Hyperthyroidism is a disease that affects a lot of cats. It is most common in older cats (over 10 years old), but shows no preference for breed or sex. Catching it early can make hypothyroidism in cats much easier to treat.

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What is hyperthyroidism?

Hyperthyroidism in Cats - Thyroid Gland

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The thyroid is two glands that, together, make a butterfly-shape and sit at the base of the neck just below the Adam’s Apple. It  produces and regulates hormones that control the body’s metabolism and organ functions. The thyroid itself is regulated by the brain.

Hyperthyroidism in cats occurs when the cat’s thyroid becomes over active and produces too much of the hormone thyroxine (also known as T4). The source of this overactive thyroid is a tumor that develops within the thyroid gland. In the vast majority of cases of hyperthyroidism in cats, the tumor is benign and referred to as a thyroid adenoma. However, in 2-3% of cases, the tumor will be cancerous (called a thyroid carcinoma). The cause of the tumors is still under investigation.

The overproduction of thyroxine causes the cat’s body to speed up. It is very stressful for the heart, kidneys, liver, nervous system, etc to be working at full speed constantly. If left untreated, it can cause the gradual wasting away of the cat’s body.

Symptoms of Hyperthyroidism in Cats

The symptoms of hyperthyroidism in cats are closely linked to the overworked organs in the cat’s body. There are several other illnesses such as diabetes, irritable bowel disease, intestinal cancer, and chronic kidney failure that can have very similar symptoms. If you think that your cat may have hyperthyroidism, it is very important that you contact your veterinarian to get a definitive diagnosis. This information is not meant to be used in lieu of veterinary assistance. Symptoms can vary widely, but common symptoms include:

  • Increased appetite
  • Weight loss despite eating normally
  • Increased activity/restlessness
  • High blood pressure
  • Increased heart rate (pounding heart)
  • Increased thirst
  • Unkempt appearance
  • Vomiting
  • Diarrhea

Diagnosis and Treatment of Hyperthyroidism in Cats

Hyperthyroidism in Cats - At the Vet's 4 - Explored September 17, 2014

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Most veterinarians will diagnose hyperthyrodism in cats by doing a complete blood panel. This test will tell them just how much thyroxine is circulating in the cat’s blood stream as well as other important information. Other tests may include palpating the cat’s throat to feel for an enlarged thyroid, urinalysis, or Thyroid Scintography.

There are a few different options for treatment of hypothyroidism in cats; thyroid medication, surgery, radio active iodine, and restricted iodine diets. Each has it’s pros and cons, so it is really important to have a good talk with your veterinarian before making a treatment choice. Holistic veterinarians may also have some homeopathic or natural remedies available too.

Thyroid Medication

How it works: 

Commonly a medication called Methimazole is used. This medication inhibits the thyroid from making thyroxine. Therefore, the thyroid makes less thyroxine and less is found int he blood stream.


  • Methimazole can normalize thryoxine levels in just a few weeks of treatment (2-3 weeks).
  • The dosage can be moved up, down, or completely discontinued to accommodate the cat’s needs.
  • Methimazole is a relatively inexpensive treatment option.


  • There are a number of side effects that cats have while taking Methimazole, such as allergic reactions, intense, itchy facial rashes, vomiting, depression, bleeding, and decreased appetite. There is a transdermal formulation (cream applied to the ear) of Methimazole that can bypass some of the gastrointestinal side effects.
  • You will have to pill your cat 2 times every day.
  • This is a lifelong therapy – your cat will have to take this medication for the rest of his/her life.
  • The tumor will continue to grow while using Methimazole. At some point the medication may not work anymore. Also, as time wears on, the chances increase that the tumor will become cancerous.

Thyroid Surgery

Hyperthyrodism in Cats - Bolli after his operation

A kitty right after thyroid surgery. Photo Credit: Annie Mole via Flickr

How it works:

A skilled veterinarian will surgically remove the tumor from the thyroid gland. With the tumor gone, the thyroid can go back to functioning normally.


  • Once the surgery is done, the thyroid is cured of it’s disease.
  • Removes the tumor, eliminating the chances of it becoming cancerous.


  • The cat must be placed under anesthesia, which is not easy on cats.
  • The cat’s body may already be under too much stress to handle a surgery.
  • If the surgeon accidentally removes too much of the healthy thyroid tissue or removes the attached glands, there can be lifelong side effects.

Radio Active Iodine

How it works:

A veterinarian at a facility that is licensed to use radioactive medicines will give the cat a small injection of I-131 (radio active iodine) under the skin. The cat’s body will soon absorb the I-131. The thyroid uses the iodine to create thyroxine and the tumor is destroyed by the radiation.


  • Radio active iodine usually kills the whole tumor in one treatment.
  • After this treatment, the cat is cured of hyperthyroidism.


  • Treatment can be expensive.
  • The cat must remain at the hospital for several days until the levels of radio active matter in their urine and feces come down enough to go home with you.
  • Once the cat is home, you may still have to limit your exposure to the cat for up to 2 weeks.

Restricted Iodine Diets

How it works:

The most common of these is the prescription cat food, Hill’s y/d. This diet reduces the amount of iodine found in the cat’s diet in attempt to give the thyroid less iodine to build thyroxine. In turn, the production of thyroxine is slowed.


  • This diet can normalize thyroxine numbers within 3 weeks.
  • This treatment can be easily removed if there are any negative side effects.


  • The cat must solely eat this diet ( no treats, no human food, no other cat foods, etc) in order to get the promised results.
  • The thyroid tumor can continue to grow while the cat is on this diet. This increases the chance of the tumor becoming cancerous.
  • Some veterinarians believe that iodine restricted diets leave the entire body deficient of iodine and can result in goiter (an enlarged thyroid gland).

Have you ever known a kitty with a hyperactive thyroid?

Resources & Digging Deeper

Feline Hyperthyroidism – Lisa A. Pierson, DMV

Hyperthyroidism – ASPCA

Hyperthyroidism in Cats –  Cornell University Veterinary Hospital

What is Feline Hyperthyroidism? -Feline Hyperthyroid Treatment Center

Feline Hyperthyroidism – Dr. Karen Becker, DVM