Hyperthyroidism is one of the most common conditions in older cats and one of the most frequently missed, because the early stages look like a cat doing well.
An older cat that becomes livelier, eats enthusiastically and slims down reads as rejuvenated. What is actually happening is that its metabolism has been driven into overdrive.
This is orientation for a conversation with your vet rather than a diagnostic guide.
What it is
The thyroid glands in the neck become overactive, almost always because of a benign enlargement, and produce excessive thyroid hormone.
That hormone regulates metabolic rate. Too much of it makes the whole body run too fast — heart rate, digestion, energy use and blood pressure all rise.
It is overwhelmingly a disease of older cats, typically over ten, and it is very rarely cancerous.
The signs
| Sign | Why it happens | How often noticed |
|---|---|---|
| Weight loss despite a good appetite | Metabolism burning through calories | The classic sign |
| Increased appetite, sometimes ravenous | Energy demand | Often mistaken for good health |
| Restlessness, more vocal, more active | Hormonal overstimulation | Read as rejuvenation |
| Increased thirst and urination | Overlaps with kidney disease | Common |
| Vomiting or diarrhoea | Accelerated gut transit | Common |
| Poor, unkempt coat | Reduced grooming, metabolic effects | Common |
| Rapid heart rate | Cardiac stimulation | Found on examination |
| Aggression or irritability | Overstimulation | Sometimes |
The combination that should always prompt testing is weight loss with a normal or increased appetite in a cat over about eight. Very few conditions produce that pattern.
The Merck Veterinary Manual pet owner section covers the clinical picture in detail.
Why it is dangerous if left
Untreated hyperthyroidism does real damage, mostly through the cardiovascular system.
- High blood pressure, which damages kidneys, eyes and brain.
- Thickening of the heart muscle, which can progress to heart failure.
- Sudden blindness from retinal detachment caused by hypertension.
- Progressive muscle wasting.
- Worsening of any concurrent kidney disease.
Retinal detachment is the one that shocks owners. A cat can lose its sight abruptly from untreated hypertension, and it is frequently irreversible.
Diagnosis
A blood test measuring thyroid hormone usually confirms it, alongside a general panel to assess kidneys, liver and blood count.
Your vet will also check blood pressure and listen to the heart, since both are commonly affected and both change the treatment plan.
One complication is worth knowing: hyperthyroidism can mask kidney disease by increasing blood flow through the kidneys. Treating the thyroid sometimes reveals kidney disease that was already present.
That is not the treatment causing harm — it is the underlying picture becoming visible. Our guide to kidney disease in cats covers what that means.
The four treatment options
| Option | How it works | Upside | Downside |
|---|---|---|---|
| Daily medication | Blocks hormone production | Cheap to start, reversible | Lifelong, twice daily, side effects possible |
| Radioactive iodine | Destroys overactive tissue | Usually curative, single treatment | High upfront cost, isolation period |
| Surgery | Removes the gland | Often curative | Anaesthetic risk, less commonly used now |
| Prescription iodine-restricted diet | Limits hormone substrate | No pills | Cat must eat nothing else, ever |
Radioactive iodine has the best long-term outcomes and is usually curative, but the upfront cost is substantial and the cat stays at a licensed facility for a period afterwards.
Medication is the most common route, and it is a reasonable one — particularly for very old cats or where cost rules out iodine treatment.
The dietary option is the least reliable in practice, because it only works if the cat eats absolutely nothing else. In a multi-cat household or with any outdoor access, that is rarely achievable.
Living with daily medication
If you go the medication route, the practical question is whether you can reliably pill a cat twice a day for the rest of its life.
- Ask about transdermal gel applied to the ear, which suits cats that resist tablets.
- Ask about flavoured compounded formulations.
- Establish a fixed routine at the same times daily.
- Arrange backup — whoever covers holidays must be able to medicate.
- Expect regular bloodwork to adjust the dose.
Transdermal gel is genuinely useful and frequently not mentioned unless you ask. Our guide to giving a cat a pill safely covers technique for tablets.
What recovery looks like
Once hormone levels are controlled, most cats regain weight, settle behaviourally and improve in coat condition over weeks to months.
Appetite usually normalises, which occasionally worries owners who had become used to a ravenous cat. That is the treatment working.
Ongoing monitoring continues regardless of route — bloodwork, weight and blood pressure. Our guide to how often a cat should see the vet covers senior monitoring generally.
Why routine screening matters here
This is a condition where early detection changes outcomes substantially, because the cardiac and hypertensive damage accumulates over time.
A cat over seven having annual bloodwork will usually be picked up before any of that damage is established. A cat that only sees a vet when visibly unwell frequently will not.
Weighing monthly at home is the cheapest early detection available. Our guide to checking body condition covers how to track it meaningfully.
If your older cat is eating well and losing weight, book the appointment. That specific combination is worth taking seriously.
Blood pressure is part of the picture
Hypertension accompanies hyperthyroidism often enough that blood pressure should be checked at diagnosis and monitored afterwards.
Untreated high blood pressure damages the kidneys, the brain and, most visibly, the eyes. Sudden blindness from retinal detachment is the outcome owners find hardest, and it is frequently irreversible.
Ask specifically whether blood pressure has been measured. It is not always done as standard, and it changes both the urgency and the treatment plan.
Our guide to how often a cat should see the vet covers what senior monitoring should include.
The eye complication is the one owners find hardest, and it can be sudden. Our guide to cat eye problems covers why abrupt blindness is an emergency and sometimes reversible if caught quickly.
Frequently Asked Questions
What is the classic sign of hyperthyroidism in cats?
Weight loss despite a normal or increased appetite, in a cat over about eight. Very few conditions produce that combination, and it should always prompt a blood test.
Why is hyperthyroidism often missed?
Because early stages look like good health. An older cat that becomes livelier, eats enthusiastically and slims down reads as rejuvenated, when its metabolism is actually running dangerously fast.
What happens if hyperthyroidism is not treated?
High blood pressure damages kidneys, eyes and brain, the heart muscle thickens and can fail, and sudden irreversible blindness from retinal detachment can occur. Muscle wasting progresses throughout.
What are the treatment options for feline hyperthyroidism?
Daily medication, radioactive iodine, surgery, or a prescription iodine-restricted diet. Radioactive iodine is usually curative with the best long-term outcomes but has a high upfront cost.
Does treating hyperthyroidism cause kidney disease?
No, but it can reveal it. Hyperthyroidism increases blood flow through the kidneys and can mask existing kidney disease, which becomes visible once thyroid levels normalise.
Is there an alternative to pilling a hyperthyroid cat twice daily?
Yes — ask about transdermal gel applied to the ear, which suits cats that resist tablets, or flavoured compounded formulations. Neither is always offered unless you ask.