Hyperthyroidism in Cats

Have you been noticing that your 13-year old cat has gotten thinner? Maybe acting a little more affectionate or is hungry all the time? You may want to write these symptoms off to the fact that your cat is “getting older”…but don’t be fooled, there could be a problem with her thyroid gland.

Did you know that cats can suffer from thyroid conditions, like humans and dogs? It is unfortunate, but more and more cats are being diagnosed with thyroid disease as they age.

What is the thyroid?

It is important to know a little bit about the thyroid gland before taking a look at how things go wrong. The thyroid gland is located in the neck of cats and other mammals. The major thyroid hormone, thyroxine, is produced here and helps control many body functions, such as body temperature, heart rate and muscle strength.

What is hyperthyroidism?

Hyperthyroidism means high thyroid hormone levels. Hypothyroidism means low thyroid hormone levels. Hyperthyroid is more common in cats, while hypothyroid is more common in dogs. Humans can suffer from either one or the other.

Experts are not sure what causes hyperthyroidism in cats. Some think it is genetic, while there is some evidence that environmental pollutants are to blame. A recent study showed a link between flame retardants and thyroid disease in cats. Flame retardants, such as PBDEs (polybromated diphenyl ethers) became more and more common in our homes during the last 30 years. In this same span of time, feline hyperthyroidism went from being a very rare disease to being one of the most common. It is worthwhile to note that indoor cats are exposed to other chemicals in the home, so it could be a combination of factors causing the disease.

Signs of Hyperthyroidism

The signs of hyperthyroidism are similar to signs of other diseases. Talk to your veterinarian if you are concerned about your cat’s health. A few key symptoms include:

  • Weight loss
  • Increase in appetite
  • Decrease in appetite
  • Drinking lots of water
  • Vomiting
  • Diarrhea
  • Hyperactivity
  • Increase in vocalization (meyowing all the time)
  • Demands attention
  • Increase in blood pressure
    High heart rate
  • Heart murmur
  • Dilated pupils
  • Sudden blindness


If you suspect that your cat may have hyperthyroidism, see your veterinarian. Your veterinarian will perform a physical examination at your visit. Important aspects of the exam will focus on the hair coat, body condition, heart rate, heart sounds and thyroid palpation. Sometimes your veterinarian can palpate, or feel a nodule on the thyroid gland in the neck. While this is not confirmation of thyroid disease, it is a significant indicator that advanced testing needs to be done.

Most veterinarians will perform a T4 blood test. This is a “screening” test in cats and is important for ruling-out the disease. If the T4 test is normal, then your cat most likely does not have the disease. T4 levels can fluctuate throughout the day and can be abnormally low if the cat is sick from something else.

If the T4 level is high or border-line, a more specific test should be done. The additional “confirmation” test is called a Free T4 by Equilibrium Dialysis. The method of Equilibrium Dialysis is a little more expensive than other methods, but it gives the most reliable results.

Treatment Options


Hyperthyroid kitties are some of the most rewarding patients to treat because veterinarians can CURE this disease! In over 90% of cases, hyperthyroidism can be completely cured.

The “cure” is a form of radioactive iodine treatment, called I-131. Before you get all nervous about the “radioactive” part of that statement, this treatment is used in humans and cats with safe, reliable results. Basically, a radioactive form of iodine is injected under your cat’s skin. It is taken up by the thyroid, where the radioactive element ‘kills’ the sick part of the thyroid. The radioactivity will linger for a couple of weeks after treatment and does not have any other effect on the cat.

I-131 treatment is expensive, ranging from $1200-2500 depending on where you live and how long the cat must be hospitalized. Only certain regulated facilities can perform the treatment, many being feline-only hospitals. Your cat will be admitted to the facility and given the injection after being found otherwise healthy. He or she will then be pampered in a special boarding facility for 4-7 days while being “radioactive.” The boarding is necessary to avoid exposure to humans, especially children. The radioactivity wanes quite quickly and then the cat is allowed to go home. Special care must be taken while at home, with specific instructions on how to handle the cat’s waste. The cost and extra effort is worth it to many, with good success.


Methimazole is an oral medication that is given to reduce thyroid levels. It is an inexpensive option if I-131 treatment is unavailable or cost-prohibitive. It is very effective but is not tolerated by all cats and can cause liver problems. If you cannot give your cat a pill, or if the pill causes an increase in vomiting or diarrhea, a cream can be applied to the hairless portion of the inner ear. Methimazole topical gel or cream is available through veterinary compounding pharmacies.


Thyroid surgery can be done to remove the diseased parts of the thyroid. This treatment is about the same cost of I-131 therapy, so it has decreased in popularity in recent years. Other considerations include anesthetic risk and damage to the parathyroid gland. Sometimes not all the diseased tissue is removed and the cat remains hyperthyroid.

With any of these treatments, the cat could potentially become hypothyroid. If this happens, thyroid hormone must be supplemented orally.

Other Issues

Sometimes kidney disease happens concurrently with hyperthyroidism. The high level of thyroid hormone increases the cat’s blood pressure, which can improve kidney function in the short-term. Often, when hyperthyroidism is treated and controlled, your veterinarian may notice a change in the kidney function on routine bloodwork. This is not due to the thyroid treatment, it simply is being “unmasked.”

The Bottom Line

Many cases of hyperthyroidism are caught at the cat’s annual or bi-annual wellness visit. Many veterinarians begin screening middle-to older cats with bloodwork on a yearly basis and this screening includes a T4 level. Sometimes the cat may not show many symptoms (maybe an increase in water consumption and some weight loss) and a high T4 flags the issue. In these cases, diagnosis can be made quickly and treatment initiated. It is always best to catch thyroid disease early, as it can have serious consequences if untreated. The most common issues secondary to untreated hyperthyroidism include heart failure and blindness.

Always remember to ask about thyroid screening if your veterinarian does not routinely offer it. Your cat may thank you!

Dr. Nichole Agarwal (Claremont Montclair Veterinarian)