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Hyperthyroidism and Thyroid Problems in Cats: What You Need to Know

Usually hyperthyroidism presents with a skinny, hyperactive old cat with a ravenous appetite. But not always.

thyroid problems in cats
Hyperthyroidism is the most common thyroid problem in cats. Katie, the cat pictured here, was one of the estimated 1 in 10 cats aged 10 years or older who are diagnosed with hyperthyroidism. Photo: aturkus

Question: What common condition in cats was not recognized before 1979?
Answer: Hyperthyroidism in cats.

As a veterinarian, I find this amazing. Mention hyperthyroidism to most people with cats, and they’ve either had a cat with hyperthyroidism or know of someone who has.

Indeed, take a straw poll of the cats in my own house, and a third of them are hyperthyroid, while the stats tell us 1 in 10 cats aged 10 years or older are hyperthyroid.

Thyroid Problems in Cats

If hyperthyroidism was first spotted in 1979, does this mean it didn’t exist before then?

As it turns out, this seems to be a genuine hike in the numbers of skinny cats who are desperately hungry and vocal about it.

This has the hallmarks of a newly emerged condition. Retrospective analysis of postmortems from cats before the 1980s shows very few cats back then had hyperthyroidism.

So what triggered the rise of hyperthyroidism in cats? What is the cause?

thyroid problems in cats
One of the symptoms of thyroid problems in cats is excessive thirst. Photo: suju

Causes of Hyperthyroidism in Cats

Quite simply, no one is certain of the cause.

However, fingers point toward several factors that hyperthyroid cats tend to have in common. This is the so-called “guilt by association” — in other words, if the cat is hyperthyroid, then one of these factors likely applies:

  • Cats use a litter box: Is there something in certain kitty litters — such as dust, fragrance or chemicals — that causes the thyroid to be overactive?
  • Eating canned foods: There’s suspicion that BPA (bisphenol A), used to coat the inside of cans, may play a role.
  • Exposure to flame retardant chemicals (PBDE): These are present in fabric (such as a couch or curtains).
  • Foods containing giblets, liver and fish: PBDEs are also present in these foods and can affect thyroid function.
  • Iodine levels in food: Do high iodine levels trigger the problem?

OK, now let’s circle back to the beginning by understanding hyperthyroidism.

What Is Hyperthyroidism in Cats?

Quite simply, put together hyper (meaning “over”) and thyroid and you have hyperthyroid — meaning overactive thyroids.

What does the thyroid gland do?

Thyroid hormone (thyroxine) governs metabolic rate.

  • When there’s not enough, the metabolism runs slow, meaning a fat, lazy cat.
  • When there’s too much, the cat is overactive and very skinny.

Think of thyroxine as gas in a car. Push down on the gas pedal and you flood the engine, causing it to rev hard and burn through fuel like nobody’s business. This is what happens to the body when too much thyroxine is present.

Where are the thyroid glands?

Cats have 2 thyroid glands, but in the normal cat they are too small to feel.

The glands sit on either side of the windpipe, in the mid-neck area. When they become overactive, these glands enlarge and often can be felt as swellings in the neck.

What are the symptoms of thyroid problems in a cat?

Are you worried your cat has overactive thyroid glands?

Hyperthyroidism usually happens in older cats, typically in those over 10 years of age. The older the cat, the greater the chance of this condition developing.

What often fools people is that in the early stages the cat seems very well. Indeed, they are often kittenish and unusually full of energy. This is down to the car-engine-revving effect, with the thyroxine boosting their metabolism.

But just as it’s not good to keep revving the car engine, so the body starts to show signs of wear and tear. These symptoms include:

  • A big appetite
  • But the cat remains skinny
  • Indeed, the cat eats a lot but loses weight
  • They are thirsty
  • And there are big puddles in the litter box
  • The cat is hyper and follows you around all day
  • The cat is vocal, usually yowling at night when you’re trying to sleep
  • Sickness and diarrhea
  • In rare cases, cats can go the other way and be apathetic and lack an appetite
  • Much later, the cat develops complications such as:
    • Heart disease resulting in fainting, collapse and heavy breathing
    • Sudden-onset blindness due to high blood pressure
How Long Can Cats Live With Thyroid Problems?
Cats diagnosed with hyperthyroidism can live for years. Photo: yairventuraf

How Long Can Cats Live With Thyroid Problems?

When treated, hyperthyroid cats can live for years. Even untreated, they have months or years ahead of them.

If you decide against treatment, just be sure not let the cat suffer when they develop complications.

Actually, wondering how long cats can live with this problem is important. It could alter the treatment decision you make. For example, those tiny thyroid pills are expensive.

Price out the costs to medicate a cat for several years, and it’s going to be $4,000–$6,000 for the pills.

And then there are the monitoring bloods tests and repeated veterinary visits to factor in. All of which makes the one-off cost of around $2,000 for radioactive iodine treatment seem not too bad a deal.

What happens if an overactive thyroid is left untreated?

The answer depends on how sick the cat is when diagnosed.

Hyperthyroidism is a slowly progressive disease. Many cats have mild symptoms for months or even a year or so, before it’s picked up. So right there you have a survival time of around 1 year.

Then after diagnosis, a lucky cat who doesn’t have complications can cope just fine for months or years — albeit getting more and more skinny.

Sadly, eventually the condition will catch up with them. Driving their metabolism so hard for so long leads to raised blood pressure, heart disease and kidney failure. These guys are then in trouble. And often they suddenly become very sick.

Signs your cat is dying of thyroid disease:

  • Becoming blind overnight because of high blood pressure
  • Collapsing with stroke-like symptoms
  • Breathing difficulties due to heart disease
  • Seizures
  • Extreme weight loss and emaciation
  • Extreme thirst
  • Urinary incontinence

My advice is to always treat the cat as soon as the diagnosis is made.

But if for some reason this isn’t possible, then keep a close eye on their quality of life. Weigh the cat regularly. Decide on a “line in the sand” when you would check in with a veterinarian to see if euthanasia is needed (such as if the cat lost one-third to one-half of their original body weight).

So, is hyperthyroidism in cats fatal?

Yes, it is. But it takes a while before the cat becomes really sick.

In this video, Dr. Justine Lee, DACVECC, DAVT, talks a bit more about thyroid problems in cats:

YouTube player

How Do You Treat a Cat With Hyperthyroidism?

The good news: There are lots of treatment options. Actually, the sheer range of choices can be baffling at times.

Your vet may give you the shorthand version and suggest what they think is the most appropriate treatment.

If something doesn’t sit right with you treatment wise, such as your cat won’t take their pills, then let your vet know. There will be another treatment option.

It may be helpful to jot down a list, answering the following questions, to guide the vet’s suggestions:

  • Do you have 1 or multiple cats? (Single cats can do well on a low-iodine diet.)
  • Is the cat easy to pill? (If not, there are liquid formulation or transdermal gels to choose from.)
  • Can you commit to regular vet checks and blood tests? (If not, then surgery may be the answer.)
  • Is the cat relatively young and otherwise healthy? (If yes, then the “cure” of radioactive iodine is a great solution.)

Hyperthyroid Treatments

  • Surgery: Surgical removal of the overactive gland puts the cat back into a normal state. However, an anesthetic is involved, and things can get complicated if both glands are overactive at the same time.
  • Medication: Many drugs normalize thyroid hormone levels. Typically these are a very small, easy-to-give pills. This is the option I use with my cat, as she’s easy to pill and appears on time when her next dose is due. On the downside, the dose climbs steadily over time, and those little pills are relatively pricey. Options include:
    • Pills
    • Liquid formula added to food
    • Transdermal gel applied to the cat’s ear
  • Radioactive iodine therapy: Also known as I-131 therapy, this is the gold-standard treatment. It used to be the cat was away from home for weeks, but now it’s just a few days’ stay at a treatment center. This can be close to a cure, with the majority of cats being “normal” after treatment.
  • Iodine-depleted food: Specifically, Hill’s y/d diet. It’s a great option in single-cat households. Simply feed this food, and the thyroid can’t overproduce hormone. The disadvantage? Your cat must not eat anything else. And I do mean anything.

Treatment Cost

Of course, deciding on a treatment can be influenced by cost.

  • Entry level is going to be the Hill’s y/d since you have to feed the cat anyway.
  • Pills come in at around 50 cents to $2 per day, depending on the dose. The liquids and transdermal gels are more expensive.
  • Surgery can cost $700–1,000, or higher if complications develop. Plus, repeat surgery for the other gland is often needed later.
  • Radioactive iodine therapy is a once-and-for-all fix for most cats. The initial cost is big, around $1,500–2,000. But break this down over 4 years (a reasonable life expectancy after successful treatment in an otherwise fit cat) and it seems more reasonable.

All in all, a diagnosis of hyperthyroidism is nothing to fear. Treat it early, and your cat may go on for years. But it’s worth discussing the costs with your vet, because sometimes the best option for your cat is not the obvious one.


vet-cross60pThis pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed June 18, 2019.

If you have questions or concerns, call your vet, who is best equipped to ensure the health and well-being of your pet. This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis or treatment. See additional information.