Hyperthyroidism
Hyperthyroidism is a condition in which the thyroid gland produces excessive thyroid hormone, causing the metabolic rate to accelerate beyond normal. In cats, it is the most common endocrine (hormonal) disorder and primarily affects middle-aged to older cats. The classic presentation — a cat that eats voraciously but keeps losing weight — is one of its most recognisable features.
Most common endocrine disorder in cats: Hyperthyroidism.
Understanding Hyperthyroidism
The thyroid gland, located in the neck, regulates metabolism. In hyperthyroid cats, a benign tumour (adenoma) on one or both thyroid lobes causes chronic overproduction of thyroid hormone. The resulting metabolic overdrive affects nearly every organ system: the heart races and can develop dangerous thickening (hypertrophic cardiomyopathy), blood pressure rises, the kidneys can be masked from apparent failure, and the gastrointestinal system accelerates, causing vomiting and diarrhoea.
The classic signs in cats are: dramatic weight loss despite a ravenous appetite; increased activity and restlessness; rapid heart rate; vomiting; and unkempt coat. Some cats vocalize excessively, particularly at night, and may be hyperactive rather than lethargic. A diagnosis is confirmed through a blood test measuring T4 (total thyroxine) levels, which is typically significantly elevated.
Treatment options are highly effective. Radioactive iodine therapy (I-131) is the gold standard — a single injection permanently destroys abnormal thyroid tissue with minimal side effects and achieves a cure rate over 95%. Daily oral medication (methimazole) controls the condition without curing it and requires lifelong administration. A prescription iodine-restricted diet (Hill's y/d) can also control thyroid hormone levels. Surgery to remove the thyroid gland is a fourth option, less commonly used.
Key Facts
- Most common endocrine disorder in cats
- Hyperthyroidism
- Classic presentation
- Weight loss + increased appetite
- Gold standard treatment
- Radioactive iodine (I-131) — 95%+ cure rate
- Also affects
- Heart, blood pressure, kidneys
Frequently Asked Questions
My older cat is eating more than ever but losing weight — could this be hyperthyroidism?
This is the most classic presentation of feline hyperthyroidism. Weight loss despite normal or increased appetite in a cat over 7–8 years old should always prompt a thyroid test at the vet. This is a straightforward blood test, and the condition is very treatable once confirmed.
Does treating hyperthyroidism affect the kidneys?
Yes — this is an important consideration. Elevated thyroid hormone artificially increases blood flow to the kidneys, masking underlying kidney disease. When hyperthyroidism is treated, kidney function may appear to 'worsen' as the masking effect is removed. This is why many vets monitor kidney values carefully after starting treatment.
Is hyperthyroidism in dogs the same as in cats?
No. Dogs rarely develop hyperthyroidism and when they do, it is most commonly associated with thyroid cancer rather than the benign adenomas typical of cats. Hypothyroidism (underactive thyroid) is actually the common thyroid disorder in dogs — the opposite condition.
Medical disclaimer: This glossary entry is for educational purposes only and does not constitute veterinary advice. Always consult a licensed veterinarian for diagnosis and treatment of your pet's health conditions.