Your senior cat has been eating more than usual but losing weight anyway. Maybe drinking more water, maybe a little more restless at night. It’s easy to put down to age, but together these changes often mean hyperthyroidism, the most common hormone disorder in cats.
What hyperthyroidism is
The thyroid gland sits in the neck and produces hormones (T4 and T3) that regulate metabolism. In hyperthyroidism, the gland grows larger and overactive and makes too much thyroid hormone. Metabolism speeds up, the heart works harder, and the cat burns calories faster than they can eat them.
In about 97-99% of cases the cause is a benign overgrowth of thyroid tissue; a malignant tumor accounts for only 1-3%. It’s a disease of older cats, usually diagnosed around 12-13 years old and rare before 8.
About 1 in 10 cats over 9 develop it. Siamese and Himalayan cats seem to have a slightly lower risk, though no one knows why.
Signs to watch for
The hallmark is weight loss while eating as much as ever, or more. Seventy to ninety percent of hyperthyroid cats show this at diagnosis, and it’s unusual enough in an older cat that it’s worth a vet visit.
Other common signs include:
- Increased thirst and urination
- Vomiting or diarrhea
- Restlessness or irritability
- Fast heart rate
- Unkempt, greasy, or matted coat
At the exam, your vet may also hear a heart murmur or gallop rhythm, or feel an enlarged thyroid in the neck. An older cat who suddenly races around and yowls at night while losing weight is a classic picture. The cat sleep guide covers other sleep changes worth noticing.
About 5-10% of cats show the opposite, called “apathetic hyperthyroidism”: they become lethargic and lose their appetite. Because it doesn’t match the usual picture, it’s easier to miss.
A cat over 8 who is losing weight while eating the same or more, and is also drinking more or vomiting, should definitely be checked. The same signs can mean diabetes, and older cats sometimes have both, so bloodwork usually looks for both.
How it’s diagnosed
The usual first step is a blood test for total T4. In most hyperthyroid cats it’s clearly high, and in moderate to severe cases the test picks up almost all of them (about 91-98%).
Early or mild cases are trickier: total T4 may sit at the high end of normal. Another illness at the same time can also pull T4 down into the normal range, which is called sick euthyroid syndrome. If your vet still suspects hyperthyroidism, they may retest in a few weeks or run additional thyroid tests.
A thyroid scan (scintigraphy) shows exactly where the overactive tissue is, even if some sits outside the usual spot. It’s mainly used when planning surgery or radioactive iodine.
Your vet will also run a full blood panel. Liver values are raised in about 60-75% of hyperthyroid cats and usually settle after treatment. Kidney values (creatinine, BUN, SDMA) matter a lot when choosing a treatment. A blood pressure check and a heart check may also be suggested, because untreated hyperthyroidism can thicken the heart muscle, which often improves once thyroid levels are back to normal.
Treatment options
There are four main approaches, and none is best for every cat. The choice depends on your cat’s health, your circumstances, and what’s available nearby, and you’ll make it together with your vet.
Radioactive iodine (I-131)
Many veterinary hormone specialists consider this the best way to cure hyperthyroidism. Radioactive iodine destroys only the overactive thyroid tissue and leaves the normal tissue alone, and one treatment cures about 95% of cats.
The main drawbacks are practical. The cat needs to be hospitalized in a licensed radiation facility for several days (requirements vary by country; in some regions it can be 1-3 weeks). Not all areas have facilities that offer it, and the cost is significant. A small number of cats end up with too little thyroid hormone afterward and need a supplement. Because the treatment can’t be undone, vets usually control the thyroid with medication for a while first to see how the kidneys respond.
Anti-thyroid medication
Medication is the most common first step. It reduces how much hormone the thyroid makes without destroying the gland, so it has to be given for life.
Most cats’ thyroid levels are back to normal within a few weeks. The big advantage is that it can be adjusted: if the kidneys get worse once the thyroid is under control, the dose can be changed or stopped.
Some versions are rubbed onto the inside of the ear. Wear gloves or finger cots when applying them, because the medicine is absorbed through human skin, and anyone pregnant should avoid touching it.
Cats on medication need regular rechecks and blood tests to keep an eye on thyroid levels and watch for side effects.
Surgery
Removing the thyroid gland can cure the disease, but it has its own risks. The parathyroid glands, which control blood calcium, sit right next to the thyroid, and damaging them during surgery can cause a dangerous drop in calcium. Anesthesia is also riskier for older cats with heart changes. The thyroid is usually brought under control with medication before surgery.
Iodine-restricted diet
A prescription food very low in iodine, which the thyroid needs to make its hormone, can bring T4 down. It has to be the only thing the cat eats: no treats, no scraps, no prey, and even sneaking from another cat’s bowl can undo it.
This option is more debated. Many cats’ T4 returns to normal on it, but the thyroid overgrowth itself isn’t treated and keeps growing, and it’s very hard to manage in a multi-cat home. It’s usually considered when the other options don’t fit. If your vet suggests it, ask about its limits.
Why hyperthyroidism and kidney disease are linked
Before treating hyperthyroidism, the kidneys need checking too.
Too much thyroid hormone increases blood flow to the kidneys, which makes their filtration rate (GFR) look better than it really is. If a cat already has early kidney disease, this hides it, and kidney blood tests come back looking more normal than the kidneys actually are.
Once hyperthyroidism is treated and thyroid hormone returns to normal, kidney blood flow drops back to its true level and the hidden kidney disease shows up. The disease was there all along, and the treatment didn’t damage the kidneys.
About 15-40% of hyperthyroid cats show worse kidney values within a few months of their thyroid levels returning to normal. Cats whose thyroid levels are pushed too low by treatment do worse still, with faster kidney decline and shorter survival.
That’s why, before irreversible treatments like I-131 or surgery, most vets control the thyroid with medication for a while to see how the kidneys respond. If kidney disease emerges, the plan can be adjusted to balance thyroid control against protecting the kidneys. Our kidney disease guide covers kidney disease itself, including staging.
What you can do at home
You can’t diagnose or treat hyperthyroidism at home, but you can help catch it early.
Weigh your cat regularly. It’s the easiest way to catch problems early, and not only hyperthyroidism. A cat losing weight slowly over months looks the same from day to day; only the scale shows it. Weigh every 2-4 weeks and write the numbers down.
Watch for signs that come together. An older cat losing weight while eating more, or losing weight while drinking more, should see the vet.
Keep up with checkups. Cats 7 and older should have yearly bloodwork that includes T4, and cats over 10 a checkup every six months. The earlier it’s found, the more treatment options you have.
If your cat is already being treated, keep to the recheck schedule your vet sets; both thyroid levels and kidneys need ongoing monitoring.
Frequently Asked Questions
What are the first signs of hyperthyroidism in cats? The classic sign is weight loss while eating as much as ever or more, seen in 70-90% of cases. Other common signs are drinking and peeing more, vomiting, restlessness, a fast heart rate, and an untidy coat. About 5-10% of cats show the opposite, lethargy and poor appetite, called apathetic hyperthyroidism.
Can hyperthyroidism in cats be cured? Yes. Radioactive iodine (I-131) cures about 95% of cats with one treatment, and surgery can also cure it. Medication controls the disease but has to be given for life. The best option depends on your cat’s overall health, especially the kidneys, and is a decision to make with your vet.
Why does treating hyperthyroidism sometimes reveal kidney disease? Too much thyroid hormone increases blood flow to the kidneys, making them filter faster than they really can and hiding kidney disease. After treatment, thyroid levels return to normal, kidney blood flow drops to its true level, and the hidden kidney disease shows up. That’s why vets usually try medication first before any irreversible treatment.
References
- Carney, H. C., Ward, C. R., Bailey, S. J., et al. (2016). 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. Journal of Feline Medicine and Surgery, 18(5), 400-416.
- Peterson, M. E. (2012). Hyperthyroidism in cats: what’s causing this epidemic of thyroid disease and can we prevent it? Journal of Veterinary Internal Medicine, 26(4), 963-975.
- Peterson, M. E., & Broome, M. R. (2015). Thyroid scintigraphy findings in 917 cats with hyperthyroidism. Journal of Veterinary Internal Medicine, 29(5), 1412-1417.
- Peterson, M. E. (2013). Diagnostic testing for feline thyroid disease: total T4. Journal of the American Animal Hospital Association, 49(4), 243-250.
- Peterson, M. E., & Becker, D. V. (1995). Radioiodine treatment of 524 cats with hyperthyroidism. Journal of the American Veterinary Medical Association, 207(11), 1422-1428.
- Edinboro, C. H., Scott-Moncrieff, J. C., Janovitz, E., Thacker, H. L., & Glickman, L. T. (2004). Epidemiologic study of relationships between consumption of commercial canned food and risk of hyperthyroidism in cats. Journal of the American Veterinary Medical Association, 224(6), 879-886.
- Williams, T. L., Elliott, J., & Syme, H. M. (2010). Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. Journal of Veterinary Internal Medicine, 24(5), 1086-1092.
- DiBartola, S. P., Broome, M. R., Stein, B. S., & Nixon, M. (1996). Effect of treatment of hyperthyroidism on renal function in cats. Journal of the American Veterinary Medical Association, 208(6), 875-878.
- Trepanier, L. A. (2007). Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North America: Small Animal Practice, 37(4), 775-788.
- Cornell University College of Veterinary Medicine. (2024). Hyperthyroidism in Cats. Cornell Feline Health Center
