Hyperthyroidism in Cats: What Is It and What Are the Signs?

Hyperthyroidism in cats is the most common hormonal disorder affecting middle-aged and older cats, and it’s one of the conditions the team at Cornerstone Veterinary Clinic in Piqua, OH diagnoses and manages regularly. If your cat has been losing weight despite a good appetite, becoming increasingly vocal or restless, or drinking and urinating more than usual, hyperthyroidism in cats may be the cause. The good news: with proper diagnosis and treatment, most cats with hyperthyroidism do very well.

 

 

What Is Hyperthyroidism in Cats?

Hyperthyroidism in cats occurs when the thyroid gland, a small, butterfly-shaped gland in the neck, produces excessive amounts of thyroid hormones (T3 and T4). These hormones regulate metabolism, heart rate, body temperature, and many other vital functions. When thyroid hormone levels are too high, essentially every organ system in the body is affected.

The vast majority of feline hyperthyroidism cases are caused by a benign (non-cancerous) overgrowth of thyroid tissue called adenomatous hyperplasia or thyroid adenoma. In rare cases (fewer than 3%) thyroid carcinoma (cancer) is the cause. Feline hyperthyroidism is most commonly diagnosed in cats over 10 years of age, though it can occur in younger cats as well.

What Causes Hyperthyroidism in Cats?

The exact cause of feline hyperthyroidism is not fully understood, but researchers have identified several potential contributing factors. These include dietary iodine content, environmental exposure to chemicals such as flame retardants found in furniture and carpeting, and possible associations with certain commercially canned cat foods. Genetics may also play a role. Research into the root causes of cat hyperthyroidism is ongoing, and no single definitive cause has been identified.

Signs of Hyperthyroidism in Cats

The signs of hyperthyroidism in cats develop gradually, which is one reason they’re easy to miss or attribute to normal aging. Knowing what to look for allows you to get your cat evaluated sooner, which leads to better outcomes.

Common signs of hyperthyroidism in cats include:

• Weight loss despite increased appetite: This is the most characteristic sign of feline hyperthyroidism. Cats eat more yet continue to lose muscle and body condition.
• Increased thirst and urination: Excess thyroid hormone increases metabolic rate and affects kidney function, leading to greater fluid intake and output.
• Hyperactivity and restlessness: Cats with hyperthyroidism may seem unusually active, anxious, or unable to settle, behaviors that can be misread as energy or playfulness.
• Vomiting and diarrhea: Gastrointestinal upset is common as the digestive tract is affected by elevated thyroid hormone levels.
• Increased vocalization: Many hyperthyroid cats become significantly more vocal, especially at night.
• Poor coat condition: A greasy, matted, or unkempt coat often develops as cats with hyperthyroidism groom less effectively.
• Rapid heart rate: Elevated thyroid hormones directly stimulate the cardiovascular system, causing an increased heart rate and sometimes heart murmurs or cardiac changes.

Can Hyperthyroidism in Cats Affect the Heart?

Yes. Untreated feline hyperthyroidism causes significant cardiovascular strain. The heart works harder to keep up with the increased metabolic demands, which over time leads to hypertrophic cardiomyopathy, a thickening of the heart muscle. This is why early diagnosis and treatment of cat hyperthyroidism matters so much: catching it before significant cardiac changes occur leads to better outcomes overall.

How Is Hyperthyroidism in Cats Diagnosed?

Diagnosing hyperthyroidism in cats is generally straightforward. At Cornerstone Veterinary Clinic, our veterinarians begin with a physical examination, during which we often palpate an enlarged thyroid gland in the neck area. Blood testing confirms the diagnosis.

Blood Tests for Feline Hyperthyroidism

A measurement of total thyroxine (T4) in the blood is the standard screening test for cat hyperthyroidism. In most cases, an elevated T4 level confirms the diagnosis. In cats with early or mild hyperthyroidism, T4 may be in the high-normal range; in these cases, additional testing such as free T4 measurement or a repeat test in a few weeks may be needed to confirm the diagnosis.

The Relationship Between Hyperthyroidism and Kidney Disease

One important diagnostic consideration is the relationship between hyperthyroidism and chronic kidney disease in cats. Hyperthyroidism can artificially increase blood flow to the kidneys, masking underlying kidney dysfunction. Once hyperthyroidism is treated, kidney disease that was previously hidden may become apparent. Our veterinarians at Cornerstone Veterinary Clinic discuss this possibility with every cat owner before beginning treatment, so there are no surprises.

Treatment Options for Hyperthyroidism in Cats

There are four main treatment options for feline hyperthyroidism, each with distinct advantages and considerations. The right choice depends on your cat’s overall health, your lifestyle, and your preferences. Our veterinarians take time to explain each option thoroughly so you can make an informed decision.

• Methimazole (oral medication or transdermal gel): This antithyroid medication is the most commonly used medical treatment for cat hyperthyroidism. It controls, but does not cure, the condition and must be given for the rest of the cat’s life. Available in pill form or as a gel applied to the inner ear flap.
• Radioactive iodine therapy (I-131): Considered the gold standard treatment for feline hyperthyroidism, radioactive iodine is a one-time treatment that cures hyperthyroidism in the vast majority of cats. It requires a short hospitalization at a specialty facility until radiation levels are safe.
• Surgical thyroidectomy: Surgical removal of the affected thyroid tissue can cure hyperthyroidism in cats but is performed less commonly due to anesthetic risks in older cats and the availability of other effective treatments.
• Prescription iodine-restricted diet: A special therapeutic diet that limits dietary iodine can control thyroid hormone production in cats that eat exclusively this diet. Convenient for some owners but requires strict dietary compliance.

Life With a Hyperthyroid Cat: What to Expect

With the right treatment, most cats with hyperthyroidism regain weight, become calmer, and return to a much better quality of life. Regular monitoring is an important part of managing feline hyperthyroidism long-term. For cats on medical therapy, T4 levels and kidney values are rechecked two to four weeks after starting treatment, then every three to six months to ensure the dose remains appropriate.

At Cornerstone Veterinary Clinic, we partner with cat owners to develop a monitoring schedule that fits their cat’s specific situation and treatment choice. Our goal is to help your cat feel their best for as long as possible.

Is Your Cat Showing Signs of Hyperthyroidism?

If your cat is over 10 years old and showing any of the signs described above, especially weight loss with increased appetite, restlessness, or increased thirst, it’s time to schedule a veterinary visit. Feline hyperthyroidism is very treatable, and the sooner it’s diagnosed, the better.

At Cornerstone Veterinary Clinic in Piqua, OH, we diagnose and manage hyperthyroidism in cats with a compassionate, individualized approach. Call us today at (937) 773-8060 or book an appointment online and give your cat the care they deserve.