Hyperthyroidism is one of the most common endocrine disorders in older cats, affecting their metabolism, energy levels, and overall well-being. This condition arises when the thyroid gland—located in the neck—produces excessive amounts of thyroid hormones (T4 and T3), leading to a hypermetabolic state. While treatable, untreated hyperthyroidism can cause serious secondary complications, including heart disease, high blood pressure, muscle wasting, and kidney damage.

At Fort Mill Animal Hospital, we believe that informed pet parents are empowered pet parents. This guide breaks down everything you need to know about feline hyperthyroidism—from recognizing early warning signs to navigating treatment and long-term care.

Schedule an Appointment with Fort Mill Animal Hospital

What Causes Hyperthyroidism in Cats?

How the Thyroid Works—and What Goes Wrong

The thyroid gland plays a critical role in regulating metabolism through the production of two hormones: T3 (triiodothyronine) and T4 (thyroxine). In cats with hyperthyroidism, this gland becomes overactive—often due to a benign tumor known as a thyroid adenoma. In rare cases (less than 2% of cases), the underlying cause is thyroid carcinoma, a malignant form of cancer.

Read more about Feline Hyperthyroidism Guidelines – AAHA

Risk Factors

  • Age: Most affected cats are over 10 years old.
  • Dietary Factors: Some studies suggest certain canned food ingredients may increase risk.
  • Environmental Exposures: Chemicals such as PBDEs (flame retardants) have been linked to higher incidence.

Symptoms of Hyperthyroidism in Cats

Recognizing the early signs of hyperthyroidism can lead to faster diagnosis and better treatment outcomes.

Early Signs to Watch For

  • Increased appetite despite noticeable weight loss
  • Hyperactivity or restlessness
  • Increased thirst and urination
  • Occasional vomiting or diarrhea
  • Messy or greasy coat, especially around the back and flanks
  • Rapid heart rate or a newly detected heart murmur

Discover why Regular Veterinary Visits are Important

Advanced Symptoms and Potential Complications

When left untreated, the condition may progress to more serious health issues:

  • Hypertrophic cardiomyopathy (HCM): a thickening of the heart muscle
  • Systemic hypertension (high blood pressure): which can lead to retinal damage or blindness
  • Chronic kidney disease (CKD): masked by hyperthyroidism and unmasked post-treatment

Explore more about Heart Disease Diagnosis – Cummings

How Is Hyperthyroidism Diagnosed?

Diagnostic Steps Your Veterinarian Will Take

Diagnosis typically begins with a physical exam and is confirmed through laboratory testing:

  • Palpation of the neck for an enlarged thyroid gland
  • Blood tests, including:
    • Total T4: Elevated levels usually confirm the diagnosis
    • Free T4: Used when Total T4 results are inconclusive
  • Blood pressure measurement
  • Heart and kidney function tests to assess overall health and determine treatment suitability

Read more about Hyperthyroidism in Cats – Cornell Feline Health Center

Treatment Options for Hyperthyroidism in Cats

The best treatment for your cat will depend on their age, overall health, and presence of other conditions like kidney disease. Fortunately, there are multiple effective options.

1. Methimazole (Tapazole)

  • How it works: Blocks thyroid hormone production
  • Forms: Pill or transdermal gel (applied to the ear)
  • Pros: Non-invasive and relatively inexpensive
  • Cons: Requires daily treatment and regular blood monitoring; may cause side effects like vomiting or liver changes

2. Radioactive Iodine Therapy (I-131)

  • How it works: A single injection destroys overactive thyroid tissue
  • Pros: Curative in over 95% of cases
  • Cons: Requires hospitalization in a licensed facility and initial isolation post-treatment

3. Prescription Diets (Hill’s y/d)

  • How it works: Severely limits iodine intake to suppress hormone production
  • Pros: Non-invasive and suitable for cats that can’t tolerate medication
  • Cons: Requires strict feeding compliance—no treats, table food, or other pet’s food allowed

4. Surgery (Thyroidectomy)

  • How it works: Removes one or both affected thyroid glands
  • Pros: Potentially curative
  • Cons: Surgical risks are higher in older or medically compromised cats; may affect calcium regulation if parathyroid glands are disturbed

Long-Term Management and Monitoring

Why Ongoing Care Matters

Even after treatment, regular follow-up care is essential to monitor hormone levels and organ function. Your veterinarian may recommend:

  • Thyroid hormone testing every 3–6 months
  • Blood pressure checks to track cardiovascular risk
  • Kidney panels, especially after radioactive iodine therapy or medication

Discover why Preventive Testing is Key

Monitoring for Kidney Disease

Cats with hyperthyroidism often have increased kidney perfusion, which can mask early kidney dysfunction. Once thyroid hormone levels normalize, kidney disease may become apparent and need additional treatment.

Read more on Chronic Kidney Disease in Cats – Cornell Feline Health Center

Frequently Asked Questions

Can Hyperthyroidism Be Cured?

Yes. Radioactive iodine therapy and surgery offer curative solutions. Methimazole and diet therapy are effective for long-term management but are not curative.

Is the Condition Painful?

Hyperthyroidism isn’t directly painful, but its effects—like increased heart workload, muscle loss, and digestive upset—can lead to significant discomfort if untreated.

How Long Can My Cat Live with Hyperthyroidism?

With early diagnosis and appropriate treatment, many cats go on to live several more happy, healthy years.

Hyperthyroidism is both common and manageable. When detected early, it doesn’t have to shorten your cat’s life—or diminish their comfort. At Fort Mill Animal Hospital, our experienced team is here to support you through every step, from diagnosis and treatment to long-term management and follow-up.

If your cat has been eating more yet losing weight, or seems restless, vocal, or unkempt, don’t wait. These subtle signs could point to a larger issue.

Schedule an Appointment with Fort Mill Animal Hospital Today