Hyperthyroidism in cats is caused by benign growths on the thyroid gland that produce excess thyroid hormone. In 95 to 98% of cases, these growths are noncancerous nodules (adenomas or adenomatous hyperplasia), while only 2 to 5% involve thyroid carcinoma. The condition overwhelmingly affects older cats, with 95% of diagnosed cats being 10 years or older. What triggers those nodules to form in the first place is less clear, but researchers have narrowed down several contributing factors.
What Happens Inside the Thyroid
A cat’s thyroid gland sits in the neck and regulates metabolism. In hyperthyroidism, clusters of thyroid cells develop an excessive capacity for growth. These cells form one or more nodules, ranging from less than a millimeter to about 3 centimeters across. The cells inside these nodules are noticeably larger than the surrounding normal tissue, and they churn out thyroid hormone independently, ignoring the brain’s normal signals to slow down.
In a healthy cat, the pituitary gland releases a messenger hormone called TSH that tells the thyroid how much hormone to make. The nodules in hyperthyroid cats bypass this feedback loop entirely. Transplant studies have confirmed this: when researchers moved thyroid tissue from hyperthyroid cats into mice, the tissue kept overproducing hormone on its own. No outside stimulation was needed. This is a key distinction from Graves’ disease in humans, where the immune system drives the thyroid into overdrive. Blood tests on hyperthyroid cats have confirmed that their immune systems are not producing the antibodies responsible for Graves’ disease.
Genetic Mutations in Thyroid Cells
Several genetic mutations within the thyroid cells themselves appear to set the stage. The most well-studied involve the TSH receptor gene. In a detailed analysis of 134 thyroid nodules from 50 hyperthyroid cats, researchers found ten different mutations that cause the TSH receptor to stay permanently switched on, as if the cell is constantly receiving a “make more hormone” signal even when no signal has been sent. The most common of these mutations is called Met452-Thr.
Other mutations affect proteins that relay signals inside the cell. Some hyperthyroid cats have mutations in a gene called GNAS, which keeps the cell’s internal “accelerator” pressed down. Others show reduced levels of a protein (called Gαi2) that normally acts as a brake on hormone production. When that brake is weakened, the cell produces more of a chemical messenger called cAMP, which drives hormone output up. These mutations are acquired over a cat’s lifetime rather than inherited from birth, which helps explain why the disease appears almost exclusively in older cats.
Breed and Genetic Predisposition
While the mutations that cause hyperthyroidism develop with age, a cat’s breed does influence risk. Siamese and Himalayan cats, two genetically related breeds, have a significantly lower risk of developing the condition. This protective effect held up even after researchers controlled for diet, litter use, and flea treatment products. No breed has been identified as having a dramatically higher risk. Most hyperthyroid cats are domestic shorthairs and longhairs, which simply reflects how common those cats are in the general population.
Environmental Chemical Exposure
Because feline hyperthyroidism became dramatically more common starting in the late 1970s, researchers have investigated whether chemicals introduced into household environments around that time could be to blame. The leading suspects have been flame retardants called PBDEs, which are found in furniture, carpets, and electronics. Cats groom themselves constantly and swallow household dust, making them especially exposed.
The evidence, however, has been mixed. One study that measured PBDE levels in both hyperthyroid and healthy cats found no significant difference between the two groups. The median blood level of key flame retardant compounds was actually higher in healthy cats than in hyperthyroid ones. Dust does appear to be the main route of PBDE exposure for indoor cats, but a direct link to thyroid disease has not been established.
Bisphenol A (BPA), a chemical used in the lining of canned food containers, has also drawn scrutiny. Earlier studies flagged canned cat food as a potential risk factor for hyperthyroidism, and BPA is a known endocrine disruptor in humans. But when researchers measured BPA levels in 69 clinically healthy older cats, every single cat had detectable BPA in its blood, and there was no association between BPA levels and thyroid hormone levels. The chemical is clearly present in cats’ systems, but the direct connection to thyroid disease remains unproven.
Dietary Factors and Soy
What cats eat may influence their thyroid function in subtler ways. Soy-based ingredients are common in commercial cat foods, and soy contains compounds called isoflavones (including genistein and daidzein) that can interfere with thyroid hormone processing. In a controlled study, cats fed a soy-containing diet had significantly higher levels of the thyroid hormones T4 and free T4 compared to cats on a soy-free diet.
The mechanism likely involves soy blocking an enzyme that converts one form of thyroid hormone (T4) to another (T3), or speeding up how quickly the body clears T3 from the bloodstream. Either way, the body compensates by ramping up T4 production. Over years of eating soy-containing food, this chronic stimulation of the thyroid could theoretically promote the kind of cellular overgrowth that leads to nodules. Soy was identified as a goitrogen (a substance that disrupts thyroid function) more than 70 years ago, though the exact mechanism in cats is still being worked out.
Why It’s Almost Always Older Cats
The 10-and-older pattern isn’t a coincidence. The mutations that drive hyperthyroidism are somatic, meaning they accumulate in individual thyroid cells over time rather than being present from birth. A cat’s thyroid cells divide and replicate for years, and with each division there’s a small chance of acquiring a mutation that locks the cell into overdrive. The longer a cat lives, the more opportunities those mutations have to occur and the more time a tiny nodule has to grow large enough to affect hormone levels.
This also explains why the disease has become more common over recent decades. Cats today live significantly longer than they did 40 or 50 years ago, thanks to improvements in nutrition, veterinary care, and indoor living. More cats reaching age 12, 15, or 18 means more cats living long enough for these slow-developing thyroid nodules to become a clinical problem. The rise in feline hyperthyroidism is likely driven by a combination of longer lifespans, possible environmental exposures, and better diagnostic testing that catches cases that would have gone undetected in earlier decades.
How It Gets Diagnosed
The standard screening test measures total T4 in the blood. When that level is repeatedly elevated, treatment is recommended to prevent damage to the heart, kidneys, and other organs. The tricky part is that cats with other illnesses can have misleadingly normal T4 readings, because chronic disease tends to suppress thyroid hormone levels. In those cases, a free T4 test, which measures the unbound and active portion of the hormone, can catch hyperthyroidism that the standard test misses. TSH levels alone are not sensitive enough to diagnose the condition and need to be interpreted alongside T4 results.