Hyperthyroidism in Cats: What is it?

fluffy gray cat with green eyes in nature

As a cat lover, you want to understand everything you can about your feline friend’s health and wellness. 

Hyperthyroidism affects about 10% of senior cats, so it’s an essential issue to keep on your radar.1

Hyperthyroidism describes a hyper or overactive thyroid. It can occur in humans, cats, and canines alike. The thyroid plays a role in the production of hormones. When it’s off-balance, the first signs are changes in appetite, urination, and thirst.

Whether your cat’s been diagnosed with hyperthyroidism or you’d like to better understand potential reasons for shifts in weight and behavior, this is your guide to hypothyroidism in cats.

What Causes Hypothyroidism?

The thyroid is an essential part of the mammalian endocrine system. Located near the base of the neck, the thyroid gland produces hormones that regulate the metabolism:2

  • Triiodothyronine, aka T3
  • Thyroxine, aka T4

T3 and T4 hormones circulate throughout the body via the bloodstream, interacting with many other organ systems. As a result, the thyroid and its hormones play a role in the following bodily functions:

  • Appetite
  • Energy levels
  • Digestion
  • Weight loss and weight gain
  • Heart rate
  • Skin health
  • Internal temperature
  • Mood

When the thyroid gland is off-balance, it can produce too little hormone (hypothyroidism) or too much (hyperthyroidism). 

While hypothyroidism is less common in cats, hyperthyroidism is relatively common in middle-aged and older kitties. Most often, one or both thyroid glands become enlarged and begin to overproduce T3 and T4 hormones.

What causes thyroid glands to become enlarged in the first place? 

According to the Cornell College of Veterinary Medicine, there are two potential direct causes.3

  1. Adenomas – Non-cancerous tumors called adenomas can arise on one or both sides of the thyroid gland. These are the more common causes of hyperthyroidism.
  1. Adenocarcinomas – Less often, the thyroid is enlarged due to a malignant (cancerous) adenocarcinoma tumor. 

Risk Factors

What puts your cat at risk for developing malignant or non-malignant thyroid growth?

Like many health issues, a kitty’s risk increases with age. The average age for cats to develop hyperthyroidism is 13 years old.4 

Beyond age, veterinarians speculate that lifestyle can play a role.

  • Diet – As you probably already know, making good choices about your cat’s diet can help preserve their health in the long term. A deficit of essential nutrients or a surfeit of harmful substances can affect overall health.
  • Chemical exposure – Also, some scientists speculate that exposure to chemicals can harm the thyroid. After all, they’re much smaller than we are, and a small amount of a toxic substance could have a more significant impact on their health. 

One ingredient that shows up in food and on the list of potentially harmful chemicals is polybrominated diphenyl ether (PBDE)5. This class of flame-retardant compounds is used in household textile, including sofa coverings. It’s also significantly prevalent in seafood-flavored cat food. Yet another reason to choose natural and organic ingredients!

Other chemicals that can affect your cat include:

  • Pesticides
  • Chemical cleaning agents
  • Synthetic fragrances

Hyperthyroidism in Cats Symptoms

How do you tell if your cat has hyperthyroidism?

Veterinarians suggest that you bring your cat in at least once a year for a full physical examination. At this time, your cat will undergo blood testing that can identify elevated levels of thyroid hormones (especially T4).

Haven’t been to the vet in a while? It can be helpful to familiarize yourself with the most common symptoms of thyroid issues.

These include:

  • Changes in thirst and appetite – If your cat’s drinking and eating habits suddenly shift, it’s time to check in with your vet to rule out underlying issues. Hyperthyroidism tends to result in increased appetite and thirstiness.
  • Digestion issues – If you notice your cat vomiting or suffering from cat diarrhea even though their diet hasn’t changed, hormone levels could be the cause.
  • Weight loss – Hyperthyroidism can send your kitty’s metabolism into overdrive. This means they could lose weight even if they’re eating more. 
  • Changes in litter box use – As cats drink more water, you might notice more frequent urination. 
  • Off mood– Increased activity, aggression, and depression can all be signs of fluctuating hormone levels. 
  • Poor coat – Cats with hyperthyroidism can start to appear unkempt and oily. They may even develop matted knots in their fur.

If your cat has any of these symptoms, we recommend checking in with your vet.


As we noted, most cats with hyperthyroidism have elevated levels of T4. In addition, your vet will likely palpate your cat’s neck and throat to check for enlargement of the thyroid gland.

However, in some cases, cats with mild hyperthyroidism will not have elevated T4 levels. In that case, your vet can use other tests to confirm the diagnosis.

  • A T3 suppression test can tell whether your cat’s thyroid and pituitary glands are responding to T3 as expected.
  • A nuclear medicine scan uses radioactive material to test for hyperactivity of the thyroid.


We’ve already laid out the symptoms of an overactive thyroid. However, it’s also helpful to keep your eye out for the opposite signs.

While hypothyroidism is rarer in cats, the following signs might also require a trip to the vet:6

  • Weight gain
  • Lethargy
  • Dry coat 

Hyperthyroidism in Cats Treatment

Rest assured that hyperthyroidism isn’t the end of the road with your furry friend.

However, it’s essential to create a treatment plan as soon as you have a diagnosis. Otherwise, your cat could face long-term health issues like the following:

  • Anorexia
  • Bloody stools
  • Heart issues
  • Kidney failure

Because hyperthyroidism is so common, there are several well-established methods for treatment.

These include7:

  • Hormone replacement therapy – Anti-thyroid medication can override and replace the hormones created by your cat’s thyroid. Cat parents who choose this option can administer oral medication twice daily or opt for a gel absorbed through the skin.

    Potential side effects include digestive issues and mood changes. If you go this route, be prepared to stick to a daily routine and monitor your cat’s health with regular check-ups.
  • Thyroid removal Removing an overactive thyroid can eliminate the issue of excess hormone production. However, this comes with the risk of any surgery. Your cat would need to undergo general anesthesia (which is contraindicated by some other health issues) and have a smooth surgery and recovery.
  • Dietary changes – Iodine is necessary for T3 and T4 production. Therefore, feeding your cat a low- or no-iodine diet can reduce their levels of hormone production. However, keep in mind that iodine is needed for some other bodily functions. A deficit may hurt some cats’ health.
  • Radioactive iodine therapy – Pet owners looking for a less invasive approach may opt for radiation. The thyroid will absorb the iodine, and its radiation will destroy the abnormal tissue within the gland. Most cats who undergo this treatment see a full recovery from their hyperthyroidism within three months. The significant potential downsides are the cost and the need to board your cat for three to five days while their radiation levels come down.

Be sure to consult with your veterinarian to understand which option is right for you and your pet. 

Best Practices For Cat Owners

Once you’ve got your cat’s hyperthyroidism under control, maintain a plan for keeping them in excellent health.

Regular check-ups and vaccinations are essential for cats of all ages. Some states require domestic cats to have yearly rabies vaccinations. Beyond what’s legally mandated, though, annual appointments can help identify health issues before they become more serious problems.

We recommend the following steps:

  • Splurge on the good stuff – There are numerous cat foods on the market, but as we’ve noted, some contain potentially harmful chemicals. Cats are carnivorous by nature, so grain-heavy formulas can also be a detrimental choice. Consult with your veterinarian to understand your cat’s nutritional needs.
  • Play with your cat – Cat owners know that their furry friends are independent—no leashes and walks required! At the same time, cats need daily exercise to stay in good health. Provide toys for independent play, and make time for using the laser pointer, feather wand, or another toy of your cat’s choice. It’ll be worth it, both for the potential adorable cat videos and the extra bonding time together.
  • Pay attention to behavioral changes – As cat owners ourselves, we know that felines get up to some curious stuff. However, there’s a difference between the occasional spell of cat quirkiness and potential signs of ill health, such as diet, litter box use, and mood changes. If you notice a change, call your vet.
  • Support overall health – Many people find that CBD for pets can help support their cat’s mood and overall health. How does it work? If you’ve tried CBD yourself, you know that it soothes nerves and enables you to deal with daily aches. With more mental and physical energy, your body has that much more energy to send resources where they’re most needed. The same is true for many cats.

Canna-Pet for Cats of Every Stripe and Spot

Are you interested in adding CBD to your cat’s daily routine?

If your kitty gets nervous before vet visits, CBD could potentially help. Likewise, it can be an ally for aging cats. 

At Canna-Pet, we’re leaders in organic, ethically sourced hemp. We know that any trace of chemical pesticides or additives could harm your cat’s thyroid and overall health, which is why we produce all-natural pet products. 

Check out our line of CBD & Hemp for Cats and read up on our blog to better understand your pet’s long-term health.


  1. New York Times. The mystery of the wasting house cat. https://www.nytimes.com/2017/05/16/magazine/the-mystery-of-the-wasting-house-cats.html#:~:text=Today%2C%20senior%20cats%20are%20routinely,while%20sparing%20the%20healthy%20tissue.
  2. Endocrine Web. What are T3, T4, and TSH? https://www.endocrineweb.com/thyroid-what-are-t3-t4-tsh
  3.  Cornell College of Veterinary Medicine. Hyperthyroidism in cats. https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/hyperthyroidism-cat
  4. Washington State University. Hyperthyroidism in Cats. https://www.vetmed.wsu.edu/outreach/Pet-Health-Topics/categories/diseases/hyperthyroidism-in-cats
  5. The Advocate. Feline hypothyroidism is a very complex disorder. https://www.theadvocate.com/baton_rouge/sports/lsu/article_be413057-64b5-5aef-9ec9-263dfc29e47f.html#:~:text=The%20most%20common%20and%20noticeable,coat%20and%20listlessness%20and%20lethargy.

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