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Hyperthyroidism, or overactive thyroid gland disease, is common in cats that are middle-aged and older. It occurs when the thyroid gland overproduces the thyroid hormones T3 and T4, which are then released into the bloodstream. The thyroid gland has two lobes (right and left) that are found on the neck on either side of the windpipe. The hormones produced by the thyroid gland control the metabolism process.

Thyroid hormones help control the body's metabolic rate and activity level, so cats with hyperthyroidism tend to burn energy quickly and lose weight despite having an increased appetite and eating more.

In many cases, hyperthyroidism is caused by a benign (non-cancerous) hormone-producing tumor. A malignant (cancerous) tumor known as a thyroid adenocarcinoma can also cause hyperthyroidism, but this occurs only rarely (in only one to two per cent of all cases).

Cats generally respond well to treatment, especially if the condition is identified early. If left untreated, feline hyperthyroidism can lead to many complications as the thyroid hormone impacts the heart rate by increasing it. Hyperthyroidism can also cause heart failure, heart murmurs, high blood pressure, kidney damage and blindness. Felines with hyperthyroidism can also have raised liver enzyme levels.


  • weight loss despite increased appetite
  • increased thirst and urination
  • increased irritability
  • restlessness
  • rapid heart rate
  • un-kept coat
  • mild to moderate diarrhea
  • vomiting
  • heat intolerance (seeking cool place to sit)
  • panting when stressed

Additional Support

Visit your vet if you notice any of the symptoms listed above. Your vet will make his or her diagnosis based on your pet's history, symptoms, and a physical examination confirmed by a blood test to measure hormone levels. It is important to note that feline renal failure and diabetes may have similar symptoms, so do not self-diagnose your pet.

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