The thyroid gland sits in the neck and produces hormones that regulate metabolism, the rate at which the body converts food into energy. In hyperthyroid cats, one or both thyroid lobes develop benign growths (adenomas) that overproduce thyroid hormone (T4). This excess hormone pushes the body into overdrive, causing the heart to beat faster, weight to drop despite a ravenous appetite, and organs to work harder than they should.

Hyperthyroidism affects approximately 10% of cats over the age of ten. The exact cause remains unknown, though researchers have investigated links to canned food diets, flame retardants in household environments, and iodine content in cat food. What we do know is that left untreated, the condition can lead to serious complications including heart failure, kidney damage, dangerously high blood pressure, and retinal detachment causing blindness.

The good news is that hyperthyroidism is one of the most treatable conditions in feline medicine. Multiple effective treatment options exist, and many cats return to a normal, healthy life once their thyroid levels are controlled.

Hyperthyroidism develops gradually, and many owners initially attribute the changes to normal aging. However, the constellation of symptoms, once you know what to look for, is quite distinctive.

The most common symptoms include:

• Weight loss despite increased appetite: Your cat may be eating more than ever but still losing weight. This is often the first sign owners notice.
• Increased thirst and urination: The elevated metabolism increases water turnover.
• Hyperactivity or restlessness: Some cats become unusually vocal, pace at night, or seem unable to settle.
• Vomiting and diarrhea: The rapid metabolism can cause gastrointestinal upset.
• Unkempt or greasy coat: Despite their best efforts, hyperthyroid cats often look disheveled.
• Rapid heart rate: Your vet may detect a heart rate above 240 beats per minute, sometimes with a heart murmur or gallop rhythm.

Less commonly, some cats present with what is called apathetic hyperthyroidism, showing lethargy, loss of appetite, and weakness instead of the classic hyperactive pattern. This variant is more common in cats with concurrent illnesses and can be harder to diagnose without bloodwork.

If your older cat is losing weight despite eating well, a simple blood test measuring T4 levels can confirm or rule out hyperthyroidism. This test is inexpensive and should be part of every senior cat's routine bloodwork.

Four main treatment approaches are available for feline hyperthyroidism, each with its own advantages and considerations.

Daily medication (methimazole/felimazole): This is the most common initial treatment. The medication blocks thyroid hormone production and is given as a pill, liquid, or transdermal gel applied to the ear. It is effective and relatively affordable, but it requires lifelong administration and regular blood monitoring. Some cats experience side effects including vomiting, appetite loss, or facial itching, though these are usually manageable.

Radioactive iodine therapy (I-131): Widely considered the gold standard, this treatment involves a single injection of radioactive iodine that selectively destroys overactive thyroid tissue while sparing normal tissue. It cures hyperthyroidism in approximately 95% of cats with a single treatment. The drawback is cost (typically $1,000 to $2,500) and a required hospitalization period of one to two weeks while radioactivity levels decrease.

Surgical thyroidectomy: Surgical removal of the affected thyroid lobe(s) can be curative. It is less commonly performed today due to the availability of I-131 therapy, but remains a good option for cats who are good surgical candidates. Risks include damage to the parathyroid glands and the need for general anesthesia.

Prescription diet (Hill's y/d): This iodine-restricted diet can control thyroid hormone production without medication. It must be the only food the cat eats, which can be challenging in multi-cat households. It is best suited for cats who cannot tolerate other treatments.

An important consideration with any treatment is the relationship between the thyroid and kidneys. High thyroid levels can mask underlying kidney disease by increasing blood flow to the kidneys. When thyroid levels are normalized, previously hidden kidney problems may become apparent. Your vet will monitor kidney values closely during the first few months of treatment.

With appropriate treatment, most hyperthyroid cats have an excellent prognosis. Cats treated with radioactive iodine often live as long as cats without thyroid disease, and those managed on medication can do very well for years with regular monitoring.

Ongoing veterinary care is essential regardless of which treatment you choose. Cats on methimazole need bloodwork every three to six months to ensure thyroid levels remain in the normal range and to check for side effects. Even cats cured by I-131 or surgery benefit from regular senior wellness exams to catch other age-related conditions early.

The heart changes caused by hyperthyroidism, known as thyrotoxic cardiomyopathy, are often reversible once thyroid levels normalize. Many cats with thickened heart walls and rapid heart rates see significant improvement within a few months of treatment. However, cats who were hyperthyroid for a long time before diagnosis may have some permanent cardiac changes.

If your cat has been diagnosed with hyperthyroidism, take heart in knowing that this is a condition with genuinely good treatment outcomes. With your commitment to their care and your vet's guidance, your cat can continue to enjoy a comfortable, happy life.