Older cats are renowned for spending most of their time sleeping in a sunny spot in the house. They might even put on weight. But sometimes, these relaxed, cuddly creatures suddenly get very thin, stressed, and hungry.

These signs are typical of feline hyperthyroidism, one of the most common endocrine diseases in cats. The first questions pet parents often ask are, "What is hyperthyroidism in cats? Is hyperthyroidism in cats painful? How can you prevent it, and what can you do if you suspect your cat has it?"

Key Takeaways:

  • Hyperthyroidism in cats is a common disease that causes weight loss, increased appetite, and other symptoms.
  • The exact cause of hyperthyroidism in cats is unknown, but factors like age and diet may play a role.
  • Diagnosis involves physical examination and blood tests, and treatment options include medication, radioactive iodine therapy, or surgery.

What Is Hyperthyroidism in Cats?

In cats, as in humans, hyperthyroidism occurs when an abnormally-functioning thyroid gland produces and secretes excessive thyroid hormone. These hormones affect multiple organs in the body, causing increased metabolism, a fast heart rate, high blood pressure, weight loss, and an insatiable appetite and thirst. Untreated, this common cat disease can lead to a shortened lifespan.

What Causes Cats to Have Hyperthyroidism?

There are no known causes of cat hyperthyroidism, but there are contributing factors such as:

  • Age, as the condition primarily affects cats over six years old.
  • An excess or deficiency of some elements in the cat's diet, particularly in some canned diets.  Excess iodine could be a contributor.  
  • Exposed to chemicals that can disrupt hormone production. These chemicals, such as flame retardants or organic pollutants, can be found in food or the environment.

Signs & Symptoms of Cats with Hyperthyroidism

Symptoms of hyperthyroidism in cats include:

Weight loss

Cats with hyperactive thyroid glands experience increased appetite but drastic weight loss due to the high metabolic rate and fast burning of calories. This is often one of the first signs of hyperthyroidism in cats.

Increased thirst and urination

Increased metabolism and effects on other organ systems can make your cat thirsty, causing them to drink more and urinate more frequently.

Hyperactivity or restlessness

Since thyroid hormones affect metabolism, an accelerated metabolism makes your cat more active, which tires them out.

Poor coat condition and excessive shedding

Disrupted hormones can affect the hair coat, making your cat's fur dry, matted, and quickly shed.

Vomiting and diarrhea

The increase in metabolism can affect the digestive system, causing vomiting and diarrhea.

Rapid or irregular heartbeat

Overproduction of hormones triggers the heart to beat faster and can cause an irregular heartbeat. This may lead to high blood pressure and heart disease.

Changes in behavior or temperament

Observe your cat's body language. Hyperthyroid cats may seem moody due to the physical symptoms they're experiencing.  They can also be more vocal and seem more needy. 

Diagnosis of Hyperthyroidism in Cats

If your veterinarian suspects your cat might have hyperthyroidism, they will:

  • Perform an initial assessment, including a physical exam and palpation (feeling) of the cat's neck to assess the size of the thyroid gland.
  • Take a pet blood test to confirm or rule out the preliminary diagnosis, evaluate the thyroid hormone levels in the bloodstream, and see if there are signs of other illnesses.

Treatment of Cats with Hyperthyroidism

Cats diagnosed with hyperthyroidism may receive the following treatments:

  • Antithyroid medication to control thyroid hormone production
  • Radioactive iodine therapy to destroy abnormal thyroid tissue
  • Less commonly, surgical removal of the thyroid glands

Effects of Untreated Hyperthyroidism

It’s important to treat hyperthyroidism in cats to avoid these health consequences:

  • A decline in health due to weight loss, which can trigger other health problems
  • Development of hypertension (high blood pressure) and heart problems that may lead to blood clots, heart failure, stroke, and death
  • Altered activity in other organs can lead to kidney disease and liver problems.
  • A decrease in your cat's quality of life

Prevention of Cats with Hyperthyroidism

Are you wondering how to prevent hyperthyroidism in cats in the first place? Here are ways to reduce the chance of your cat developing hyperthyroid symptoms:

  • Feed your cat the right food and avoid exposing them to environmental factors that possibly could contribute to the development of hyperthyroidism.
  • Have a veterinarian check your kitty regularly to pick up on early symptoms.
  • Follow your veterinarian’s instructions related to medications and checkups to prevent symptoms from recurring. To effectively control hyperthyroidism in cats, it is important to follow treatment instructions to ensure an adequate response.

Cost of Treatment

There are a number of treatment options, and each comes with its own short and long-term costs.

Diagnostics usually cost $75–$350, depending on the examinations and blood tests required. In the long-term, your cat might need blood tests every 3–6 months, but the early months are typically more intense as your cat will need repeat testing until their hormones have stabilized.

Medications can range from $20–$50 a month, depending on the dose your cat needs. These costs will be life-long and may increase if your cat needs a higher dose as time goes by.

Other solutions are more expensive up-front but can be more economical in the long term as you save on ongoing medication costs. Radioactive iodine therapy can cost anywhere from $2,500–$3,000. Thyroidectomy or surgical thyroid gland removal costs $750–$3,000, depending on whether one or both lobes need to be removed.


Knowing the symptoms of hyperthyroidism in cats means you can get immediate veterinary attention for your cat, which may be life-saving. The veterinarian will conduct tests to confirm the diagnosis, give practical advice, and recommend effective treatment so your cat can live a long, happy life.

Frequently Asked Questions

When to put a cat to sleep with hyperthyroidism?

A massive decline in health, extreme pain, and unresponsiveness from treatment may prompt pet parents and veterinarians to discuss end-of-life care.

How often should you feed a cat with hyperthyroidism?

An increased appetite is a symptom of the disease, so it should settle as their thyroid hormone levels come under control. Limiting a hyperthyroid cat's food intake while they are stabilizing is not usually recommended, as this can contribute to excessive weight loss.

What are the behaviors and signs of cats with hyperthyroidism?

A hyperthyroid cat may be stressed, vocal, and agitated. They are often fixated on food, drink more than usual, and have unkempt fur.

What happens if you leave cats with hyperthyroidism untreated?

Untreated feline hyperthyroidism puts excess stress on the body systems, affecting the cat’s health and quality of life.

How long can a cat live with hyperthyroidism?

With good health intervention, a cat with hyperthyroidism may live a normal life expectancy.