What It Is
Hyperthyroidism is one of the most common endocrine disorders in older cats. It occurs when the thyroid glands (in the neck) produce too much thyroid hormone, which speeds up the cat’s metabolism. Think of it as the body’s engine running in overdrive, burning fuel faster than it should.
It’s typically caused by a non-cancerous tumor on one or both thyroid glands. While the condition is serious, it’s very treatable, and cats diagnosed early often live normal, happy lives.
🐾 Common Symptoms
Because hyperthyroidism affects metabolism, symptoms often involve weight loss despite a good (or even increased) appetite.
- Weight loss
- Increased appetite (polyphagia)
- Increased thirst and urination
- Hyperactivity or restlessness
- Vomiting
- Diarrhea
- Poor coat condition
- Rapid or irregular heartbeat
- Muscle wasting
- Panting or shortness of breath
In some cases, cats may become cranky or more vocal than usual.
🦠 Causes
Hyperthyroidism is usually caused by a benign (non-cancerous) tumor on the thyroid gland, called an adenoma. The exact cause isn’t fully understood, but theories include:
- Dietary factors (excess iodine or certain food preservatives)
- Environmental influences
- Genetics
- Age (most cats diagnosed are over age 10)
Rarely, it can be caused by thyroid cancer (thyroid carcinoma).
🔍 Diagnosis
Your vet will likely run the following tests:
- Physical exam (feeling for enlarged thyroid glands)
- Bloodwork (especially total T4 levels, which are elevated in most cases)
- Additional testing (free T4, T3 suppression test, or thyroid scan if results are borderline)
- Blood pressure measurement (high BP is common with hyperthyroidism)
- Chest X-rays or ultrasound (in advanced cases, to check for heart changes)
💊 Treatment
There are four main treatment options, and your vet can help you decide based on your cat’s age, health, and your budget.
- Medication (Methimazole)
- Oral or topical
- Controls hormone levels but doesn’t cure
- Requires daily dosing and regular blood monitoring
- Radioactive Iodine Therapy (I-131)
- The gold standard – curative in over 95% of cases
- Involves a one-time injection and short hospital stay
- Not always available in every area
- Surgery (Thyroidectomy)
- Removal of one or both thyroid glands
- Can be curative
- Higher risk in older cats or those with heart issues
- Prescription Diet (Hill’s y/d)
- Very low in iodine to reduce hormone production
- Works only if used exclusively without any other treats or food
- Best for cats who can’t take meds or undergo procedures
🩺 Prognosis
With proper treatment, most cats do very well and live for years after diagnosis. If left untreated, hyperthyroidism can lead to:
- High blood pressure
- Blindness
- Heart disease (thyrotoxic cardiomyopathy)
- Weight loss and organ failure
Monitoring is essential. Treated cats need regular bloodwork to track hormone levels and assess kidney function.
💡 Cat Owner Note
A cat that’s suddenly shedding pounds but still eating like a lion? That’s a red flag. Hyperthyroidism is one of those conditions that sneaks up quietly, but the good news is it’s highly treatable. You’ve got options. Your vet can help you choose what’s best for your cat (and your budget).
❓ Frequently Asked Questions About Hyperthyroidism in Cats
Is hyperthyroidism in cats curable?
Yes, with radioactive iodine therapy (I-131) or surgery. Medication and diet control the disease but don’t cure it.
Can hyperthyroidism come back after treatment?
Sometimes, especially with medication or diet-based management. Recurrence after I-131 treatment is rare but possible.
How long can a cat live with hyperthyroidism?
With treatment, many cats live 2–5+ years or longer. Early detection and proper management are key.
Do cats with hyperthyroidism need a special diet?
Not always, but a prescription iodine-restricted diet is one treatment option. Your vet will recommend the best course based on your cat’s needs.
Can untreated hyperthyroidism kill a cat?
Yes, over time. It can cause heart damage, high blood pressure, and multi-organ failure if left untreated.
How often do I need to test my cat’s blood?
Every few weeks during the first 2–3 months of treatment, then every 3–6 months long-term to monitor hormone levels and kidney function.
Is methimazole safe long-term?
Yes, for most cats. But some may develop side effects like vomiting, liver issues, or low white blood cell counts. Regular bloodwork is a must.