Hyperthyroidism in Cats: Symptoms, Causes, and Treatment

In the large majority of cats the cause is a benign, non-cancerous enlargement of one or both thyroid lobes. A cancerous thyroid tumour is possible but uncommon. That is reassuring, because a benign, hormone-driven disease is exactly the kind that responds well to treatment when it is caught in time.

The tricky part is that hyperthyroidism creeps in slowly in a senior cat, and the early signs look a lot like "just getting old." A cat losing weight while eating well is easy to miss until the change is obvious. That is why any older cat that is dropping weight deserves a blood test rather than a wait-and-see.

Worried your cat has an overactive thyroid? You can check the symptoms free in the Omelo app in about 30 seconds and see whether they point to a vet visit. In India, you can also book a video consultation with a licensed vet for ₹349 at vet.beomelo.com if you want a professional to look at what you are seeing.

• Weight loss despite a normal or increased appetite. This is the single most telling sign. A hyperthyroid cat is often hungry, even ravenous, yet keeps getting thinner. Owners frequently notice a sharp spine and ribs before anything else.
• Increased thirst and urination. You may be refilling the water bowl more often or scooping a heavier litter tray.
• Restlessness and hyperactivity. An older cat that suddenly seems wired, pacing, or unable to settle.
• A poor coat. The fur may look greasy, matted, or unkempt because grooming falls off.
• Vomiting or diarrhoea. The gut speeds up too, and some cats vomit more or pass looser, more frequent stools.
• Increased vocalising, often at night. Loud yowling in the small hours is a common complaint.
• A fast heart rate. A vet will often pick up a racing or pounding heartbeat on exam.
• Occasionally the opposite: some cats become dull and lose their appetite rather than gaining one. This is called apathetic hyperthyroidism and is easy to mistake for other illness.

You will rarely see the whole list. In an older cat, weight loss with a good appetite is enough on its own to justify a thyroid blood test.

What is clear is who tends to get it. Age is the strongest link, with most cases in cats past 10 years. It is not a disease of kittens or young adults. It also is not something an owner causes or could have prevented through better care, so there is no reason for guilt if your cat is diagnosed. The useful takeaway is simply that senior cats deserve routine wellness bloodwork, because that is how this disease is usually caught early.

• Sudden blindness or a cat bumping into things, with widely dilated pupils. High blood pressure can detach the retinas, and fast treatment offers the only chance to save sight.
• Laboured, fast, or open-mouth breathing, which can signal heart strain or fluid on the lungs.
• Collapse, fainting, or sudden severe weakness.
• A cat that abruptly stops eating and drinking, or that becomes very lethargic.

These are complications rather than the disease itself, and they need a clinic right away.

Vets almost always run full bloodwork and a urine test at the same time, and this matters. Hyperthyroidism and chronic kidney disease often exist together in older cats, and an overactive thyroid can mask early kidney disease by artificially boosting blood flow through the kidneys. Once the thyroid is treated, hidden kidney trouble can surface. Knowing about it up front lets your vet plan treatment safely rather than being caught out later. This is one reason not to treat at home or guess at the diagnosis.

• Anti-thyroid medication (methimazole). A daily tablet or a gel applied to the ear that lowers hormone production. It controls the disease well and is widely used, but it is lifelong, needs periodic blood monitoring, and does not cure the underlying problem.
• Prescription iodine-restricted diet. A special therapeutic food that limits the iodine the thyroid needs to make hormone. It can manage the disease, but only if the cat eats that diet and nothing else, which is hard in multi-cat homes or for cats that go outdoors.
• Surgery (thyroidectomy). Removing the affected thyroid tissue can cure the disease. It needs anaesthesia in an older cat and careful case selection, so it suits some cats better than others.
• Radioactive iodine therapy (I-131). Widely considered the gold standard. A single injection destroys the overactive tissue and cures most cats, with no daily medication afterwards. It requires a short stay at a licensed facility, and availability varies by location.

Whichever route you take, follow-up bloodwork is part of the deal. The aim is to bring hormone levels back to normal without tipping the cat into an underactive thyroid, and to keep an eye on the kidneys and heart along the way.

At home, keep track of your cat's weight, appetite, thirst, and energy, because you are the one who will notice a drift before the next visit. A simple monthly weigh-in on a kitchen scale is genuinely useful. Provide easy access to fresh water, quiet resting spots, and a calm routine, since these cats are often older and may have other age-related conditions too. With good control, many hyperthyroid cats feel like themselves again, put weight back on, and settle back down.

Omelo does not diagnose, treat, or prescribe, and it cannot confirm hyperthyroidism. Only a vet with a blood test can do that. Think of the app as the step that helps you decide to book the visit, not a replacement for the clinic.

In India, if you would like a licensed vet to weigh in before you travel, you can book a 15-minute video consultation for ₹349 at vet.beomelo.com. A vet can talk through the changes you are seeing in an older cat and help you plan the right tests and next steps.

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