Hyperthyroidism in Cats: The Overactive Thyroid, Explained
Hyperthyroidism in cats causes weight loss despite a big appetite. The signs, how it is diagnosed, the four treatments, and the (good) outlook.
Hyperthyroidism in cats is the great impersonator of old age. A cat with an overactive thyroid often eats well, sometimes ravenously, and keeps getting thinner anyway, so the one thing that should be reassuring, a healthy appetite, turns out to be part of the problem. It is the most common hormonal disorder in older cats. It is also, and I say this as someone who has spent a lot of time reading about the harder feline diseases, one of the more fixable things on the senior-cat list. A great many cats are cured of it outright.
This is a guide to what an overactive thyroid looks like and how it is handled. It is not a diagnosis of your cat, which belongs to your vet and one straightforward blood test.
What an overactive thyroid actually does
The thyroid is a small gland in the neck that sets the body's metabolic pace. In hyperthyroidism it makes too much hormone, and the whole engine runs too fast. The cat burns through calories and energy quicker than she can take them in, which is why she loses weight while eating more, not less. In the large majority of cats the cause is a benign enlargement of the gland. VCA notes that only 1-2% of hyperthyroid cases in cats involve malignant (cancerous) thyroid gland tumors. It is a disease of older cats: VCA notes that the average age of affected cats is approximately 12 years.
The signs
The signature is weight loss despite a normal or increased appetite. Around that you tend to see increased thirst and urination, restlessness or a new short temper, more vocalizing (especially at night), the occasional bout of vomiting or diarrhea, a fast heart rate, and a coat that stops looking cared-for.
These overlap heavily with kidney disease and diabetes, which is why a senior cat drinking more and losing weight is a reason to book a vet visit rather than to guess at home.
How it is diagnosed
Often your vet can feel the enlarged thyroid as a small nodule in the neck. The confirmation is a blood test for thyroid hormone, total thyroxine, or TT4; borderline cases get a free T4 or a thyroid scan. As these things go, it is one of the more straightforward diagnoses in an older cat.
The four ways to treat it
There are four established treatments, and the right one depends on your cat, your budget, and how permanent a fix you want.
| Treatment | What it is | Permanent fix? | Worth knowing |
|---|---|---|---|
| Methimazole | A daily pill or ear gel that blocks hormone production | No, it controls it for life | Cheapest to start; needs bloodwork every 3 to 6 months; some cats get side effects |
| Radioactive iodine (I-131) | A single injection that destroys the overactive tissue | Yes, usually a cure | Over 95% success and spares the other organs; needs a specialist center and a short hospital stay |
| Iodine-limited diet (y/d) | A prescription food that starves the gland of iodine | No, it controls it while fed | Only works if the cat eats it and nothing else, ever; no pills |
| Surgery (thyroidectomy) | Removing the affected gland | Often a cure | Needs anesthesia; may take two operations if both sides are affected |
Radioactive iodine is widely considered the gold standard when it is available and affordable, because it is a one-time treatment that leaves the rest of the cat alone. VCA reports that recurrence of hyperthyroidism is rare after I-131 therapy, with a success rate of greater than 95%. Methimazole is the common starting point, partly because it is reversible, which matters for the reason in the next section.
Hyperthyroidism and the kidneys
Hyperthyroidism and chronic kidney disease often turn up in the same older cats, and the two interact. An overactive thyroid pushes extra blood through the kidneys, which can hide kidney disease that is already there, and treating the thyroid can then bring it to the surface. This is why Cornell advises checking the heart and kidneys at diagnosis and monitoring kidney function during treatment, and why many vets begin with reversible methimazole and recheck kidney values before and after, rather than going straight to a permanent cure. If your cat has already been diagnosed with chronic kidney disease, this overlap is worth understanding.
What the outlook is
Good, genuinely. VCA puts it plainly: the outcomes following most hyperthyroid therapies are usually excellent, and most cats have a very good chance of returning to a normal state of health. Left alone, an overactive thyroid strains the heart and the kidneys over time, so it is worth catching. Caught and treated, it is one of the senior-cat diagnoses most likely to end with a cat who feels like herself again.
When to see the vet
Book a visit if an older cat is losing weight while eating well, has turned newly restless or loud at night, or is drinking and urinating more than she used to. If you are trying to gauge her thirst, the cat water intake calculator gives a normal daily range for her weight. None of this is an emergency in the way that refusing all food is, and it is not a wait-and-see either.
When Biscuit first started losing weight, an overactive thyroid was on the list of things it might have been, right beside the kidney disease she turned out to have. I remember hoping, a little guiltily, that it would be the thyroid, because that one so often has a happy ending. It was not, for us. But for a great many cats it is, and if your older cat is eating everything in sight and still fading, hyperthyroidism in an old cat is a hopeful thing to rule in or out.
Sources
- VCA Animal Hospitals, Hyperthyroidism in Cats
- Cornell Feline Health Center, Hyperthyroidism in Cats