Signs of Hyperthyroidism in Cats: What Every Owner Should Know

Signs of Hyperthyroidism in Cats: What Every Owner Should Know

Hyperthyroidism is the most common hormonal disorder in cats and one of the most common diseases in senior cats overall. It's also one of the most treatable — but only if it's caught. Because the signs develop gradually and overlap with other conditions, many cats go undiagnosed for months or years. Knowing what to look for can help you get your cat diagnosed and treated before serious complications develop.

What Is Hyperthyroidism?

Hyperthyroidism occurs when the thyroid glands produce excessive thyroid hormone, accelerating the body's metabolism. In cats, this is almost always caused by a benign tumor (adenoma) of one or both thyroid glands. It's most common in cats over age 10, with the average age of diagnosis around 13 years.

The Classic Signs of Hyperthyroidism in Cats

1. Weight Loss Despite Ravenous Appetite

The most characteristic sign of hyperthyroidism. The accelerated metabolism burns calories faster than the cat can consume them, causing progressive weight loss even in cats that are eating voraciously — or even more than usual. A cat that seems constantly hungry but is losing weight is a classic hyperthyroid presentation.

2. Increased Appetite (Polyphagia)

Hyperthyroid cats are often ravenously hungry, demanding food constantly, stealing food from other pets, or eating much more than their normal amount. This increased appetite combined with weight loss is the hallmark combination.

3. Increased Thirst and Urination

Elevated thyroid hormone increases kidney blood flow and metabolic rate, leading to increased water intake and urine production. The litter box may need more frequent changing, and the water bowl may empty faster than usual.

4. Hyperactivity and Restlessness

Thyroid hormone is a metabolic stimulant. Hyperthyroid cats are often unusually active, restless, or agitated for their age. They may vocalize more, pace, or seem unable to settle. This can be mistaken for anxiety or cognitive dysfunction in senior cats.

5. Vomiting

Frequent vomiting — often of undigested food — is common in hyperthyroid cats. The accelerated gut motility means food moves through the digestive system too quickly to be fully absorbed.

6. Diarrhea or Increased Defecation

Accelerated gut motility also causes loose stools or more frequent defecation. Some hyperthyroid cats produce large volumes of soft, poorly formed stool.

7. Poor Coat Condition

Despite being active, hyperthyroid cats often have a dull, unkempt, or matted coat. They may groom excessively in some areas while neglecting others, or the coat may simply look rough and unhealthy.

8. Enlarged Thyroid Gland (Goiter)

In many hyperthyroid cats, the enlarged thyroid gland can be felt as a small lump or nodule on the neck. Your vet will palpate the neck during examination. This is not always detectable at home but is a key diagnostic finding.

Cardiovascular Signs

Thyroid hormone has direct effects on the heart. Hyperthyroidism commonly causes:

• Rapid heart rate (tachycardia): Heart rate over 240 beats per minute
• Heart murmur: New murmur detected on examination
• Hypertrophic cardiomyopathy (HCM): Thickening of the heart muscle from chronic overstimulation
• Rapid or labored breathing: From heart failure or pleural effusion in advanced cases

Cardiovascular complications are one of the most serious consequences of untreated hyperthyroidism.

Hypertension Signs

Hyperthyroidism commonly causes high blood pressure, which can lead to:

• Sudden blindness (retinal detachment)
• Dilated pupils that don't respond to light
• Disorientation or sudden behavioral changes
• Nosebleeds

Sudden blindness in a senior cat is a hypertensive emergency — seek veterinary care immediately.

Behavioral Changes

• Increased vocalization, especially at night
• Aggression or irritability (unusual in a previously gentle cat)
• Anxiety or inability to settle
• Cognitive changes that may resemble dementia

How Hyperthyroidism Compares to Other Senior Cat Conditions

At-Home Monitoring

For senior cats, regular at-home monitoring helps catch hyperthyroidism and related conditions early. The KIT4CAT CheckUp At-Home Cat Wellness Test Kit monitors urine glucose, protein, pH, and blood — useful for tracking overall metabolic health in senior cats. Weigh your cat weekly and keep a log of appetite, water intake, and behavior changes.

When to See the Vet

Schedule a vet appointment promptly if your senior cat shows:

• Weight loss despite normal or increased appetite
• Ravenous hunger in a previously normal eater
• Increased thirst and urination
• Hyperactivity or restlessness unusual for their age
• Frequent vomiting or diarrhea
• Rapid breathing or heart rate
• Sudden blindness or behavioral changes

Diagnosis requires a blood test measuring total T4 (thyroid hormone) levels. Annual bloodwork for cats over 7 — and twice-yearly for cats over 10 — is the best way to catch hyperthyroidism before complications develop.

Final Thoughts

Hyperthyroidism is common, serious, and highly treatable. The combination of weight loss with ravenous appetite in a senior cat is the most important red flag to recognize. Early diagnosis and treatment can prevent heart disease, hypertension, and kidney damage — and most treated cats return to excellent quality of life.