Hyperthyroidism in Cats — Cat Glossary

Q: How is hyperthyroidism treated in cats?

Four main options exist: (1) Methimazole / carbimazole — oral medication that blocks thyroid hormone synthesis; effective but requires lifelong daily dosing and regular blood monitoring; (2) Radioactive iodine (I-131) — considered the gold standard; single treatment, high cure rate (~95%), no long-term medication; requires a brief hospital stay for radiation safety; (3) Surgical thyroidectomy — effective but carries anaesthetic risk in older cats; (4) Dietary iodine restriction (Hills y/d) — manages but does not cure; requires strict dietary compliance. The AAHA and most feline specialists recommend I-131 as first-line where accessible.

Hyperthyroidism (feline)

Hyperthyroidism — overproduction of thyroid hormone (T3/T4) by an enlarged thyroid gland, almost always due to a benign adenoma. The most common endocrine disorder in cats over 10. Causes weight loss despite increased appetite, elevated heart rate, and hyperactivity. Highly treatable with medication, radioactive iodine, or surgery.

Prevalence and why it matters

Feline hyperthyroidism was first described as a clinical entity in the 1970s and is now recognised as affecting approximately 10% of cats over 10 years of age, making it the most common endocrine disorder in older cats. It is also one of the most treatable — with early diagnosis, the prognosis is excellent and most cats return to full health after appropriate treatment.

The challenge is that signs develop gradually and are easy to attribute to normal ageing. By the time a cat is visibly unwell, secondary organ changes (cardiac hypertrophy, hypertension, kidney stress) may already be present. Annual senior wellness bloodwork is the most reliable way to detect it early.

Classic signs

Weight loss

Despite a dramatically increased appetite (polyphagia). Muscle wasting is common. A cat eating more than ever while losing weight is the textbook presentation.

Increased thirst / urination

Polyuria and polydipsia — often mistaken for diabetes or kidney disease. Blood and urine tests distinguish these.

Hyperactivity / restlessness

An older cat that suddenly seems agitated, vocal at night, or unable to settle. Elevated thyroid hormone acts as a metabolic stimulant.

Heart changes

Elevated heart rate, gallop rhythm, or murmur detectable on examination. Long-term, untreated hyperthyroidism leads to hypertrophic cardiomyopathy.

Unkempt coat

Reduced grooming is a general sign of illness; in hyperthyroid cats the coat often becomes greasy, matted, or dull.

Vomiting / diarrhoea

GI signs are common. Increased gut motility from elevated thyroid hormone causes loose, frequent stools and intermittent vomiting in many affected cats.

Treatment options

All four main treatment options are effective; choice depends on the cat's overall health, owner preference, and what is accessible in the region.

Methimazole / carbimazole

Daily oral medication (or transdermal gel applied to the ear). Effective at controlling T4 levels but does not cure — lifelong treatment required. Allows renal function to be monitored before committing to permanent treatment. Side effects (facial excoriation, blood count changes) occur in a minority.

Radioactive iodine (I-131)

Single injection of radioactive iodine selectively destroys overactive thyroid tissue. Cure rate approximately 95%. Considered gold standard by most feline specialists. Requires a few days in hospital (for radiation safety) but no ongoing medication. Available at specialist centres.

Surgical thyroidectomy

Removal of the affected thyroid lobe(s). Effective and potentially curative. Carries anaesthetic risk in older, potentially cardiac-compromised cats; typically used when I-131 is unavailable or the cat cannot be medically stabilised.

Iodine-restricted diet (Hills y/d)

Restricts dietary iodine, limiting thyroid hormone synthesis. Manages but does not cure. Requires strict dietary compliance — even small amounts of other food can reduce efficacy. Suitable for cats unable to tolerate other treatments.

The kidney complication. Hyperthyroidism increases renal blood flow and can mask underlying chronic kidney disease. When thyroid levels are brought down, kidney function may apparently worsen — reflecting pre-existing CKD that was hidden. A methimazole trial before permanent treatment lets vets assess whether a cat\'s kidney function is acceptable at normal thyroid levels. This is standard practice in specialist feline medicine; see IRIS guidelines.

For full diagnostic and management guidance, see International Cat Care's hyperthyroidism resource and the Merck Veterinary Manual chapter on feline hyperthyroidism.

Frequently asked questions

What causes hyperthyroidism in cats?

In over 98% of cases, feline hyperthyroidism is caused by a benign tumour (adenoma) of one or both thyroid lobes causing excess thyroid hormone production. Malignant thyroid carcinoma is rare. The exact cause of thyroid adenoma development is not fully understood, but dietary iodine levels, environmental factors (including certain fire retardant chemicals), and genetic predisposition have been proposed. The epidemiology is reviewed in Merck Veterinary Manual's hyperthyroidism chapter.

What are the signs of hyperthyroidism in cats?

Classic signs: weight loss despite increased appetite (sometimes dramatically increased — polyphagia), increased thirst and urination, hyperactivity or restlessness, vomiting or diarrhoea, unkempt coat, and a palpable thyroid lump in the neck. Heart changes (gallop rhythm, murmur, hypertension) are common secondary effects. Because many owners attribute weight loss in older cats to "just getting old," hyperthyroidism is often caught late. See the International Cat Care hyperthyroidism guide for a full symptom list.

How is hyperthyroidism treated in cats?

Four main options exist: (1) Methimazole / carbimazole — oral medication that blocks thyroid hormone synthesis; effective but requires lifelong daily dosing and regular blood monitoring; (2) Radioactive iodine (I-131) — considered the gold standard; single treatment, high cure rate (~95%), no long-term medication; requires a brief hospital stay for radiation safety; (3) Surgical thyroidectomy — effective but carries anaesthetic risk in older cats; (4) Dietary iodine restriction (Hills y/d) — manages but does not cure; requires strict dietary compliance. The AAHA and most feline specialists recommend I-131 as first-line where accessible.

Will treating hyperthyroidism affect my cat's kidneys?

Yes — this is an important and well-documented phenomenon. Hyperthyroidism increases blood flow to the kidneys, which can mask pre-existing chronic kidney disease (CKD). When thyroid hormone levels are normalised, the kidney's true function level may be revealed, sometimes showing CKD that was invisible before treatment. This is why pre-treatment kidney assessment and a methimazole trial period (before permanent treatment) is recommended to identify cats whose renal function deteriorates significantly when thyroid levels are controlled. The IRIS CKD staging guidelines are used in this context.

How often should older cats be screened for hyperthyroidism?

Annual blood and urine tests for cats over 10 years are recommended by most feline veterinary organisations, including International Cat Care and the AAHA Feline Life Stage Guidelines. A basic blood panel including T4 (thyroxine) is the primary screen. Routine senior wellness visits allow early detection when treatment options are broadest and secondary organ damage (heart, kidneys) is minimal.