Feline Hyperthyroidism vs. Hypothyroidism: What’s the Difference?

WeightQ&AMeongsiljang Veterinary Advisory Board

Here’s a comparison table outlining the causes, symptoms, weight changes, and treatment options for feline hyperthyroidism and hypothyroidism.

Feline Hyperthyroidism vs. Hypothyroidism: The Key Difference in One Line

A veterinarian gently palpating the thyroid gland in the neck area of an older cat.

Feline thyroid disease is an endocrine disorder caused by an imbalance of hormones produced by the thyroid gland in the front of the neck, leading to metabolic issues. The direction of weight change is opposite depending on the condition. In hyperthyroidism, excessive hormone production causes weight loss despite a good appetite, whereas in hypothyroidism, insufficient hormone levels lead to decreased activity and appetite, resulting in weight gain. Hyperthyroidism is overwhelmingly common in cats over 10 years old, while hypothyroidism is very rare.

Different Causes — Starting with the Underlying Reasons

These two conditions have different underlying causes. Hyperthyroidism is most often caused by benign adenomatous hyperplasia, which is an abnormal enlargement of the thyroid tissue. According to textbooks, about 98% of cases are due to this benign hyperplasia, and hyperthyroidism is so common that it is considered the most prevalent endocrine disease in older cats. In contrast, cases caused by thyroid carcinoma are rare, accounting for only about 2% of cases. On the other hand, hypothyroidism is known to occur as an iatrogenic condition following thyroid treatment (surgery or radioactive iodine therapy), and naturally occurring cases in cats are very rare.

At a Glance — Hyperthyroidism vs. Hypothyroidism

Item	Hyperthyroidism	Hypothyroidism
Hormone status	Excessive secretion	Deficient
Body weight	Loss (despite increased appetite)	Gain
Activity level	Hyperactive·irritable	Lethargic·increased sleep
Appetite	Ravenous level	Decreased
Heart rate	Increased (tachycardia)	Decreased
Coat condition	Hair loss·coarse	Hair loss·dry
Age of onset	Senior cats 10 years and older	After treatment or congenital
Frequency	Most common endocrine disease in senior cats	Very rare

Based on The Cat: Clinical Medicine and Management, 2nd edition

Symptom Check — Don’t Miss These Signs

Behavioral changes are just as important to watch for as weight changes.

Possible signs of hyperthyroidism: Constantly begging for food

Possible signs of hyperthyroidism: Drinking excessive amounts of water and urinating frequently

Possible signs of hyperthyroidism: Pacing and vocalizing more often, even at night

Possible signs of hyperthyroidism: Rapid heart rate and labored breathing

Possible signs of hypothyroidism: Reluctance to move and excessive sleeping

Possible signs of hypothyroidism: Intolerance to cold and seeking out warm spots

Possible signs of hypothyroidism: Dry skin and poor hair growth

A senior cat with a robust appetite yet a lean appearance

When to Go Straight to the Vet

If your pet is breathing rapidly or has pale gums, seek emergency care immediately. Prolonged hyperthyroidism can place increasing strain on the heart, leading to a rapid heart rate (tachycardia) and high blood pressure, which may result in serious complications. In hypothyroidism, warning signs include low body temperature, severe lethargy, and complete loss of appetite. Additionally, cats that refuse to eat for more than 24 hours can deteriorate quickly, so do not wait and watch; a veterinary visit is essential.

How Is It Diagnosed?

The primary diagnostic test is a blood test to measure total T4 hormone levels. According to veterinary internal medicine textbooks, over 90% of cats with hyperthyroidism can be diagnosed with this single test. If the results are borderline or inconclusive, additional tests such as free T4, T3 suppression tests, or thyroid scans may be performed. In cases of hypothyroidism, T4 levels are low, but they can also be reduced due to other systemic illnesses (euthyroid sick syndrome), making differential diagnosis challenging. Therefore, TSH stimulation tests are often conducted alongside T4 measurements.

Image of a cat thyroid hormone blood test

Treatment Options — The Choices Vary Widely

There are four treatment options for hyperthyroidism. - Radioactive iodine therapy: A curative, non-invasive treatment that is recommended as the first choice in textbooks. - Methimazole/Carbimazole: A lifelong medication that is the most accessible option. - Thyroidectomy: Less commonly used nowadays due to the burden of anesthesia. - Iodine-restricted prescription diet: A supportive method. In contrast, the standard treatment for hypothyroidism is levothyroxine supplementation, with the dosage adjusted by the veterinarian based on weight and test results.

Precautions for Long-Term Methimazole Use

Methimazole (carbimazole) is highly effective, though it can occasionally cause side effects. For this reason, veterinary textbooks recommend rechecking total T4 (TT4), a complete blood count (CBC), serum biochemistry, and urinalysis one month after starting treatment, while also assessing weight gain and improvement in clinical signs. Thereafter, regular monitoring is typically performed every 4 to 6 months. If drug side effects are a concern or lifelong medication is difficult to manage, it is advisable to discuss with your veterinarian the possibility of switching to radioactive iodine therapy, a non-invasive treatment that can offer a cure.

Key Points for Home Care

The most important indicator for both conditions is regular weight monitoring. Weigh your pet once a week at the same time and keep a record.

Hyperthyroidism care: Provide additional water bowls to ensure adequate hydration.

Hyperthyroidism care: Maintain a quiet environment to reduce cardiac stress.

Hypothyroidism care: Provide a warm resting area to help maintain body temperature.

Hypothyroidism care: Administer medication at the same time every day on an empty stomach for optimal absorption.

If your pet’s weight changes by more than 5% over two weeks, contact your veterinarian immediately.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Can a cat develop both hyperthyroidism and hypothyroidism at the same time?

It's very rare for both conditions to occur at the same time. However, if the medication for hyperthyroidism is too strong or if the thyroid gland is removed, it can lead to a temporary hypothyroidism. This is called iatrogenic hypothyroidism.

Can young cats also develop thyroid disease?

Hyperthyroidism almost exclusively affects senior cats aged 10 years and older. Hypothyroidism is a rare congenital condition in kittens, characterized by delayed growth and a disproportionately large head relative to the body.

Does eating a lot but losing weight always mean hyperthyroidism?

While this is one of the most common causes, diabetes mellitus, chronic kidney disease, and malabsorption syndromes can present with similar symptoms. A blood test is necessary to differentiate between these conditions.

Which cats are recommended for radioactive iodine therapy?

This option is primarily considered when medication side effects are severe, lifelong drug therapy is difficult to manage, or heart complications have already begun. Since only a limited number of veterinary hospitals in Korea offer this treatment, it’s important to research and confirm availability in advance.

If your pet has a thyroid condition, how should you change their diet?

For hyperthyroidism, iodine-restricted prescription diets can be used, but their effects are mild and serve mainly as a supportive measure. In hypothyroidism, there’s no need for any special restricted diet. Always consult your veterinarian before switching to a prescription diet.

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References

[1] Little SE. The Cat: Clinical Medicine and Management, 2nd Edition. Chapter on Feline Hyperthyroidism

[2] Fuentes VL et al. Textbook of Cardiovascular Medicine in Dogs and Cats — Feline Hyperthyroidism

[3] Hill P, Warman S, Shawcross G. 100 Top Consultations in Small Animal General Practice (2011)

[4] Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. J Vet Intern Med. 1988;2:150