Causes, Symptoms And Treatments For Hyperthyroidism In Cats

Here's what cat parents should know about hyperthyroidism.

cat at the vet

If your cat’s suddenly losing weight for seemingly no reason, you might be wondering what’s going on.

In some cases, there could be an underlying condition, like hyperthyroidism, to blame.

Here’s what DodoVet experts had to say about the causes, symptoms and treatments for hyperthyroidism in cats so you’ll know exactly what to look out for.

What is hyperthyroidism?

Hyperthyroidism occurs when a cat’s body starts overproducing thyroid hormones, which regulate metabolism. Too much of these hormones causes the body’s metabolism to speed up, affecting many of a cat’s organs.

According to Trisha Mumford, CVT, a registered veterinary nurse with DodoVet, this is the most common hormone imbalance in cats.

“Either one or both [thyroid] glands can be affected,” Mumford told The Dodo. “Hyperthyroidism mainly affects older cats, with the average age for diagnosis being around 13 years.”

Causes of hyperthyroidism in cats

According to Mumford, the swollen thyroid glands are usually caused by a benign (non-cancerous) tumor. “It is very rare when the tumor is found to be cancerous — usually about 1 to 2 percent [of cases],” Mumford added.

These non-cancerous tumors make the thyroid glands (located in the neck) get bigger and produce extra hormones.

It’s unknown what causes this to happen, but “there have been studies that show an increase in [diagnoses] of cats fed an all wet food diet,” Mumford said. This could be because food made with seafood contains higher levels of iodine, which is required for thyroid function, but the jury’s still out on this.

Symptoms of hyperthyroidism in cats

The most common symptom of hyperthyroidism is weight loss without a decrease in your cat’s appetite (or even with an increase in appetite).

Other symptoms include:

  • Excessive thirst
  • Vomiting
  • Diarrhea
  • Inappropriate urination
  • Difficulty swallowing
  • Restlessness
  • Aggression
  • Unkempt coat

Cats can also develop high blood pressure or heart disease from hyperthyroidism because the faster metabolic rate means that the heart has to work harder to keep up.

Diagnosing hyperthyroidism in cats

Your vet will first check your cat for enlarged thyroid glands by feeling his neck. They’ll also check your cat’s blood pressure and test his blood to help them determine if he has hyperthyroidism.

“There is a specific blood test that looks at thyroid levels called a T4,” Dr. Hilary Jones, chief veterinary officer at DodoVet, told The Dodo. “It’s important to do this test along with a full set of blood work looking at organ function markers, like liver and kidney values, as the thyroid has effects on other organ systems as well.”

An ultrasound of the heart is sometimes performed, too.

Treatment for hyperthyroidism in cats

According to Mumford, the treatment your cat will need depends on a few factors: the severity of the disease, your cat’s age and his overall health.

Medications and changes to your cat’s diet can manage the disease, while surgery and radioactive therapy can cure it.


According to Dr. Jones, medication is the most common treatment for this condition.

A medicine called methimazole is often prescribed and given either orally or as a topical solution to help block the production of thyroid hormones.

“It can be just as effective as other treatments, as long as the medication is continued to be given daily for the remainder of the cat's life,” Mumford said.


Cats can eat a prescription diet that has low levels of iodine to manage their symptoms. Vets often prescribe this dry or wet Hill’s Prescription Diet food.

“It takes up to 12 weeks for the cat to no longer be [considered] hyperthyroid, and the diet does not work in 10 percent of cats,” Mumford said. “No other food or treats can be given to a pet on this diet.”


For surgery, your vet will remove the affected parts of the thyroid.

“As most cats [with hyperthyroidism] are older and may not be healthy enough for surgery, this treatment option is less common,” Mumford said.

Radioactive therapy

This treatment involves injecting radioactive iodine. While that sounds scary, it’s similar to what happens when your cat gets vaccines.

“This is the gold standard but can be expensive and requires a speciality clinic, so unfortunately it isn’t always available to everyone,” Dr. Jones said.

“Cats then need to be boarded at the facility where treatment is being provided until radiation levels within the cats’ urine and feces are below a hazardous level (usually one to three weeks),” Mumford added.

What’s the prognosis for cats with hyperthyroidism?

“Cats whose hyperthyroidism is well managed can have great quality of life!” Dr. Jones said.

Since most cats are diagnosed with the condition when they’re already middle- to old-aged, it’s hard to tell how hyperthyroidism affects a cat’s lifespan.

“Cats who have multiple chronic conditions may have a shorter lifespan than those with just [hyperthyroidism],” Dr. Jones added.

While your cat receiving a diagnosis of hyperthyroidism can be scary, the good news is that it’s easily manageable with medication and diet and can even be cured in some cases.

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