🐾 Quick answer: Cat hyperthyroidism causes weight loss despite good appetite, hyperactivity and vomiting in older cats. Learn the signs, diagnosis, and all treatment options from daily medication to curative radioactive iodine.
Hyperthyroidism is the most common hormonal disorder in cats — particularly those over 10 years old. The good news is that it’s one of the most treatable conditions in feline medicine, with several effective options available.
What Is Hyperthyroidism?
The thyroid gland produces hormones that regulate metabolism. In hyperthyroidism, a benign tumour (adenoma) on one or both thyroid glands causes overproduction of thyroid hormone, dramatically speeding up the cat’s metabolism.
Signs of Hyperthyroidism
- Weight loss despite increased appetite — the classic presentation
- Increased thirst and urination
- Hyperactivity, restlessness — especially at night
- Vomiting and diarrhoea
- Poor, unkempt coat
- Rapid heart rate
- Vocalising at night
- Increased faecal frequency
Diagnosis
A simple blood test measuring T4 (thyroxine) levels confirms the diagnosis in most cases. Your vet will also check blood pressure, kidney function, and heart health — all commonly affected by hyperthyroidism.
Treatment Options
Medication (Felimazole/methimazole) — daily tablets or transdermal gel applied to the ear flap. Effective and affordable. Requires lifelong administration and regular blood monitoring. Some cats develop side effects.
Radioactive iodine (I-131) — the gold standard treatment. A single injection cures 95% of cats permanently. Requires a short hospital stay (for radiation safety reasons). Not available everywhere but highly effective.
Surgical thyroidectomy — removal of the affected thyroid tissue. Very effective but carries anaesthetic risk in older cats.
Prescription iodine-restricted diet (Hill’s y/d) — controls hyperthyroidism without medication when fed exclusively. Requires strict compliance — no other food.
The Kidney Complication
Treating hyperthyroidism can unmask underlying kidney disease — hyperthyroidism artificially maintains kidney blood flow, and when treated, borderline kidney disease may become apparent. Your vet will monitor kidney function carefully after starting treatment.
- Weight loss with increased appetite in a cat over 8
- Any senior cat with increased thirst, restlessness, or vomiting
- Annual senior blood tests — catch it early
Hyperthyroidism is very treatable. Most cats respond well to treatment and regain weight, energy, and quality of life quickly. Early diagnosis and prompt treatment give the best outcomes.
The PawPulse Team
Researched using current veterinary guidelines. Always consult your vet for medical advice about your pet.