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Feline Hyperthyroidism

What is Hyperthyroidism?

Hyperthyroidism is one of the more common diseases in the senior cat. It is caused by a tumor (which is usually non-cancerous) of the thyroid gland. The enlarged thyroid gland secretes excessive amounts of thyroid hormone. The thyroid hormone regulates the metabolism of your cat’s body, so the elevated levels speed everything up in your cat’s body.

Clinical Signs

The most common clinical sign of hyperthyroidism is weight loss despite an increased appetite. Other signs are increased thirst and urination, vomiting, diarrhea, and hyperactivity.
Hyperthyroidism can also lead to heart disease. Affected cats may have a heart murmur, cardiac arrhythmias, rapid heartbeat or trouble breathing.


Hyperthyroidism is diagnosed by a blood test called “T4”. Cats with hyperthyroidism have elevated T4’s. It is also important to run a blood chemistry panel and urinalysis to evaluate the health of other major organs. Kidney disease in particular may complicate hyperthyroid treatment. Other tests that may be run are chest radiographs and EKG if concurrent heart disease is suspected.


There are four ways to treat hyperthyroidism:

  1. New! y/d diet made by Science Diet/Hill’s
    A therapeutic food has been developed that is low in iodine and has been shown to normalize thyroid levels in hyperthyroid cats. This food is available in dry and canned forms. Your cat will need to eat this food 100% of the time to control hyperthyroidism. Your other normal healthy cats can eat a portion of y/d, but it should not be their complete diet.
    If your cat has been taking thyroid medication, then she can be started on y/d and weaned off of her meds.
    Blood tests are run after 3 weeks of consuming the diet and then every 6 months. These tests cost about $100.
  2. Anti-thyroid medication – Felimazole (oral) / Transdermal medication (methimazole)
    These medicines work by decreasing the amount of thyroid hormone produced. The hormone levels normalize and your cat will gain her weight back and the other clinical signs will disappear. The medicines are usually given twice daily, and they must be given for the rest of your cat’s life, since they do not cure the disease, but just keep it under control.
    Possible side affects of the oral Felimazole are vomiting, poor appetite and lethargy. These usually resolve in 1-2 weeks. Other, more severe side effects can be intense itching, liver problems, or bleeding problems, but these are very rare.
    Transdermal medication (methimazole) has fewer side effects and may be easier to give than felimazole. It also has to be given for the rest of your cat’s life.
    Both medicines cost about $35-45 per month. In addition periodic blood tests need to be performed to make sure the drug is working and no side effects are occurring. Blood tests are run after 2 weeks of initiating them, and then every 4-6 months, depending on the cat. These tests cost about $100.
  3. Surgical removal – Thyroidectomy
    Surgical removal of the enlarged thyroid gland(s) will cure hyperthyroidism. Cats with concurrent renal disease are not candidates for this surgery. There are also risks associated with surgery, such as increased anesthesia risk, hypocalcemia, and re-growth of the glands. The cost for surgery is around $1200-1400.
  4. Radiation Therapy
    Radiation treatment also cures hyperthyroidism. It is the “Gold Standard” for treatment. Again, cats with kidney disease are not candidates for this treatment. Radiation therapy is done at a referral center. Your cat will stay there for about 4-5 days. The cost is around $1,500 – 2,000.

Curative treatments (surgery and radiation therapy) should be seriously considered in younger cats (less than 15 years old) with hyperthyroidism. “Young” hyperthyroid cats can live for several more years after diagnosis. The cost of anti-thyroid medicine, prescription foods, and blood tests adds up over the years and may be more expensive in the long run than a curative treatment. In addition, some cats given anti-thyroid medicine for several years will develop malignant cancer in their thyroid glands which is very difficult to manage. It is currently unknown if feeding y/d for many years will lead to a malignancy.