Here is the short version: if an older cat is losing weight while eating like a teenager, hyperthyroidism is near the top of the list. A small gland in the neck starts overproducing thyroid hormone, which pushes the whole body into overdrive, burning through weight, straining the heart, and quietly damaging other organs. The encouraging part is that this is one of the most treatable serious diseases a cat can get, and one of the treatments actually cures it. This guide covers the signs, how it is diagnosed, the four ways to treat it, and the kidney and heart complications worth understanding.
The most common hormone disease in older cats
Hyperthyroidism is the most common endocrine disorder of middle-aged and older cats.3 The professional guidelines put its prevalence at up to about 10 percent of cats over 10 years old, and it is primarily a disease of cats from around age seven upward.2 The cause is almost always reassuringly benign: a non-cancerous overgrowth of the thyroid gland. A malignant thyroid tumor is rare, fewer than 3 percent of cases.3
AAFP feline hyperthyroidism guidelines, 2016
What too much thyroid hormone does
Thyroid hormone sets the body's metabolic speed. When there is too much, every system runs hot. The signature sign is weight loss despite a good or even voracious appetite, because the cat is burning fuel faster than it can eat.1 Around that core sign, watch for:
- Increased thirst and urination.
- Hyperactivity, restlessness, or a cat that seems newly "wired."
- Vomiting or diarrhea, and larger stools.
- A rapid heart rate.
- An unkempt, greasy, or matted coat in a cat that used to groom well.
Many owners read these as a cat simply slowing down or getting quirky with age, which is exactly why it goes unnoticed. An older cat that is eating more but weighing less deserves a blood test, not a bigger food bowl.
How it is diagnosed
Diagnosis is usually straightforward: a blood test for total thyroxine (total T4), which is elevated in most affected cats and confirms the disease.3 There is one wrinkle worth knowing. A small percentage of genuinely hyperthyroid cats have a total T4 that still reads in the normal range, so if the suspicion is strong, a vet may add a free T4 test or simply repeat the bloodwork a couple of weeks later.3 Because the disease shows up in senior cats, this test is often bundled into routine senior wellness panels.
Four ways to treat it, including a cure
There are four established treatments, and they range from daily management to a one-time cure.1
Radioactive iodine (the cure)
A single injection of radioactive iodine (I-131) is taken up only by the overactive thyroid tissue and destroys it, leaving healthy tissue alone. It cures roughly 95 percent of cats with one treatment and is widely considered the treatment of choice.1 The cat stays a few days at a licensed facility while the radioactivity clears. It is the closest thing to a true fix.
Methimazole (daily medication)
Methimazole is a drug, given as a pill or a gel rubbed inside the ear, that blocks hormone production. It controls the disease well but does not cure it, so it is given for life and the cat needs periodic monitoring.3 It is inexpensive to start and a good fit for many cats and households.
Prescription diet
A strictly iodine-restricted prescription diet can control hormone levels, because the thyroid needs iodine to make hormone. It only works if the cat eats that food and nothing else, which is a real constraint in multi-cat homes.1
Surgery
Surgical removal of the affected thyroid tissue is also an option and can be curative, though it carries the usual anesthetic and surgical considerations and is chosen less often now that radioiodine is widely available.1
What it costs
Radioactive iodine is a larger one-time cost, commonly around $1,500 to $3,500 depending on the facility and what diagnostics are included, after which there is usually nothing more to pay for the thyroid.4 Methimazole is cheap per month but is a lifelong, recurring expense plus regular rechecks, so over years the costs can converge. These are US market estimates that vary by region. As with much on this site, the more permanent fix often looks expensive up front and reasonable over a lifetime.
The kidney and heart catch
Two complications make hyperthyroidism more than a weight problem. First, the heart: chronically high thyroid hormone speeds and thickens the heart and raises blood pressure, and left untreated it can lead to heart failure.1 If your cat also has a heart murmur, our guide on feline heart disease is worth a read.
Second, and less intuitive, the kidneys. Hyperthyroidism increases blood flow through the kidneys and can mask underlying chronic kidney disease, making the kidneys look healthier on paper than they are. Treating the thyroid can then unmask that kidney disease as the values rise.3 This is not a reason to avoid treatment, it is a reason vets monitor kidney values closely during it, and it is why our piece on chronic kidney disease in cats so often shares a chart with this one.
What to do this week
- If your cat is 7 or older, make sure a thyroid (T4) test is part of its annual or twice-yearly senior bloodwork.
- Weigh your cat, or have the vet do it, and take "eating well but losing weight" seriously rather than as normal aging.
- If hyperthyroidism is diagnosed, ask your vet to compare all four treatments for your cat, including whether radioiodine is available near you.
- Whatever treatment you choose, ask how the kidneys and heart will be monitored along the way.
Hyperthyroidism is a good-news disease in a hard category. It is common, it is easy to test for, and it has a genuine cure. The catch is simply noticing it, because the early signs look so much like an old cat being an old cat. Watch the scale and the food bowl together, and this is a disease most cats move past.