Behavior Eating/Drinking Health Hygiene

7 Facts About Hyperthyroidism in Cats

Learn six important facts about hyperthyroidism in cats, including symptoms and treatments.

Hyperthyroidism in cats is a common but usually treatable condition affecting many older cats. I have had one foster and one resident cat with it, and I fully expect to deal with it more in the future.

Here are some basic facts about hyperthyroidism in cats so you can better understand the condition and make the best decision on treatment for your personal situation and cat!

1. The thyroid is a gland in the neck that produces the hormones thyroxine (T4) and triiodothyronine (T3), which control metabolism.

Thyroids contain two lobes, the left lobe and the right lobe, and are connected by a bridge, giving the entire gland a butterfly shape.

The thyroid makes T4 and T3, but T4 is also converted into the more potent T3. T3 is then ultimately responsible for metabolism (or how fast cells work. This is important because hyperthyroidism affects the amount of T4 that is produced, in turn affecting T3.

2. Hyperthyroidism in cats occurs when there is a benign growth, an adenoma, on the thyroid, but the cause of this tumor is unknown.

What causes the adenoma to grow is unknown, but it is benign in at least 95% of cats. It’s also found in outdoor and indoor cats, meaning a possible hypothesis is there is something about the cat’s physiology that makes them more predisposed. This means commercial cat food is likely not the cause of the condition, or it would be found much less in outdoor cats and strays.

The tumor causes the thyroid to produce more T4, which results in more T3, speeding up the metabolism.

Of note, hyperthyroidism is very rare in dogs, and is almost always cancerous.

3. Common symptoms of hyperthyroidism in cats include increased appetite, vocalization and irritability; diarrhea and vomiting; weight loss; and changes in grooming habits.

Most of these symptoms are a result of a hyperactive metabolism. The vocalization is one of the more noticeable symptoms.

Changes in grooming habits can include not grooming or excessive grooming, including pulling fur out.

Black and white cat on bed
Pepe had hyperthyroidism, which is a commom condition in senior cats.

4. Additional effects on the body include changes in the heart, liver, and kidneys.

Possible Heart Issues

  • Heart murmur (common) – Extra sounds created from blood not flowing smoothly.
  • Hypertrophic cardiomyopathy – An increased heart rate can cause the heart muscle to thicken, which can make it harder for the heart to pump blood. This is one of the biggest reasons to treat hyperthyroidism.
  • Cardio arrhythmia – Irregular heartbeat.
  • Systemic hypertension – High blood pressure.

Possible Liver Issues

Increase liver enzymes – Because T4 is metabolized in the liver, the increase in this hormone causes an increase in ALT and APL liver values in more than 90% of cats with hyperthyroidism.

Possible Kidney Issues

Mask kidney disease, or speed up progression of disease – Because hyperthyroidism increases renal blood flow, it can make the kidneys appear like they’re working better than they are. At the same time, untreated hyperthyroidism in cats can accelerate kidney disease. The thyroid and kidneys work closely together, plus the impact on the heart can further impact the kidneys.

5. Typically a senior disease, the average age for cats with hyperthyroidism is 13 years old.

Only 5% of cats with hyperthyroidism are under 10 years old, which makes it predominantly a senior disease. Other common senior conditions include diabetes and kidney disease.

6. There are three treatments for hyperthyroidism in cats: medication or a prescription diet to manage it, or radioactive iodine (I-131) therapy to cure it in 95% of cats.


Methimazole is the medicine used to treat hyperthyroidism in cats. It interferes with the pathway used to make hormones in the thyroid, resulting in less hormone production.


  • Easy to administer
  • Low cost
  • Dosage is adjustable as needed


  • Need frequent blood work, starting with 2-4 weeks after starting meds, then 3-6 months for life
  • Doesn’t cure condition or remove adenoma – stop med and condition returns
  • Can cause liver disease in very small percentage of cats, along with other side effects like GI issues, bone marrow toxicity and facial scratching
  • Chance of resistance over time
  • Increased chance of adenoma turning carcinogenic in some cats (not necessarily related to use of methimazole, but related to adenoma)


Hill’s y/d prescription food has a lower level of iodine than other commercial or prescription diets. Remember, iodine is one of the parents of T4 and T3, so less iodine means less hormones.


  • Convenient
  • No medication required


  • Not good for picky eaters
  • Not good for multicat households because hyperthyroid cat can never have regular food and other cats cannot eat this food
  • Need blood work in first 4-8 weeks, then every 6 months for life
  • Doesn’t cure condition – stop giving food and condition returns
Snuggling bonded pair: Pepe and Rigsby were rescued from a hoarder along with nearly 30 other cats. They both needed full mouth extractions because their teeth were terrible.

I-131 aka Radioactive Iodine Therapy

As a result of hyperthyroidism, the healthy part and cells in the thyroid shut off and the overactive part stays on. Iodine is processed in the thyroid, but the thyroid can’t distinguish between different types. During I-131, radioactive iodine is injected into the cat and it kills the hyperactive cells and abnormal parts of the thyroid. Then the healthy part of the thyroid “turns on” and works again. Radioactive particles are removed through waste, and it takes 3 weeks from the first day of treatment for radioactive particles to be completely out of the cat’s body.


  • Cures 95% of cats with hyperthyroidism within 3-6 months (typically sooner)
  • Usually one and done, but may require an additional treatment if not cured
  • No anesthesia needed, just a simple injection


  • High upfront cost (over $1,000 in most geographic locations), with necessary diagnostics before procedure
  • Requires short hospital stay
  • Cat must stay in a separate area with limited interaction with owner until radioactive particles are completely gone. Must use flushable litter and wear gloves when scooping. Litter box must be kept in a separate bag after and disposed of 3 months after.
  • Does not work in small percentage of cats – still require meds
  • Risk of hypothyroidism developing after treatment

7. Although rare, methimazole can cause liver toxicity. The hospital visit and treatment costs associated with this side effect end up making it more expensive than I-131 therapy.

Only 2% of cats are said to have issues with the drug methimazole, but it did happen to my foster cat Pepe. He was totally fine for about 6ish weeks and his thyroid normal. Then, he started acting odd and lost his appetite. He quickly became jaundice, with his skin and the white of his eyes turning yellow. An ultrasound and blood work showed an inflamed liver and high liver values. The methimazole was stopped.

After a hospital stay and nearly dying, he remained on supportive care and lots of meds (anti-nausea, antibiotics, steroids, you name it). We put him on liver supplements for a number of weeks, but he still needed his thyroid treated. Since the prescription diet wasn’t an option (he was bonded with another cat who wouldn’t be able to eat it), the only option was I-131. After the therapy, all levels were normal and he’s doing great nearly a year later.

Tabby cat and black and white cat snuggling on cat bed
Pepe and Rigsby found their happily ever after together.

If you are choosing meds over I-131, just remember that treating liver toxicity is expensive and stressful. The costs of a hospital stay and meds far exceeds the cost of I-131. Keep in mind, even if methimazole works for a cat, there is still a cost of the meds, frequent blood work, vet visits, plus travel time and expenses. If a cat lives another 1.5-2 years on the meds, you’ll likely have paid more than the I-131 therapy.

Good Luck

Always consider all of your options, pros, cons, and upfront and long-term costs when you’re making treatment decisions for cats with hyperthyroidism. Remember the most important part is that the issue is managed to prevent other more serious health issues.


Feline Radioactive Thyroid (I-131) Treatment, by VCA Animal Hospitals.

Hyperthyroidism in Cats, by Cornell Feline Medical Center.

Radioactive Iodine Therapy (I-131) for Hyperthyroid Felines, by Virginia Veterinary Centers.

The Thyroid Gland in Animals, by Merck Veterinary Manaul.

By LizsKittyBootCamp

Hi, I'm Liz, and I'm a cat behaviorist who provides advice and insights on cat behavior.

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