If your older cat is eating more than ever but losing weight, hyperthyroidism is one of the first things your vet will check. It’s the most common hormonal disorder in older cats. The good news is that it’s very treatable. Here’s what to know about hyperthyroidism in cats and your treatment options.
Quick Answer: Hyperthyroidism happens when the thyroid gland makes too much thyroid hormone, usually because of a benign growth. Signs include weight loss, increased appetite, thirst, vomiting, hyperactivity and a poor coat. A blood test diagnoses it. Treatments include daily medication, a prescription iodine-restricted diet, radioactive iodine therapy and, less often, surgery.
What Is Hyperthyroidism?
The thyroid glands in the neck control metabolism. In hyperthyroid cats, they become enlarged and overactive, speeding up nearly every body system. Most cases are caused by a benign tumor called an adenoma.
The Cornell Feline Health Center notes that hyperthyroidism is common in middle-aged and older cats, typically over 10 years old.
Signs of Hyperthyroidism
| Sign | Notes |
|---|---|
| Weight loss | Despite eating well |
| Increased appetite | Often dramatic |
| Increased thirst and urination | Can overlap with kidney disease |
| Vomiting or diarrhea | Common |
| Hyperactivity or restlessness | Vocalizing at night |
| Unkempt or greasy coat | Poor grooming |
| Fast heart rate | Heard by your vet |
| Lump in the neck | Enlarged thyroid, felt by your vet |
Complications if Untreated
- Heart disease (thickened heart muscle)
- High blood pressure, which can damage the eyes, kidneys and brain
- Muscle wasting
- Worsening weight loss
Diagnosis
Your vet will examine your cat and run:
- Total T4 blood test, the main screening test
- Free T4 in some borderline cases
- Kidney values, because hyperthyroidism can mask kidney disease
- Blood pressure
- Heart evaluation if needed
Treatment Options
| Treatment | How it works | Pros | Cons |
|---|---|---|---|
| Methimazole | Daily pill or transdermal gel to reduce hormone production | Widely available, adjustable | Lifelong; possible side effects |
| Iodine-restricted diet | Prescription food limits iodine | No pills | Cat must eat only this food |
| Radioactive iodine (I-131) | One-time injection destroys overactive tissue | Often curative | Special facility; short hospital stay |
| Surgery | Removes the affected gland | Can be curative | Anesthesia risk; less commonly used now |
Your vet will help you choose based on your cat’s health, temperament and your preferences.
Possible methimazole side effects
- Vomiting or poor appetite
- Facial itching
- Blood cell changes
- Liver changes
Your vet will monitor bloodwork, especially in the first few months.
The Kidney Connection
Hyperthyroidism increases blood flow to the kidneys, which can hide underlying chronic kidney disease. Once treatment brings thyroid levels down, kidney values may rise. Your vet will monitor both closely.
Living With a Hyperthyroid Cat
- Give medication consistently.
- Weigh your cat monthly.
- Keep up with blood tests as advised.
- Watch for vomiting, appetite changes or itching.
- Check blood pressure regularly.
See caring for a senior cat and signs your cat is in pain.
When to Call Your Vet
Book a visit if your cat:
- Is losing weight but eating well
- Is drinking more, vomiting more or restless at night
Get urgent care if your cat:
- Has sudden blindness (possible high blood pressure)
- Is breathing fast or having trouble breathing
- Stops eating
Frequently Asked Questions
Is hyperthyroidism painful?
It isn’t usually described as painful, but it makes cats feel unwell and can damage organs.
Can hyperthyroidism be cured?
Radioactive iodine therapy and surgery can be curative. Medication and diet control the disease but don’t cure it.
Is hyperthyroidism hereditary?
Its exact cause isn’t fully known. Diet and environmental factors are being studied.
How long do hyperthyroid cats live?
With treatment, many live for years.
Can I use the gel instead of pills?
Yes. Transdermal methimazole applied to the ear is an option for cats that are hard to pill, though absorption can vary.
Can dogs get hyperthyroidism?
It’s rare in dogs. Dogs are more likely to have an underactive thyroid.
Should my cat eat a special diet after radioiodine?
Most cats return to a normal diet. Your vet will check thyroid and kidney values after treatment.
Will my cat gain weight back after treatment?
Most hyperthyroid cats regain weight and muscle once thyroid levels are controlled.
Does the iodine-restricted diet work if my cat goes outside?
It only works if your cat eats nothing else, including treats, prey or other pets’ food. Indoor-only cats do best on this option.
How often are blood tests needed?
Usually a few weeks after starting treatment, then every few months once stable.
Can hyperthyroidism come back after radioiodine?
It’s uncommon. Most cats are cured with a single treatment, but your vet will recheck thyroid levels to be sure.
This article is for general information only and isn’t a substitute for professional veterinary advice. If you’re worried about your pet’s health, contact your veterinarian or an emergency animal hospital.
