About an overactive thyroid in cats

Updated November 21, 2016

Hyperthyroidism, or an overactive thyroid, is the most common hormone abnormality in cats. A disease of older cats, it's usually first diagnosed in cats that are at least 10 years old.


Hyperthyroidism in cats is usually caused by a benign growth on the cat's thyroid glands. The growth causes the glands to produce too much thyroxine or T4, the hormone that regulates the speed of the cat's metabolism.


Signs of an overactive thyroid in cats include weight loss, a ravenous appetite, increased drinking, increased urination, increased activity and restlessness, fast heart rate, poor hair coat, diarrhoea, occasional vomiting, aggressiveness or irritability, and howling, especially at night. Some hyperthyroid cats seem listless, weak or depressed.


An overactive thyroid takes a toll on the cat's heart and kidneys and can cause elevated liver enzyme levels. Many untreated hyperthyroid cats also have high blood pressure.


Hyperthyroidism is diagnosed with blood tests that measure the amount of T4 in the cat's blood. In addition, a veterinarian can usually feel a mass on the cat's neck.


Methimazole (trade names Tapazole or Felimazole) and carbimazole can regulate a cat's overactive thyroid and control hyperthyroidism. The medication is available as tablets or can be compounded into a flavoured liquid or gel to rub inside the cat's ear tip.

Radioactive Iodine

An injection of radioactive iodine can cure a hyperthyroid cat. Treatment is done at a specialised veterinary centre. The cat must be hospitalised until the amount of radiation it releases reaches safe and legal levels, usually three to five days.

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