Pet Care Blog

Hyperthyroidism in Cats: Symptoms & Treatment

Dr. Ricky Walther
sick cat lying on top of red car

Hyperthyroidism (overactive thyroid) is the most common endocrine disorder in cats, usually affecting older pets. The condition is caused by an excess release of thyroid hormones, which can have a significant impact on your pet’s health as these hormones regulate metabolic processes in the body.

If you are worried that your pet may have feline friend hyperthyroidism, in this guide you will find everything you need to know about this condition so that you can recognize the symptoms and get appropriate treatment for your pet.

Table of Contents:

Pro tip: Diagnosis and treatment for feline hyperthyroidism can be quite expensive, but a good pet insurance plan can help you manage the costs. It can also cover your cat for other common illnesses, as well as injuries and accidents.

cat resting on bed (Image source: PetMD)

What is hyperthyroidism in cats?

In felines, the thyroid gland is made up of two parts, one located on either side of its windpipe. These glands produce hormones that help regulate the metabolic rate. With hyperthyroidism, the thyroid glands release excess levels of thyroid hormones, speeding up the body's metabolism and, as a result, causing stress to your pet’s internal organs, negatively impacting its overall health.

The thyroid glands produce two iodine-containing hormones, T3 and T4, which affect a number of processes in the cat’s body, including:

  • Metabolism of carbohydrates and fats
  • Body temperature regulation
  • Weight loss and gain
  • Cardiac output and heart rate
  • Brain development and growth in young kittens
  • Nervous system function
  • Skin condition
  • Muscle tone
  • Reproduction

What causes hyperthyroidism in cats?

There’s no known genetic predisposition for hyperthyroidism in cats, but it’s very common. It can occur in any breed of cat, female or male, but it is most common in older ones. The possible age range of diagnosis is 4 to 20 years, even though seeing the condition in young cats is rare. Less than 6% of cases are under 10 years of age. The average age of diagnosis is 12-13 years.

Even though no individual breed is known to be at a higher risk, Burmese, Siamese, Persian, Himalayan, and Abyssinian breeds seem to have a somewhat decreased incidence of the disease in comparison to other cat breeds.

Exposure to high dietary iodine levels may cause susceptible felines to develop hyperthyroidism. Some studies have established a link between hyperthyroidism in felines to some canned foods that contain fish, while others have pointed to PBDEs (flame-retardant chemicals) found in some carpeting and furniture cleaners.

In rare cases, the condition can be caused by thyroid cancer.

Symptoms of Hyperthyroidism in Cats

Some of the most common signs of feline hyperthyroidism include:

  • Increased appetite
  • Excessive thirst or excess drinking
  • Weight loss
  • Unkempt coat which might appear matted, dull, or greasy
  • Thickened nails
  • Diarrhea
  • Vomiting
  • Changes in behavior like not sleeping at night, aggression, restlessness, or hyperactivity
  • Urinating more than usual
  • Difficulty breathing
  • Rapid breathing
  • Abnormal heartbeat (“gallop rhythm”)
  • Heart murmur (heart sounds produced when blood is pumped across a heart valve)
  • Enlarged thyroid gland (feels like a lump on the cat’s neck)

Some hyperthyroid cats (less than 10%) will exhibit signs like weakness, loss of appetite, poor appetite, and depression.

Diagnosing Hyperthyroidism in Cats

Diagnosing hyperthyroidism in felines is generally quite straightforward as there will be high thyroid hormone levels in the bloodstream. However, in some cats, thyroid hormone levels might be within the normal range (especially in the early stages of the disease), which makes the diagnosis more complicated.

If your feline friend is manifesting the typical clinical signs but the blood test results are inconclusive, your vet will want to perform additional blood tests or they might recommend a thyroid scan. The symptoms of hyperthyroidism in cats can sometimes be similar to those of chronic hepatic disease, diabetes, chronic kidney failure, and some types of cancer (usually intestinal lymphoma). In order to exclude these conditions, your vet will use thyroid function tests - a series of blood tests used to measure how well the cat’s thyroid gland is working - along with routine lab findings.

thyroid gland position in cats (Image source: VCA Animal Hospital)

How to treat hyperthyroidism in cats?

There are several ways to treat your cat's hyperthyroid condition. Here is a look at some of the most common options your vet might recommend.

Methimazole Medication

Methimazole is one of the most common treatment choices in hyperthyroid cats. This medicine is often given before radioiodine therapy or surgery in order to control your pet’s symptoms. However, methimazole is only effective in stabilizing the clinical signs and doesn’t cure the condition, which means that your kitty will have to receive it for the rest of the life. If your pet is under ten years old, the cost of this medication for a lifetime could exceed radioiodine therapy or surgery.

Methimazole is administered orally in pill form or it can be formulated into a transdermal gel that’s applied to the animal’s ear. The medication rarely causes adverse side effects, but when it does, these side effects can be quite severe. For that reason, you will need to make regular monitoring appointments with your vet.

Radioiodine Therapy

Vets usually prescribe radioiodine treatment (radioactive iodine therapy or I131) which can successfully cure the disease in most cases. This therapy uses radioactive iodine to destroy the affected tissue in the thyroid gland and most felines are cured with only one treatment. This treatment is only offered in certain speciality veterinary hospitals so check with your vet to see if it is an option in your area.

After treatment, the pet’s thyroid levels are monitored. Recurrence isn’t common after the treatment, but it’s possible and can be managed with daily medication.

As the therapy is radioactive, it can only be used in confined medical facilities. This means that your feline friend will have to be admitted to a hospital for several days or weeks after treatment so that the radioactive material can leave its body before returning home. Once your kitty is home, your vet will provide instructions in order to minimize the risk of exposure to the radioactive material for other members of the household.

If radioiodine therapy is not an option due to your pet’s overall health or for financial reasons, your vet might prescribe an iodine-restricted diet or daily doses of methimazole as an alternative.

Low-Iodine Diet

An iodine-restricted diet is a relatively new alternative for treating hyperthyroidism in cats. A pet on this diet cannot be given other cat food, human food, or treats. If other cats in the household eat this food, they’ll need supplemental nutrients that can provide adequate iodine.

Similar to the methimazole treatment, an iodine-restricted diet won’t cure the disease and your feline friend will require lifelong treatment. The results of these diets are variable, so check with your vet to see if they believe it's in your cat's best interest.


Like radioactive iodine treatment, surgical removal of the diseased thyroid gland can be curative.

This surgery is best performed when only one of the thyroid glands is affected, as removing both of them could result in hypothyroidism, in which case decreased thyroid hormone levels result in a slower metabolic rate.

Another possible complication to expect after the procedure is hyperactivity of the remaining thyroid gland.

What if feline Hyperthyroidism is left untreated?

This condition can be easily missed since felines tend to hide when they are not feeling well or mask symptoms of illness. In addition, hyperthyroidism can be overlooked because it usually occurs in senior pets who might already have other health problems with similar clinical signs.

As mentioned earlier, thyroid hormones affect almost all of the organs in the body and if the disease is left untreated, it can lead to serious health issues or even death. For example, hyperthyroidism can cause increased heart rate which, over time, can obstruct the normal functioning of the heart and result in cardiac arrest.

Thyroid hormones can also cause high blood pressure (hypertension) and cause damage to the kidneys, brain, and eyes. In addition, kitties with untreated hyperthyroidism have a lower quality of life, and some might even endure pain due to the symptoms of the disease.

Feline hyperthyroidism is commonly diagnosed along with renal (kidney) disease. Kitties suffering from both conditions might need treatment for both. The diagnosis of renal disease in a pet suffering from hyperthyroidism can also influence the prognosis.

Pro tip: Hyperthyroidism in cats must be treated as quickly as possible using the most suitable treatment plan prescribed by a vet. If you’re worried about unexpected costs, taking out a pet insurance policy can help put those worries to rest. With the right health insurance plan, your vet bills will be covered and you can arrange the best care for your cat without worrying about unexpected expenses.

sick cat lying on top of bed (Image source: Soft Paws)

Follow-Up Care

Your feline friend will need to be re-examined by a vet every 3-4 weeks during the first 3 months of treatment. The vet will perform a complete blood count to check thyroid hormone levels in the blood. He will then adjust treatment in accordance with the results.

If your pet has undergone surgery, your vet will monitor its recovery and watch for complications, such as vocal cord paralysis or the development of low calcium levels in the blood. In the first week after the procedure and then every 3-6 months, the vet will also measure your pet’s thyroid hormone levels to see if there is a recurrence of thyroid gland overactivity.

The prognosis for hyperthyroid cats is generally good with appropriate therapy. The outcomes following most hyperthyroid treatments are usually very good, and most felines have a solid chance of returning to a normal state of health. Pets managed with medication or iodine-restricted diet generally do well so long as their medication is administered regularly or their feeding is consistent, and follow-up tests are performed as scheduled. In some cases, complications involving other organs might affect the prognosis.

Can hyperthyroidism in cats be prevented?

Early diagnosis improves the prognosis and reduces the risk of secondary health issues. Older felines should get a complete physical exam by a vet every 6 months, paying special attention to the thyroid glands. Urine and blood tests should be performed annually in all cats over the age of 6 in order to detect hyperthyroidism before more serious damage occurs.

If your feline friend is diagnosed with hyperthyroidism, your vet will help you determine the best course of action.

Key Takeaways

  • Feline hyperthyroidism refers to an overproduction of hormones in the thyroid gland.
  • The condition is very common in senior cats.
  • Symptoms can include weight loss, increased appetite, and poor coat appearance.
  • The condition can be easily diagnosed by a vet with a blood test.
  • Pet health insurance can help owners cover the cost of their cat's treatment and care.

Do you want to find the best pet insurance?

Let's analyze your pet's breed, age, and location to find the right coverage and the best savings. Ready?

Analyze My Pet

About Pawlicy Advisor

The pet insurance marketplace endorsed by veterinarians, at Pawlicy Advisor we make buying the best pet insurance easier. By comparing personalized coverage and pricing differences we can save you a ton of money, up to 83% in some instances!

Pawlicy Advisor helping a pet parent and their dog find a great deal on insurance

Instantly Compare Pet Insurance Plans

Get Quotes


How Pet Insurance Works

How To Compare Plans

Determine If Pet Insurance Is Worth It

Determine If Wellness Plans Are Worth It

Vet Visit Costs

New Puppy Checklist

Comparison Charts

ASPCA vs. Pets Best

Pets Best vs. Embrace

Embrace vs. Pumpkin

Pumpkin vs. MetLife

More Comparison Charts

Find Your State

New Jersey





New York



More States

Dog Insurance

German Shepherd




English Bulldog

French Bulldog

Great Dane





More Breeds

Ricky Walther, DVM

Dr. Ricky Walther

Ricky Walther, DVM, is a small animal general practitioner in the greater Sacramento, California area. Realizing the positive financial and medical impact that pet insurance can provide for pet parents and the profession, he lends support and advice to companies like Pawlicy Advisor "The Pet Insurance Marketplace") that simplify the process of connecting with veterinary financing resources.

More you might like

female veterinarian giving a pill to a brown labrador in a clinic
6 minute read

Metronidazole for Dogs: Uses, Dosage, and Side Effects

Cane Corso dog
8 minute read

Cane Corso Growth & Weight Chart: Everything You Need To Know

Professional veterinarian giving pill to German Shepherd dog in clinic
7 minute read

Trazodone for Dogs: How Does It Work and When Is It Prescribed?

Shiba Inu dog running in field
7 minute read

Shiba Inu Growth & Weight Chart: Everything You Need To Know

Dog waiting to take prescription medication pills
5 minute read

Benadryl for Dogs: Uses, Dosage, and Side Effects

yeast infection in a dog's ear
7 minute read

Yeast Infections in Dogs: Causes, Treatment, and Prevention

Vet holding medications for a little maltese dog
8 minute read

Tramadol for Dogs: Uses, Dosage & Side Effects

sick dog lying on couch
8 minute read

Pancreatitis in Dogs: Causes, Symptoms, Treatment, and Prevention

X-ray of Hip Dysplasia in Dog
7 minute read

Hip Dysplasia in Dogs: Symptoms, Treatment, and Prevention

dog eating a nut
5 minute read

3 Types of Nuts That Are Safe for Dogs to Eat

growing corgi puppies
7 minute read

Corgi Growth & Weight Chart: Everything You Need To Know

poodle puppy playing on the floor
8 minute read

Poodle Growth & Weight Chart: Everything You Need To Know

increased pot belly of a dog with cushing disease
6 minute read

Cushing Disease in Dogs: Symptoms, Treatment, and How to Save on Costs

Dog reaching into picnic basket for food
8 minute read

Can Dogs Eat Almonds?

dogs looking at a dish of pistachios
5 minute read

Can Dogs Eat Pistachios? Here Are The Risks!

human's hand feeding dog a carrot
6 minute read

Can Dogs Eat Carrots? Everything You Need to Know

puppy eats peanut butter off of a spoon
8 minute read

Can Dogs Eat Peanut Butter? Only If It's Xylitol-Free

dog eating banana from woman's hand
6 minute read

Can Dogs Eat Bananas: Everything You Need to Know

growing pug puppy
7 minute read

Pug Growth & Weight Chart: Everything You Need To Know

Jack Russel carrying apply in mouth
6 minute read

Can Dogs Eat Apples? Here's Everything You Need to Know

Golden Retriever looking at a pile of mangoes
5 minute read

Can Dogs Eat Mango? Here's Everything You Need to Know

growing akita puppy
7 minute read

Akita Growth & Weight Chart: Everything You Need To Know

growing great dane puppy
8 minute read

Great Dane Growth & Weight Chart: Everything You Need To Know

dog with broccoli in mouth
7 minute read

Can Dogs Eat Broccoli? Here's Every thing You Need To Know

golden retriever puppy eating watermelon
5 minute read

Can Dogs Eat Watermelon? Here's Everything You Need To Know

dog wants to eat a potato chip
6 minute read

Can Dogs Eat Potatoes? Here's Everything You Need to Know

growing chihuahua puppy
7 minute read

Chihuahua Growth & Weight Chart: Everything You Need To Know

growing Rottweiler puppy
8 minute read

Rottweiler Growth & Weight Chart: Everything You Need To Know

growing boxer puppy
7 minute read

Boxer Growth & Weight Chart: Everything You Need To Know

growing french bulldog puppy
7 minute read

French Bulldog Growth & Weight Chart: Everything You Need To Know

pets across the united states
15 minute read

Pet Ownership Statistics by State, And So Much More (Updated 2020)

sleepy english bulldog puppy
8 minute read

English Bulldog Growth & Weight Chart: Everything You Need To Know

dog being pet by the vet
9 minute read

Leptospirosis in Dogs: Symptoms, Treatment & How to Save on Costs

dog looking at a wild mushroom
8 minute read

Can Dogs Eat Mushrooms? Here's Everything You Need To Know

dog having tick removed
8 minute read

Lyme Disease in Dogs: Symptoms, Treatment & How to Cut The Costs

sick dog at the vet
9 minute read

Heartworm Disease in Dogs: Symptoms, Treatment & Reducing Costs

german shepherd puppy playing
6 minute read

German Shepherd Growth & Weight Chart: Everything You Need To Know

dog at vet with kennel cough
10 minute read

Kennel Cough in Dogs: Symptoms, Treatment & How To Save On Costs

dog and cat in need of affordable pet care
11 minute read

How to Get Affordable Pet Care in 2024

dog at vet visit for the first time
2 minute read

5 Tips For Your First Vet Visit

well cared for dog
18 minute read

39 Dog Care Tips: The Ultimate Pet Parent’s Guide

Owner gives kiss on cheek to smiling, happy dog
8 minute read

Dog Teeth Cleaning Costs: Best Ways To Save On Dental Care

dog with cataract on left eye
5 minute read

Dog Cataract Surgery Costs and How to Save

sad puppy with parvo
6 minute read

How Much Does It Cost to Treat Parvo?

dog with x-ray at veterinary clinic
5 minute read

Dog X-ray Costs and How to Save

vet visit with small dog
14 minute read

How Much Does a Vet Visit Cost? Here's Everything You Need To Know

rescue puppy pitbull
12 minute read

Bringing Home A Rescue Puppy

petfinder pets near you
6 minute read

Petfinder: Everything You Need to Know

A woman wearing face mask gets kiss from Spaniel dog
8 minute read

Keeping Pets Happy And Healthy During COVID-19: A Pet Parent's Guide

A dog concerned about toxic algae.
1 minute read

Protecting Your Pets From Toxic Algae

Back to Blog
A family with pets that are insured by Pawlicy Advisor
Pawlicy Advisor is the leading independent marketplace for finding the best coverage for your pet at the lowest rate.
Join 2,438,795+ insured dogs and cats across the US.
Get a Quote
Our pet insurance partners
ASPCA Pet Health Insurance Logo
Pets Best Pet Insurance Logo
Embrace Pet Insurance Logo
Pumpkin Pet Insurance Logo
MetLife Pet Insurance Logo
Prudent Pet Insurance Logo