Quick orientation
Hyperthyroidism is the most common hormonal disease in older UK cats — affecting roughly 1 in 10 cats over the age of ten. The signs are easy to dismiss as a normal active older cat (eating well, energetic, perhaps a bit thin). Diagnosis is straightforward and four treatment options exist, each with different trade-offs. With appropriate treatment most affected cats live normal lifespans with good quality of life.
What hyperthyroidism actually is
The thyroid gland (a small gland in the neck) regulates metabolism. In hyperthyroidism, the gland becomes overactive and produces excess thyroid hormone, putting the body into a state of "metabolic overdrive." The result is a cat burning energy faster than they can take it in, with knock-on effects on the heart, kidneys, blood pressure, and behaviour.
The underlying cause is usually a benign change to the thyroid gland (adenomatous hyperplasia or a benign tumour called an adenoma). Malignant thyroid cancer is much rarer in cats but does occur. The trigger isn't fully understood; possible contributors include diet, environmental chemicals, and ageing thyroid tissue.
Hyperthyroidism is almost exclusively a disease of cats over eight, most commonly diagnosed in cats aged 12–15. Younger cats developing thyroid problems is uncommon enough to warrant referral.
UK prevalence
Most common age at diagnosis
Treatment options available
Lifespan with good treatment
Warning signs to watch for
The paradox of hyperthyroidism is that it can look like a vigorous, healthy older cat. The clues are subtle:
- Weight loss despite a good (often increased) appetite — the most consistent sign. An older cat that's eating enthusiastically but losing weight needs investigation.
- Increased thirst and urination
- Unkempt or matted coat, particularly along the spine — the cat may have stopped grooming as effectively
- Increased activity, restlessness, or vocalisation, particularly at night
- Vomiting or diarrhoea, sometimes intermittent
- A faster heart rate (often the first thing the vet notices)
- A faster respiratory rate
- Behaviour changes — a previously placid cat becoming restless, demanding, or aggressive
- A small palpable lump in the neck (the enlarged thyroid — often felt by the vet during examination)
A single sign in isolation isn't conclusive. The combination of weight loss + good appetite in a cat over ten is the strongest pointer.
What diagnosis involves
A typical UK workup is fast and inexpensive:
- Clinical examination — weight, body condition score, heart rate, palpation of the neck for an enlarged thyroid.
- Blood test for total T4 (thyroid hormone) — the standard screening test. A high T4 in a cat with consistent clinical signs confirms the diagnosis. In about 10% of cases the T4 is in the upper-normal range — these cats need follow-up testing (free T4, repeat T4 in 4–6 weeks).
- Wider screening — a full blood profile and urinalysis, because hyperthyroidism commonly coexists with chronic kidney disease, and treating one can unmask the other.
- Blood pressure measurement — hypertension is common in hyperthyroid cats and worth checking.
- Imaging is only needed in atypical cases or when planning surgery.
Many UK general practices include a T4 test in their senior cat wellness panels, which is one reason for catching cases early.
The four treatment options
Unlike most chronic conditions, hyperthyroidism has multiple genuinely different treatment paths. The right one depends on the individual cat, owner preferences, finances, and concurrent conditions.
1. Daily medication (methimazole / carbimazole)
A tablet (or transdermal gel applied to the ear) given once or twice daily for life. Controls the disease but doesn't cure it. Requires regular blood monitoring (every 3–6 months) to adjust the dose. Reversible — stopping medication brings the disease back, which is useful for trialling treatment in cats with concurrent kidney disease.
Pros: Lowest upfront cost, most flexible, reversible.
Cons: Daily medication compliance is challenging in some cats; lifetime treatment with ongoing monitoring; doesn't cure.
2. Iodine-restricted prescription diet (Hill's y/d)
A dry or wet prescription food formulated to provide just enough iodine for the body's needs. Suitable for cats willing to eat one specific diet exclusively (no treats, no other foods, no hunting). Effective when followed strictly.
Pros: No daily pilling; no surgery; reversible.
Cons: Strict diet compliance is hard in multi-cat households or with picky cats; doesn't cure.
3. Surgical thyroidectomy
Surgical removal of the affected thyroid lobe(s). Curative in most cases. Requires general anaesthesia and carries the usual surgical risks plus a small risk of damage to the parathyroid glands (which manage calcium).
Pros: Curative; one-off procedure.
Cons: Anaesthetic risk in older cats; not always available locally; risk of recurrence in remaining thyroid tissue.
4. Radioactive iodine therapy (I-131)
A single injection of radioactive iodine that is selectively taken up by overactive thyroid tissue and destroys it. Curative in 95%+ of cases. Requires hospitalisation at a licensed facility (usually 1–2 weeks of isolation). Only available at a small number of UK referral centres.
Pros: Curative; no anaesthetic; addresses all overactive tissue including ectopic.
Cons: Highest upfront cost; isolation period; geographic access (referral usually required); not appropriate for cats with other major health concerns.
The kidney consideration
Hyperthyroidism increases blood flow through the kidneys, which can mask early chronic kidney disease (CKD). Treating the hyperthyroidism normalises kidney function tests, sometimes revealing kidney disease that was hidden. This is why many vets start with reversible treatment (medication or diet) for several weeks before committing to surgery or radioactive iodine — to confirm the cat tolerates a return to normal thyroid function without precipitating kidney disease.
Typical UK costs in 2026
- Initial consultation + T4 + senior screen: £120–£250
- Daily medication (varies by dose): £25–£60/month
- Quarterly monitoring bloods: £80–£150 per visit
- Prescription diet (Hill's y/d): roughly £60–£100/month
- Surgical thyroidectomy (one-off): £800–£1,800
- Radioactive iodine (one-off, including hospitalisation): £1,800–£3,500
Most lifetime pet insurance policies cover hyperthyroidism. Check whether your policy specifically covers radioactive iodine treatment (some have caps that may not cover the full cost) and whether ongoing medication and monitoring count toward the annual limit.
Choosing a vet for older cat care
Hyperthyroidism management is a long-term relationship. Worth looking for:
- A practice that takes feline-specific care seriously — ideally with ISFM Cat Friendly Clinic accreditation
- Routine senior cat wellness panels (annual or twice-yearly bloods including T4)
- Comfort with the various treatment options — a vet who recommends the same approach for every cat regardless of circumstances may be defaulting rather than tailoring
- For radioactive iodine specifically, awareness of and good relationships with the relevant UK referral centres
Our choosing a vet guide covers the broader framework, and the cat stress at the vet guide is particularly relevant for the regular monitoring visits hyperthyroid cats need.
Frequently asked questions
Find a vet for senior cat care
Older cat care is a long-term relationship with a practice you trust. The FetchRated directory lists UK veterinary practices with verified reviews — look for ISFM Cat Friendly Clinic accreditation in particular.
