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Feline Hyperthyroidism

Hyperthyroidism is a common disease in older cats. The thyroid glands, located in the neck and regulate the metabolism, grow and increase the production of the hormones that speed up the metabolism.


A non-cancerous tumor causes the enlargement.

In only 2% of cases, the tumor is cancerous.

If left untreated, hyperthyroidism will affect many organs. A high level of thyroid hormones will stimulate the heart to beat more rapidly. This leads to hypertension (high blood pressure), heart damage, and eventually heart failure.

Eventually, hypertension will damage the kidneys, liver, and eyes, resulting in sudden and permanent blindness.


The main signs of hyperthyroidism are increased appetite and weight loss.

Other signs may include:

  • Increased thirst and urination
  • Vomiting and diarrhea
  • Unkempt hair
  • Increased energy, restlessness
  • Increased heart rate, heart murmurs
  • Difficulty breathing (due to the developing heart disease)

If you notice any of the above signs, bring your cat to the veterinarian right away because there is danger of heart failure and death.


To properly diagnose your cat with hyperthyroidism, your veterinarian may perform the following:

  • Physical exam: thyroid and heart for protrusions and abnormalities
  • Blood tests: measure hormone levels and checks how well the liver and kidneys are functioning
  • Urinalysis: to check kidney function
  • Thyroid scan
  • Chest x-rays and cardiac ultrasound: to check the heart


Heart problems and hypertension can be treated. Typically, if blindness occurs, it is permanent.

There are 3 treatment options. Your veterinarian will choose one depending on your cat's specific situation.

  1. Radioactive Iodine: Given intravenously, it destroys the tumor without damaging other organs. Usually, the cat will have to be hospitalized for a week or two for supervision
  2. Surgery: To remove the entire thyroid gland. Some cats may need to take thyroid hormone supplements following surgery
  3. Oral medication: There is medication to suppress excess hormone production, but it doesn't destroy the actual growth and has to be given for life. Your cat will have to be monitored by your veterinarian every few months because there is a possibility of serious side effects


Cats that are promptly diagnosed and treated have a good prognosis. After surgery or radiation therapy, most cats can lead healthy lives. Cats on life-long medication can also live a normal life with regular follow-ups at the veterinarian.

Medically Reviewed by Sara Ochoa, DVM

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