Key Fact: Over 98% of feline hyperthyroidism cases are caused by benign tumors. Malignant thyroid carcinoma is exceptionally rare.

Known Risk Factors

• Age: The average age of onset is 12-13 years. It is rare in cats under 8.
• Diet: Some studies suggest a potential link to fish-based, high-iodine diets and certain canned foods.
• Environment: Exposure to environmental chemicals like flame retardants (PBDEs) may be a contributing factor.
• Breed: No strong breed predisposition exists, though Siamese and Himalayan cats may have a slightly elevated risk.

Recognizing the Symptoms of Hyperthyroidism

Classic Clinical Signs

1. Weight Loss Despite Increased Appetite

• The most common and telling sign.
• Ravenous hunger paired with steady weight loss.
• May manifest as food-stealing, constant begging, or "always hungry" behavior.

2. Hyperactivity & Restlessness

• Inability to settle, pacing, or increased vocalization (especially at night).
• Excessive grooming.

3. Gastrointestinal Issues

• Increased thirst (polydipsia) and urination (polyuria).
• Vomiting, diarrhea, or increased stool volume.

Other Common Signs

• Behavioral Changes: Irritability, aggression, anxiety, or increased nighttime activity.
• Physical Changes: Unkempt, greasy, or matted fur; rapid claw growth; noticeable muscle wasting.
• Cardiac Signs: Rapid heart rate (tachycardia), heart murmur, or breathing difficulties if heart failure develops.

Symptom Progression

If untreated, the disease typically advances through stages:

1. Early Stage: Subtle weight loss and a slightly increased appetite.
2. Moderate Stage: Obvious weight loss, clear hyperactivity, and gastrointestinal signs.
3. Advanced Stage: Severe muscle wasting, cardiac complications, and hypertension.
4. Crisis Stage: Risk of heart failure, sudden blindness from hypertension, and death.

The Diagnostic Process

1. Physical Examination

Your veterinarian will check for:

• Weight loss and poor body condition.
• A palpable "goiter" or nodule in the neck (present in ~90% of cases).
• Rapid heart rate and possible heart murmurs.
• Poor coat quality.

2. Essential Blood Testing

• Total Thyroxine (T4): The primary screening test. An elevated level confirms diagnosis in most cases.
• Additional Thyroid Tests: If T4 is borderline, your vet may recommend a Free T4 by equilibrium dialysis or a T3 suppression test for greater sensitivity.

3. Additional Recommended Tests

A comprehensive workup is crucial to assess overall health and plan safe treatment:

• Complete Blood Count (CBC) & Chemistry Panel: Checks organ function, kidney values, and liver enzymes (often elevated).
• Urinalysis: Assesses kidney concentrating ability.
• Blood Pressure Measurement: Hypertension is a common and dangerous complication.
• Imaging: Chest X-rays or an echocardiogram may be needed to evaluate heart size and function.

Screening for Concurrent Diseases

Hyperthyroidism can mask or coexist with other conditions common in senior cats:

• Kidney Disease: Treatment can unmask underlying kidney failure; assessing kidney function beforehand is critical.
• Heart Disease: The increased metabolic rate puts significant stress on the heart.
• Hypertension: High blood pressure can lead to blindness, stroke, or kidney damage.

Treatment Options Compared

Cost Breakdown (2026 Estimates)

Radioactive Iodine (I-131)

Long-Term Medication (Annual Cost)

Long-Term Management & Monitoring

Regardless of the treatment chosen, ongoing care is essential:

• Regular Blood Work: Monitor T4 levels, kidney function, and overall health.
• Blood Pressure Checks: Especially important if hypertension was present.
• Watch for Hypothyroidism: A small percentage of cats become hypothyroid after I-131 or surgery and may need temporary supplementation.

Monitoring Frequency:

• After starting treatment: Every 2-4 weeks until stable.
• Long-term: Every 6-12 months for life.

Prognosis and Quality of Life

The prognosis for treated hyperthyroidism is excellent. With appropriate management, most cats return to normal health and can expect a normal or near-normal life expectancy.

Without treatment, the disease is fatal, typically leading to severe weight loss, heart failure, and death within 1-2 years.

Prevention and Early Detection

There are no proven prevention strategies. The key to a successful outcome is early detection through regular veterinary care.

Schedule a Senior Wellness Exam if your cat shows:

• Unexplained weight loss (especially with a good appetite).
• Increased thirst or urination.
• Hyperactivity, restlessness, or behavioral changes.
• Vomiting or diarrhea.
• A poor, unkempt coat.

Veterinary Recommendation: Cats over 10 years old should have a wellness exam, including blood pressure and basic blood work, every 6 months.

Conclusion

Feline hyperthyroidism is a highly manageable disease. While the array of symptoms and treatment options can seem overwhelming, a timely diagnosis and a treatment plan tailored to your cat's specific health profile can lead to an excellent quality of life for many years.

The most important step is scheduling a veterinary visit at the first sign of symptoms. Early intervention prevents complications and opens the door to all available treatment possibilities, giving your beloved senior cat the best chance for a healthy future.