What Is 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.
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.