Cat hyperthyroidism: symptoms and management

Hyperthyroidism is the most common hormonal disease in cats and one of the most common diseases overall in cats over 10 years old. An overactive thyroid gland produces too much thyroid hormone, speeding up virtually every body process.

Classic signs — the "paradox cat"

The characteristic picture is a cat that:
- Eats ravenously but loses weight (the hallmark symptom)
- Is hyperactive, restless, or vocal at night
- Drinks and urinates more than usual
- Has a poor, unkempt coat despite grooming
- Occasionally vomits or has loose stools
- Has a rapid or irregular heart rate (audible as a gallop rhythm)

Why does this happen?

In the vast majority of cases (>95%), hyperthyroidism is caused by a benign growth (adenoma) on one or both thyroid glands that produces hormone autonomously. Thyroid cancer is very rare in cats.

Complications if untreated

Chronically elevated thyroid hormone puts enormous strain on the heart, causing hypertrophic cardiomyopathy (thickened heart walls) and hypertension. It also masks kidney disease — treating hyperthyroidism can unmask pre-existing kidney problems. This is why kidney function is always tested before and after starting treatment.

Diagnosis

A simple blood test measuring total T4 is usually sufficient. Some cats with early disease have borderline results and need repeated testing or more specific assays (free T4, T3 suppression test).

Treatment options

1. Daily medication (methimazole/carbimazole) — most common first-line treatment. Tablet, pill pocket, or transdermal gel applied to the inner ear flap. Controls but does not cure.
2. Radioactive iodine (I-131) — curative in 95%+ of cases with a single injection. Requires a hospital stay of 1–3 weeks. Not available everywhere.
3. Surgical removal of the thyroid gland (thyroidectomy) — highly effective; requires a skilled surgeon and anaesthesia.
4. Prescription low-iodine diet (Hill''s y/d) — controls hyperthyroidism through diet alone. Strict — must be the only food.