Hyperthyroidism in Cats: The Overactive Thyroid, Explained With Real Numbers

An older cat that eats like a machine and loses weight anyway is the classic picture. The AAFP's guidelines describe the typical patient in one line: a cat over 8 that is active, has a good appetite, and is losing weight.

That combination fools people, because the cat looks enthusiastic. It is also the reason this page exists: hyperthyroidism is the most common hormone disease of older cats, it is very treatable, and the earlier it is caught the less it damages the heart and kidneys along the way.

What it actually is

The thyroid glands in the neck start overproducing thyroid hormone. In cats the cause is almost always benign: a non-cancerous growth called an adenoma, or overgrowth of the gland itself. Both lobes are affected in about 70% of cases, and only about 2% of hyperthyroid cats have a malignant tumor at diagnosis, per the AAFP guidelines.

It is not the cat version of Graves' disease. Human Graves' is autoimmune; no thyroid autoantibodies have ever been documented in cats. The closer human analogue is toxic nodular goiter, a benign tissue that simply stops listening to the body's off switch.

How common it is, market by measured market

The spread is real, not measurement noise: geographic variation in this disease is documented, and nobody has fully explained it yet. In the big English cohort the cases ranged from 6 to 25 years old, and in a referral population 70% of cats were 10 or older at diagnosis. A quarter of the referral cases had been picked up incidentally, on bloodwork done for something else.

The signs

The AAFP's list, in everyday words:

• Weight loss, the headline sign, usually with a good or even ravenous appetite
• Drinking and urinating more
• More vocal, more agitated, busier than the cat used to be
• Fast breathing or a fast heart rate
• Vomiting or diarrhea
• A coat that stops looking after itself

There is also a minority pattern the checklists miss: Merck describes rare apathetic cases, where the cat is lethargic and off its food instead of wired. The weight loss usually still shows. An older cat losing weight is worth bloodwork whatever its energy level says. The same goes for a senior cat that suddenly discovers hallway sprinting: our cat zoomies guide covers when a burst of energy is play and when it is a symptom.

When to call the vet: an older cat losing weight, with or without a big appetite, is an appointment this week, not a wait-and-see. Something wrong with your cat means call your vet; out of hours, an emergency clinic.

How it is diagnosed

The confirmation is persistently elevated thyroid hormone, the blood value called T4, together with matching signs. The trap is that 5 to 10% of affected cats test normal on a single T4, per Merck, so an ambiguous case gets follow-up testing rather than a shrug.

The guidelines also ask for more than the thyroid number: a full blood panel and urinalysis to find what else is going on, and a blood pressure reading at every visit. High blood pressure is a known companion of this disease, and its quiet damage, including retinal detachment, is exactly what early monitoring exists to prevent.

The four treatments, with their real numbers

Two honesty notes on that table. On surgery, the long-term picture depends on who you read: the AAFP's summary table gives the 90%-plus bilateral cure figure, while Merck's professional manual reports that over 40% of cats followed long term after surgery are persistently or recurrently hyperthyroid. Neither names its underlying study, so we show both rather than picking one. And after radioiodine, some degree of low thyroid function occurs in up to half (Cornell) to three-quarters (AAFP, dose-dependent) of cats for a while; only a minority need lifelong supplementation.

On cost, the guideline itself says something most pages will not: over the life of the cat, radioiodine, surgery and daily medication cost roughly the same. The difference is whether the bill lands up front or spreads across years.

The kidney question, because it decides real treatments

Ask around a cat forum and you will meet the belief that treating the thyroid ruins the kidneys. The AAFP guidelines address this head on, as myths, and their wording is unusually blunt:

• Methimazole does not damage kidneys. Kidney disease that appears after treatment was already there, hidden by the overactive thyroid.
• No treatment route causes kidney damage. Any of them can unmask pre-existing kidney disease, which creates the impression of causality where none exists.
• Deliberately leaving the thyroid a little overactive to protect the kidneys is not protective. It is harmful: untreated hyperthyroidism drives blood pressure and glomerular damage, and the panel recommends treating regardless of kidney status.

There is a diagnostic wrinkle worth knowing: an overactive thyroid burns muscle, and creatinine, the standard kidney number, reads artificially low in a cat with less muscle. A normal creatinine in an untreated hyperthyroid cat does not clear the kidneys. Overtreatment is not harmless either: driving the thyroid too low worsens kidney outcomes, which is why follow-up testing is scheduled rather than optional.

What treatment buys, measured

In the study behind the guideline's survival figures, 167 treated cats: median survival was 2.0 years on methimazole alone, 4.0 years with radioiodine alone, and 5.3 years for cats treated with methimazole and then radioiodine. That 5.3-year figure belongs to that specific sequence, not to every treated cat. Cats with pre-existing kidney disease survived shorter, and the guidelines say plainly that in a well-managed cat, the other diseases of age weigh more on the outcome than the thyroid does.

What radioiodine costs, four markets, named clinics

Those are each clinic's own published numbers, not a blended average, and the Canadian line is a finding, not a gap: the named Canadian centers deliberately price per cat after a consultation, so a Canadian reader hunting for a figure online is not missing an obvious page.

What raises the risk

Honestly: nobody knows the cause. The guidelines list the studied associations, canned food, certain flame retardants, litter type, and label all of them conjecture, not proven fact. The one London case-control study that dug in, 109 cases against 196 controls, flagged canned food as its leading association. Genetics runs the other way: in the 95,629-cat English cohort, purebred cats overall had lower odds than moggies, with Burmese, Persian and Siamese lowest.

What to take away

• Hyperthyroidism is the most common hormone disease of older cats: the classic patient is over 8 and most are 10 or older.
• The classic picture is a cat that eats well yet loses weight, drinks more, and turns restless or vocal at night; a blood test (T4) answers it. A cat that suddenly sleeps far less than its own normal is showing the same flag from the other side: our cat sleep guide covers what a healthy baseline looks like.
• Four treatments exist: radioiodine and surgery aim at cure (90%-plus success in the AAFP's tables), while the daily pill and the prescription diet only control it.
• Untreated, the disease progressively damages the heart and kidneys; treated, most cats do well and usually die of something else.
• Treatment does not cause kidney failure: it unmasks kidney disease that was already there, and leaving the thyroid untreated to spare the kidneys actively harms them.
• Radioiodine costs roughly $1,900 to $3,000 in the US, £2,500 to £3,400 in the UK, and around A$1,900 in Australia (2026 figures at named clinics).