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Hyperthyroidism In Cats

Education > Cat Health & Behaviour 6th February 2019

In cats, hyperthyroidism and diabetes mellitus are the two most common endocrine (hormonal) diseases. The thyroid hormone has multiple functions, primarily involving the metabolic system, e.g. regulation of heat production (a person with hyperthyroid perspires a lot) and the control and “burning” of fat, protein and carbohydrates.

Hyperthyroidism (overproduction of thyroid hormones) is more common in older cats over 10 years old. The most common cause is a non-cancerous change to the thyroid tissue (thyroid hyperplasia) causing excessive hormones to be produced. Cancer of the thyroid glands (thyroid carcinoma) is rare.

There are two thyroid glands located in the neck on either side of the windpipe (ref: vcahospitals.com) which become enlarged in hyperthyroid patients. Too much thyroid hormone results in increased metabolism and hyperthyroidism. Too little results in hypothyroidism.


  • Weight loss despite increased appetite
  • Increased thirst and urination
  • Occasional vomiting and diarrhoea
  • Poor and unkempt coat
  • Hyperactivity or irritability
  • Tachycardia (fast heart rate)
  • Enlarged thyroid glands (goitre)


  • Reduced appetite and activity levels
  • Muscular weakness
  • Difficulty breathing
  • Thickening of heart walls
  • Arrhythmia (abnormal heart rate)
  • Blindness caused by high blood pressure


Blood tests to check for elevated levels of thyroxine (T4) in bloodstream. In early stages, total T4 level may be within normal range. Retesting should be done in 3 to 6 weeks, especially when clinical signs are present. If the cat is very ill or the owner does not wish to wait, a more sensitive free T4 concentration test can be done for diagnosis of early or mild hyperthyroidism.


Reduce production of thyroid hormone with anti-thyroid drugs, surgical removal of the glands or radioactive iodine therapy. A radioactive form of iodine is injected, usually subcutaneously, to destroy over-active tissues of the thyroid gland. Specialised facilities are required and the patient will be hospitalised. If owners are unable to administer medication or afford treatment, feeding an iodine-restricted diet has been shown to reduce thyroid hormone levels and clinical symptoms.

Hyperthyroidism may mask the signs of kidney insufficiency in cats, and when it is treated and corrected, the kidney condition becomes obvious months later. Be mindful of any difference in your cat’s appetite, thirst and weight. A cat can be sick even when he is eating and drinking very well.

Dr Gloria Lee
Mount Pleasant Vet Centre (Mandai)

Our dogs and cats age much faster than we do and are usually considered seniors when they are 7 years old. As our pets enter their golden years, twice yearly health screenings are recommended to detect age-related diseases at an early stage. We can help our pets stay healthy and pain-free for as long as possible.