Hyperthyroidism in Cats: Signs, Diagnosis & Treatment Options

Hyperthyroidism is the most common endocrine disease in older cats, affecting about 10% of cats over age 10. The classic sign: a cat that is eating voraciously but steadily losing weight.

What Is Feline Hyperthyroidism?

A benign tumor (adenoma) on the thyroid gland overproduces thyroid hormone (T4). This hormone controls metabolism — too much of it puts the body into overdrive. The heart races, calories burn too fast, and despite a ravenous appetite, the cat loses weight. Left untreated, it leads to heart failure (thyrotoxic cardiomyopathy), high blood pressure, and eventual death.

Classic Signs

Weight loss despite increased appetite (the hallmark sign), increased thirst and urination, vomiting, diarrhea, hyperactivity and restlessness, poor coat quality, rapid heart rate, and increased vocalization (especially at night). Some cats also develop hypertension which can damage the eyes and kidneys.

Diagnosis

A simple blood test measuring total T4. Most hyperthyroid cats have clearly elevated T4. If the value is borderline, free T4 by equilibrium dialysis provides more accuracy. Blood pressure measurement and kidney values are checked at the same time because hyperthyroidism can mask underlying kidney disease.

Treatment Options

Radioactive Iodine (I-131): Gold standard. A single injection. The radioactive iodine selectively destroys the overactive thyroid tissue while sparing normal tissue and parathyroid glands. 95-98% cure rate. One-time cost $1,200-$2,500. Cat must be hospitalized 3-7 days.

Methimazole (Felimazole): Daily oral or transdermal medication. Does NOT cure the disease — controls it. Requires lifelong dosing and monitoring. 20% of cats experience side effects. Cheaper upfront but ongoing costs add up.

Prescription Diet (Hill's y/d): Iodine-restricted diet. Must be fed EXCLUSIVELY — absolutely no other food, treats, or flavored medications. Normalizes T4 in most cats. Does not address the tumor.

Which Treatment Is Best?

For a young senior cat (8-12) with no other major health problems: I-131 is the best long-term choice. For an older cat with significant kidney disease: methimazole or y/d diet may be safer because you can stop treatment if it worsens kidney function. Discuss with your vet.

Living with a Hyperthyroid Cat

With treatment, cats return to normal weight, energy, and coat quality. Lifespan with treatment is normal. Without treatment, progressive wasting and heart disease lead to death within months to 2 years.