Updated 11/1/25

A uniquely feline health condition, hyperthyroidism, has risen to epidemic proportions since the first cases were diagnosed in 1979. It is a worldwide phenomenon, though it’s worse in the U.S. than other countries. Hyperthyroidism (hyper = too much, thyroid = a hormone-making gland), is seen mostly in older cats (over seven years of age).

What the Thyroid Does

In people, the thyroid gland is shaped like an “H” at the base of the throat. Cats’ glands are in the same location, but their thyroids lack the crossbar, forming two small glands, one on each side of the trachea.

The thyroid’s job is to regulate the body’s metabolism. Too much thyroid hormone is like drinking too much coffee—it revs up every process in the body.

Signs of Hyperthyroidism

Hyperthyroidism usually develops from a benign thyroid tumor. The abnormal cells continue to make hormone, flooding the bloodstream with excess thyroid hormones.

Common symptoms include:

  • Increased appetite
  • Weight loss (often despite eating more)
  • Fast heart rate
  • High blood pressure (hypertension)
  • Restlessness or anxiety
  • Yowling, especially at night
  • Increased thirst and urination
  • Vomiting or diarrhea

Not every cat shows every sign. About 20% of affected cats actually become sluggish and depressed instead of hyper. Left untreated, hyperthyroidism can lead to hypertrophic cardiomyopathy, a thickened, enlarged heart that can lead to a painful death.

High blood pressure is another risk—it can cause sudden blindness. Make sure your vet regularly checks your cat’s blood pressure. If your veterinarian doesn’t have the equipment, find one that does. This is now considered standard of care for senior cats.

Diagnosing Hyperthyroidism

Sometimes your veterinarian can feel an enlarged thyroid gland in the neck. The definitive diagnosis, however, comes from blood tests.

Cats with hyperthyroidism typically have an elevated T4 level, the inactive form of thyroid hormone. The thyroid normally releases T4 into the blood, where it’s converted to active T3 as needed. Even if T4 is within the “normal” range, a value at the high end may still indicate disease.

If results are unclear, a more sensitive equilibrium dialysis test can confirm the diagnosis. Many hyperthyroid cats also have elevated liver enzymes, which can help point to the problem.

Detecting Disease Early

Research suggests we might be able to spot thyroid disease before it develops.¹ Normally, when T4 is high, thyroid-stimulating hormone (TSH) should drop. Some cats have low or undetectable TSH even when T4 is still normal, and these cats often go on to develop hyperthyroidism later.

Current TSH tests aren’t common or accurate at low levels, but adding this test to a senior wellness panel may be helpful. A low TSH suggests future thyroid disease, while a normal TSH makes it unlikely.[i]

What Causes Feline Hyperthyroidism?

No single cause explains the rise in feline thyroid disease, but environmental and dietary factors appear to be the most likely contributors.[ii]

  1. Canned Food and Certain Ingredients

Studies show a higher risk of hyperthyroidism in cats that eat large amounts of canned food, especially fish or giblet varieties. Researchers suspect BPA and related chemicals in can linings may be part of the problem. The smaller the can, the greater the potential leaching of chemicals because more surface area is in contact with the food. Unfortunately, can linings vary widely, and manufacturers may change materials at any time. Researchers found that cans claimed to be BPA-free still contained BPA. And the chemicals used instead of BPA may be just as dangerous, if not worse.[iii]

The FDA maintains that these chemical levels are safe—but the FDA has been wrong before.

  1. Fire Retardants (PBDEs)

Another suspect: PBDEs, flame-retardant chemicals used in carpets, upholstery, and electronics. They were introduced around the same time feline hyperthyroidism first appeared. Cats spend a lot of time lounging on furniture, rugs, and warm electronics—prime PBDE territory. As they groom, they ingest contaminated dust, potentially accumulating high levels over time. More recent studies even found PBDEs in cat food, especially fish-based canned foods and some dry foods.

  1. Excess Iodine

Many cat foods contain far more iodine than needed. Pet food manufacturers add extra to ensure they meet minimum requirements, but the actual amount can vary wildly. Since excess iodine is known to trigger thyroid problems in people, it may do the same in cats.

  1. Other Possible Factors

Environmental chemicals and diet don’t tell the whole story. Outdoor cats can also develop the disease, suggesting other triggers.

Vaccines are implicated in the development of hypothyroidism in dogs, where vaccine-induced antibodies attack and damage the glands. While holistic veterinarians believe vaccines may also be involved in cats, no potential link to the disease in cats has been found.[iv]

Reducing Your Cat’s Risk

Wait—please don’t get rid of all your furniture or kick your cat outside! (The risks outdoors are far worse!) Instead:

  • Feed canned food for hydration and nutrition
  • Choose poultry, beef, or lamb flavors without fish, giblets, or by-products
  • Ask manufacturers if their cans are BPA- and BPA-analog free

Treatment Options

There are four main approaches to treating feline hyperthyroidism:

  1. Methimazole

Methimazole (brand name Felimazole) reduces thyroid hormone production. It’s available as tiny tablets or a topical gel rubbed inside the ear. Your vet will check blood levels frequently at first, then every six months once stabilized.

  1. Thyroidectomy (Surgical Gland Removal)

Surgical removal of the thyroid glands can be curative but carries risk. Cats have two thyroid glands, and both are affected in about 70% of cases. The nearby parathyroid glands—which regulate calcium—are tiny and easy to damage during surgery.

  1. Radioactive Iodine (I-131) Therapy

This is the gold standard of treatment. The thyroid naturally concentrates iodine, so a single injection of radioactive iodine destroys overactive thyroid cells without harming other tissues.

  1. Prescription Diet (Hill’s y/d)

Hill’s Prescription Diet y/d is formulated with extremely low iodine levels. Hill’s claims it can normalize thyroid hormones within 12 weeks if fed exclusively. The ingredients are the usual poor quality common to mass-market commercial foods, alth0ugh the canned food contains actual meat. Although the food does not contain enough iodine to satisfy current nutritional standards, Hill’s says it’s fine to feed it to all cats in a household.

Holistic and Supportive Care

Hyperthyroidism is challenging to manage naturally, but Traditional Chinese Medicine and homeopathy are the most powerful modalities for a true cure. Others, including many energy therapies, may help control symptoms or support overall health. You can locate a holistic veterinarian through ahvma.org.

Complications and Cautions

Because hyperthyroidism mostly affects older cats, kidney disease is a common companion problem. Interestingly, the high blood pressure caused by hyperthyroidism can help kidneys function better. Once thyroid levels are lowered, kidney function can appear to worsen—not because treatment caused damage, but because it revealed a problem that was already there.

The Bottom Line

Hyperthyroidism is now one of the most common diseases in older cats. With good monitoring and proper treatment, most cats live many happy years after diagnosis. Regular checkups, proper monitoring, and thoughtful food choices are your best bet.

(Watch Floppy Cat’s interview with Dr. Jean here; the written transcript is below the video under “show more.”)

References

[i] Wakeling J, Elliott J, Syme H. Evaluation of predictors for the diagnosis of hyperthyroidism in cats. J Vet Intern Med 2011;25(5):1057-1065.

[ii] Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Vet Clin North Am Small Anim Pract. 2007 Jul;37(4):633-45, v.

[iii] Srebny V, Henneberger L, König M, et al. Beyond estrogenicity: A comparative assessment of bisphenol A and its alternatives in in vitro assays questions safety of replacements. Environ Sci Technol. 2025 Aug 26;59(33):17457-17470.

[iv] Nguyen LQ, Arseven OK, Gerber H, Stein BS, Jameson JL, Kopp P. Cloning of the cat TSH receptor and evidence against an autoimmune etiology of feline hyperthyroidism. Endocrinology. 2002;143(2):395–402.