Hyperthyroidism can’t be completely prevented in cats, but you can significantly reduce the risk by managing a few key factors: what your cat eats, what chemicals they’re exposed to indoors, and how early you start screening. Up to 10% of cats over age 10 develop this disease, making it one of the most common conditions in older cats. The good news is that several of the biggest risk factors are things you have direct control over.
Why Canned Food Is the Biggest Dietary Risk
The strongest modifiable risk factor identified so far is canned cat food, specifically the linings inside the cans. A study published in the Journal of the American Veterinary Medical Association found that cats eating food from pop-top cans had nearly five times the risk of developing hyperthyroidism compared to cats that ate no canned food at all. Cats eating from other types of cans still had about 2.5 times the risk. The longer a cat stayed on a diet of mostly canned food, the higher the risk climbed.
The likely culprit is bisphenol A (BPA), a chemical used in the epoxy resins and flexible coatings that line most canned foods. BPA has been detected in canned cat foods, and some U.S. brands had among the highest BPA concentrations measured in any canned food product. BPA interferes with thyroid hormone signaling: it blocks the active thyroid hormone from binding to its receptors, which can trigger the body to ramp up thyroid hormone production. Over years, that constant overstimulation may drive the thyroid gland to enlarge and become overactive.
This doesn’t mean you need to eliminate canned food entirely, but reducing your cat’s reliance on it helps. Mixing in dry food, fresh food, or wet food sold in BPA-free packaging (pouches, tetra packs, or glass) lowers cumulative exposure. If your cat does eat canned food, choosing brands that specifically use BPA-free linings is a practical step.
Check Your Cat Food for Soy
Soy-based ingredients show up in a surprising number of commercial cat foods. Nearly 60% of cat foods tested in one analysis contained soy isoflavones, plant compounds that directly interfere with thyroid function. These compounds block a key enzyme the thyroid needs to produce its hormones, and they also block the enzyme that converts stored thyroid hormone into its active form.
When thyroid hormone production drops, the brain responds by sending stronger and stronger signals to the thyroid gland to work harder. Over time, this can cause the gland to grow abnormally. The effect is especially pronounced when a cat’s diet is also low in iodine, because iodine deficiency already strains the thyroid. Virtually all dry and semi-moist foods containing soy protein have isoflavone levels high enough to interfere with thyroid function.
Reading ingredient labels matters here. Look for soy protein, soy flour, soybean meal, or soy isolate on the list. Choosing soy-free formulas, particularly for cats you plan to feed the same diet long-term, removes one layer of chronic thyroid stress.
Keep Iodine Levels Balanced
Iodine is essential for thyroid hormone production, and both too little and too much can cause problems. A diet low in iodine forces the thyroid to work overtime, making it more vulnerable to the effects of other thyroid-disrupting chemicals like soy isoflavones or BPA. On the other hand, excess iodine can also overstimulate the gland.
For cats already diagnosed with hyperthyroidism, veterinary diets restrict iodine to 0.32 parts per million or less. For healthy cats, the goal is simply consistency: feeding a high-quality commercial diet from a reputable manufacturer generally provides appropriate iodine levels. The risk increases when cats eat fish-heavy diets (which tend to be high in iodine) or very cheap foods with inconsistent mineral content. If you’re rotating between multiple brands or making homemade food, inconsistent iodine intake becomes a real concern.
Reduce Indoor Chemical Exposure
Cats spend most of their time indoors, close to the ground, grooming chemicals off their fur constantly. This makes them uniquely vulnerable to household contaminants. Flame retardants called PBDEs, found in furniture foam, electronics, and carpet padding, were initially suspected as a major driver of feline hyperthyroidism because they leach into household dust. However, a matched study comparing dust levels and blood levels in hyperthyroid versus healthy cats found no significant difference between the two groups, weakening the case for PBDEs as a direct cause.
That said, the broader principle still applies. Cats are small animals with fast metabolisms and limited ability to detoxify certain chemicals. Reducing dust accumulation through regular vacuuming, choosing furniture and mattresses made without chemical flame retardants, and ventilating your home all lower your cat’s overall chemical burden. Avoiding scented plug-ins, heavily fragranced cleaning products, and pesticide sprays near areas where your cat sleeps or eats is a reasonable precaution, even if no single household chemical has been definitively proven to cause hyperthyroidism on its own.
Breed Plays a Role You Can’t Change
Genetics influence hyperthyroidism risk, though for most cat owners this isn’t something you can act on after the fact. Siamese and Himalayan cats have a significantly lower risk of developing the disease, a protective effect that holds up even after accounting for diet, litter use, and flea treatment exposure. These two breeds are genetically related, suggesting a shared trait that makes their thyroid glands more resistant to the environmental triggers that affect other cats.
Most domestic shorthairs and longhairs, which make up the vast majority of pet cats, don’t have this protection. If your cat isn’t a Siamese or Himalayan, the dietary and environmental strategies described above become that much more important.
Start Annual Thyroid Screening Early
Hyperthyroidism is typically diagnosed in cats older than 10, but elevated thyroid levels in younger cats are being found more frequently now that annual blood screening is becoming routine. The 2023 AAHA endocrine guidelines recommend tracking thyroid hormone levels over time rather than relying on a single test. In a healthy cat, levels should remain stable or gradually decline with age. A rising trend, even within the “normal” range, can be an early warning sign.
Starting annual blood panels at age 7 gives your vet a baseline to compare against. This is especially valuable because early hyperthyroidism often produces vague symptoms (weight loss despite a good appetite, increased thirst, restlessness) that are easy to dismiss as normal aging. Catching a rising trend before symptoms appear means earlier intervention, which protects the heart and kidneys from damage caused by prolonged thyroid hormone excess.
A Practical Prevention Checklist
- Minimize pop-top canned food. Substitute with pouches, BPA-free cans, or fresh-food options when possible.
- Avoid soy-containing cat foods. Check labels for soy protein, soybean meal, and soy flour.
- Feed a consistent, high-quality diet with balanced iodine from a reputable manufacturer.
- Vacuum regularly and reduce household dust, especially if you have older upholstered furniture or carpet.
- Request a thyroid panel as part of your cat’s annual bloodwork starting at age 7, and ask your vet to track the trend year over year.
None of these steps guarantee your cat won’t develop hyperthyroidism. But taken together, they address every major modifiable risk factor that research has identified so far, and they’re all straightforward changes that fit into normal cat care.