Hyperthyroidism in cats is treated by reducing or removing the effect of excess thyroid hormone. The four main options for this are medication, radioactive iodine therapy, surgery, and a low-iodine prescription diet.

Treatment plans should be made with a vet, as variables like the cat's age and general health are relevant. Hyperthyroidism in cats can be fatal if left untreated, but many cats do very well with the right treatment.

This guide explores how to treat hyperthyroidism in cats, starting from veterinary diagnosis through to finding the right treatment option.

What Is Hyperthyroidism in Cats?

Cats have two thyroid glands in their necks, which control their metabolism. The term "hyperthyroidism" means these glands are overactive, producing too much thyroid hormone. Most cases are linked to a benign thyroid enlargement or tumour, so it mainly affects middle-aged or older cats.

When a cat has too much thyroid hormone, it can have the following effects on their body:

• Their metabolism gets faster
• It puts strain on other organs
• It can contribute to weight loss, fast heart rate, high blood pressure, heart strain and kidney-related concerns

The vast majority of cases are due to benign swellings — only a very small percentage come from cancerous growths.

Signs Your Cat May Need Treatment for Hyperthyroidism

There are various symptoms of hyperthyroidism in cats. These often develop gradually and can be mistaken for normal ageing, so you need to be mindful of them when you notice changes in your cat.

Here are the key signs of hyperthyroidism to watch out for in your cat:

• Weight loss despite good or increased appetite
• Drinking more
• Urinating more
• Vomiting
• Diarrhoea
• Restlessness, hyperactivity, noisy or demanding behaviour
• Poor, greasy, matted or untidy coat
• Lethargy in some cats
• A fast heartbeat
• In some cases, a small lump or lumps in the neck

Many of these can be symptoms of other things too, so it is very important that you don't ignore them, especially in middle-aged cats. Litter Sense cat litter can give you an early warning sign by detecting changes in your cat's urine — it changes colour to let you know how healthy your cat's urine is.

If you use Litter Sense cat litter and notice a red flag colour change, or you spot any of the symptoms listed above, book a vet appointment.

How Vets Diagnose Hyperthyroidism Before Treatment

Treatment for hyperthyroidism in cats should never start without a vet diagnosis. Once you spot a possible sign of the problem, you should contact your vet and book an appointment. The steps to diagnosis will look like this:

• A discussion of symptoms
• A physical examination
• Feeling the neck for an enlarged thyroid gland
• Checking heart rate
• Checking blood pressure
• Blood test for thyroid hormone, commonly T4
• Blood chemistry and urine tests to assess general health

Cats with hyperthyroidism are not usually young, so vets will typically assess kidney function as well. Treating hyperthyroidism can sometimes reveal underlying kidney disease because the overactive thyroid can increase blood flow through the kidneys.

It is quite common for kidney disease and hyperthyroidism to occur together in older cats. Further testing may be needed if signs suggest hyperthyroidism but T4 is not clearly elevated.

The Main Treatment Options for Hyperthyroidism in Cats

There are four recognised routes to treat hyperthyroidism in cats: medication, radioactive iodine therapy, surgery and dietary therapy. The right choice depends on health, ability to medicate and financial considerations, so let's explore them in detail.

Medication for Cat Hyperthyroidism

There are medications that can help control the condition by reducing thyroid hormone production. Taking this route typically controls rather than cures hyperthyroidism, so symptoms will likely return if the medication is stopped.

Anti-thyroid medication, like methimazole, usually comes as a tablet or oral liquid and will usually need to be delivered for the rest of the cat's life, unless used temporarily before surgery or radioactive iodine.

Medication is:

• Suitable for many cats
• Often accessible and familiar to first-opinion vets
• Frequently used to stabilise a cat before surgery

Daily dosing can be stressful for some cats and owners, and regular blood tests will still be needed. Side effects can include vomiting, reduced appetite, lethargy, skin reactions, liver value changes, anaemia and diarrhoea. Always follow the vet's guidance for medication.

Radioactive Iodine Therapy for Cats

Radioactive iodine, often called I-131, is administered via injection. Any abnormal thyroid tissue in the body absorbs the iodine, so the radiation destroys the overactive thyroid tissue while sparing most of the surrounding tissue.

This is often considered the gold-standard treatment of choice when it is available. This is because it:

• Usually cures the condition
• Doesn't require daily medication afterwards
• Can be administered without general anaesthetic

When this method is used, most cats return to normal hormone levels within one to two weeks. According to Cornell Vet College, it is curative within three months in approximately 95% of cases.

The drawback is that it is only available at licensed specialist facilities, and cats must stay in hospital until their radiation levels fall. Hospitalisation is usually 3–5 days and visitors are typically not allowed during the quarantine period.

With this treatment, it is also important to note that hyperthyroidism can reoccur after the treatment.

Surgery to Remove the Thyroid Gland

This type of procedure is called a thyroidectomy. It removes the affected thyroid gland or glands, providing a long-term or permanent cure in suitable cats. Cats may need to take medication first to stabilise their thyroid levels — this can take up to 3–4 weeks.

There are risks associated with surgery:

• General anaesthetic can be risky for older cats or those with heart/kidney problems
• Accidental parathyroid gland damage can occur
• There is a risk of low calcium, also called hypocalcaemia
• The risk is higher if both thyroid glands are removed

This approach is less commonly chosen, as the medication or radioactive iodine routes are less invasive. Not every cat is a good candidate for surgery.

Low-Iodine Diet for Cat Hyperthyroidism

The thyroid needs iodine to produce thyroid hormone. Therefore, a prescription low-iodine diet limits the amount of that nutrient available for the thyroid to produce the hormone. This can help reduce levels of thyroid hormone in a cat's system, but there are challenges.

The main concern is that this diet must be fed exclusively. The cat can never be allowed to eat:

• Treats
• Other cat food
• Prey
• Scraps
• Flavoured medicines

With this in mind, outdoor cats or cats in multi-cat households are rarely good candidates. This treatment approach controls the condition rather than removing abnormal thyroid tissue. Long-term suitability should be discussed with a vet — for many, it is considered controversial.

Which Treatment Is Best for Your Cat?

You will not need to make the decision on your own, as your vet will advise you on the best course of action. There is no one-size-fits-all solution for every cat, so the different options will suit different cats at different times.

• Medication: May suit cats needing quick stabilisation or owners who want lower upfront costs
• Radioactive iodine: May suit cats where a potential cure is preferred and referral/hospital stay is possible
• Surgery: A good option if a cat is a good anaesthetic candidate
• Low-iodine diet: May suit cats where medication, surgery or radioiodine are not appropriate

When making a decision, you will need to consider age, kidney function, heart health, blood pressure, temperament, lifestyle, insurance cover, cost and more. Here's a simple table to break it down:

Treatment	Cure or Control?	Main Benefit	Main Drawback
Medication	Controls	Widely available	Lifelong dosing and monitoring
Radioactive iodine	Usually curative	High success rate, no daily tablets	Specialist referral and hospital stay
Surgery	Potential cure	Removes affected gland	Anaesthetic and parathyroid risks
Low-iodine diet	Controls	Non-invasive	Must be exclusive

Ongoing Care After Treatment

Cat hyperthyroidism treatment doesn't end at the first prescription or procedure. Depending on the type of treatment they get, your cat may need regular vet checks, repeat blood tests, kidney function checks, blood pressure monitoring, and observation of appetite, weight, and more.

With it being a lifelong treatment option, medication requires the most ongoing care. For all owners, however, it can be helpful to do the following after a hyperthyroidism diagnosis:

• Weigh your cat regularly
• Keep a note of appetite and water intake
• Report vomiting, diarrhoea, lethargy, itching and marked behaviour changes

Remember that hyperthyroidism and kidney disease often overlap in older cats, so ongoing care will be tailored to the specific circumstances of your pet.

Can Cats Live Well After Hyperthyroidism Treatment?

With good management, a cat can live for many years after treatment for hyperthyroidism. The prognosis is good with appropriate therapy, but complications involving other organs can affect the outlook. This is why early diagnosis and treatment are important.

Untreated hyperthyroidism can be damaging to:

• The heart
• Blood pressure
• The eyes
• Kidneys

Successful treatment often improves appetite, weight, coat condition and behaviour. If your cat is losing weight, eating more, drinking more or acting unusually restless, take it to see the vet.

Frequently Asked Questions About How to Treat Hyperthyroidism in Cats

Can hyperthyroidism in cats be cured?

Radioactive iodine usually cures the condition, and surgery can be a cure if the cat is a good candidate. Medication and diet usually control rather than cure the condition.

What happens if cat hyperthyroidism is left untreated?

Symptoms will only keep getting worse. It could lead to issues like high blood pressure, heart disease, eye damage and heart failure. Untreated hyperthyroidism is almost 100% fatal.

Is medication for cat hyperthyroidism lifelong?

If medication is chosen as the long-term treatment then it will be lifelong. It may also be used temporarily before surgery or radioactive iodine.

Is radioactive iodine safe for cats?

It is widely regarded as highly effective and usually curative. It does involve radiation controls, specialist facilities and a short hospital stay. You need to discuss suitability with your vet.