Hyperthyroidism in Cats: Symptoms, Diagnosis, and Treatment

Hyperthyroidism is the most common hormonal disorder in older cats: an overactive thyroid floods the body with thyroid hormone, speeding up the metabolism so a cat loses weight even though it is eating well. It almost always shows up in mature, senior, or geriatric cats, and the good news is that it is one of the most treatable diseases of old age, often curable, when it is caught in time.

If your older cat is on this journey, this article sits alongside our senior cat care guide and our overview of the signs of ageing in cats.

What hyperthyroidism is

Hyperthyroidism is an excess of thyroid hormone produced by the thyroid glands in the neck. In the great majority of cats, around 98 percent, the cause is a benign (non-cancerous) enlargement of the gland; cancerous thyroid tumours are uncommon. Thyroid hormone sets the pace of the body’s metabolism, so too much of it pushes nearly every organ to work harder than it should, including the heart.

It is a disease of older cats. The average cat is about 12 to 13 years old at diagnosis, and it is rare in cats under 7. That places it firmly in the mature adult (about 7 to 10 years) to geriatric (15 years and over) life stages, which is why it belongs on every senior owner’s radar.

Signs and symptoms

The classic sign is weight loss in an older cat that still has a healthy, often increased, appetite. The metabolism is running hot, so the cat burns calories faster than it can take them in. Owners often describe a cat that seems hungrier than ever yet feels bonier when picked up.

Common signs include:

• Weight loss despite a good or ravenous appetite
• Increased thirst and urination
• Restlessness, hyperactivity, or irritability
• A scruffy, greasy, or unkempt coat
• Occasional vomiting or diarrhoea
• A fast heart rate, which your vet may pick up on examination

I first knew something was off with my old tabby, Pickle, not from a dramatic symptom but from lifting him onto the bed one evening: he had always been a solid armful, and suddenly my hands met more spine than I expected. He was thirteen, eating like a kitten, and pacing the kitchen at 3am. The vet had it pegged within a single blood test. I have weighed my cats on the kitchen scales once a month ever since, because the number tells you what your hands can miss.

How vets diagnose it

Diagnosis is usually straightforward: a blood test measuring total thyroxine (T4). A clearly raised T4 in a cat with the right signs confirms the disease. Because thyroid levels can fluctuate, a small number of cats need a repeat test or an additional test (such as free T4) to confirm a borderline result.

Vets almost always run a broader panel at the same time, checking kidney values, liver values, and blood pressure. This matters because chronic kidney disease is very common in older cats and frequently sits alongside hyperthyroidism. An overactive thyroid increases blood flow through the kidneys and can make kidney results look better than they truly are, so the full picture is needed before settling on a plan.

Treatment options

There are four established treatments, and the right one depends on the individual cat, other illnesses, and your circumstances.

• Medication: Anti-thyroid tablets or a transdermal gel, usually given twice daily, that lower hormone production. This controls the disease well but does not cure it, so it continues for life with periodic blood tests.
• Diet: A special iodine-restricted food. The cat must eat it exclusively, with no other food or hunting, which suits some households and not others.
• Surgery: Removing the affected thyroid tissue, which can be curative, though it carries anaesthetic considerations in an older cat.
• Radioactive iodine (I-131): A single injection that targets only the overactive tissue. It cures the condition in roughly 95 percent of cats and is widely regarded as the gold standard, though it requires a stay at a licensed centre.

Each option has trade-offs in cost, convenience, and whether it manages or cures the disease. Your vet will weigh these with you, and money is a fair part of that conversation; our guide to how much a vet costs can help you plan.

Living with a hyperthyroid cat

With treatment, most cats do very well, often returning to a normal weight and a settled routine. The condition is monitored with repeat blood tests so the dose or diet can be fine-tuned and so the thyroid level is not pushed too low.

The key thing to watch after treatment is the kidneys. Because the high thyroid level can hide kidney disease, vets recheck kidney values once the thyroid is controlled; if kidney disease emerges, it is managed in parallel, and our note on feeding a cat with kidney disease is worth a read. Senior cats benefit from vet checks every 6 months rather than yearly, which is the single best way to catch problems like this early.

This guide is general information, not a diagnosis. If your cat is losing weight, drinking more, or otherwise not itself, your own vet can examine it, run the right tests, and advise on the best treatment for your cat.