine deficient diet but is only suitable for indoor cats and must be fed exclusively.


The prognosis for control is generally good and cats can usually be managed successfully especially if diagnosed in the early stages before complications with the heart or liver have arisen.