Overactive thyroid (hyperthyroidism) in the cat

What is hyperthyroidism in cats?

Hyperthyroidism occurs when the thyroid gland produces too much thyroid hormone. In cats, the cause is almost always a benign thyroid tumour (adenoma). The condition mainly affects cats older than 10 years and is the most common hormonal disorder in this age group [1][4].

Untreated, hyperthyroidism leads to severe heart problems and eventually death. Early recognition and treatment are therefore essential.

Proven treatment options

There are four recognised treatments with strong scientific evidence [1]:

Radioactive iodine

Radioactive iodine selectively destroys the overactive thyroid tissue. It is the preferred treatment due to its high success rate and permanent effect after one treatment [1]. Because the cat is temporarily radioactive, admission to a specialised clinic for several days is mandatory.

Medication (methimazole)

Methimazole inhibits hormone production and acts quickly. The product is available as a tablet or as a gel applied to the inner ear flap. Adverse effects occur in 10–20% of cats; regular blood tests are necessary [1][4].

Low-iodine diet

A strictly low-iodine therapeutic diet (exclusively Hill's y/d) lowers thyroid values in three-quarters of cats [2][3]. The required discipline is considerable: any other food source — including treats, prey or food from other pets — negates the effect. Nearly three in ten cats refuse the diet due to palatability issues [3].

Kidney disease as a hidden complication

Hyperthyroidism increases blood flow to the kidneys and thereby masks existing kidney failure. After successful treatment, chronic kidney disease may become apparent. Your veterinary surgeon will therefore monitor kidney function closely, particularly in the first weeks after treatment [1][4].

Natural and supplementary products: no evidence

There is no scientific evidence that herbal preparations, homeopathy or other alternative products effectively treat or manage hyperthyroidism in cats [4]. Using them as a replacement for proven treatments significantly increases the risk of heart disease, high blood pressure and death.

Supplementary support — such as stress reduction, a quiet environment and a palatable therapeutic diet — can promote welfare, but never replaces the necessary veterinary treatment.

Supplementary ≠ replacement

Hyperthyroidism is a progressive condition where delaying treatment causes direct physical damage. Always discuss all options — including dietary approaches — with your veterinary surgeon, who can also take kidney function and the heart into account when formulating the treatment plan [1].