Hyperthyroidism in Cats: Symptoms, Diagnosis, Causes, and Treatments

Early detection helps address thyroid imbalances for a healthier life in senior cats.

By Medha deb

Created on Sep 10, 2025

Early detection helps address thyroid imbalances for a healthier life in senior cats.

Hyperthyroidism is the most commonly diagnosed hormone disorder in senior cats, leading to an elevated metabolic rate and potentially affecting multiple organs. Recognizing early symptoms and understanding available treatments is crucial for ensuring the long-term health and well-being of your feline companion.

What Is Feline Hyperthyroidism?

Hyperthyroidism in cats is an endocrine disorder characterized by the overproduction of thyroid hormones (thyroxine, or T4, and triiodothyronine, or T3) from the thyroid glands, typically located in your catâs neck. These hormones regulate the body’s metabolismâmeaning that when they are excessively produced, nearly every organ system is forced to work harder than normal. This “overdrive” can result in a wide range of symptoms and secondary health problems.

Most frequently diagnosed in cats older than 7âespecially those over 10 years old.
Seen in an estimated 10% of senior cats.
The majority of feline hyperthyroidism cases are caused by benign growths (adenomas) in one or both thyroid glands.
Rarely, malignant (cancerous) growths, known as thyroid carcinomas, may be responsible.

Causes of Hyperthyroidism in Cats

Despite decades of research, the precise causes of hyperthyroidism in cats remain elusive. However, several key contributing factors and risk elements have been identified:

Benign Thyroid Tumors (Adenomas): These non-cancerous growths in the thyroid cause excessive hormone production.
Thyroid Cancer: Rare but possible.
Environmental Factors: Exposure to certain flame retardants (polybrominated diphenyl ethers, or PBDEs) used in furniture and household dust may increase risk.
Dietary Factors: Some reports have suggested a possible link between frequent consumption of fish-flavored canned foods and hyperthyroidism incidence, although this is still under investigation.
Advancing Age: Increasing age is consistently linked to higher risk.

Symptoms of Feline Hyperthyroidism

Your cat may exhibit a combination of the following common and atypical symptoms:

Weight Loss: Despite increased or ravenous appetite.
Increased Appetite: Frequently described as voracious or insatiable.
Increased Thirst (Polydipsia) and Increased Urination (Polyuria): Owners may notice larger or more frequent clumps in the litter box.
Increased Activity, Restlessness, Hyperactivity, or Irritability: Cats may appear more energetic or agitated than usual.
Vomiting and/or Diarrhea: Not all cats, but gastrointestinal symptoms are common.
Poor or Unkempt Coat: Hair may become greasy, matted, or shedding may be excessive.
Rapid Heart Rate (Tachycardia) and/or Increased Respiratory Rate: Detected by veterinarians during exams.
Muscle Wasting: Loss of muscle mass, especially along the spine and hind legs.
Occasionally: Lethargy, decreased appetite, depression, or weaknessâless common (<10% of cases).

Because the disorder develops gradually, many owners attribute subtle changes to normal aging and may not immediately recognize the signs.

Potential Complications and Associated Diseases

High Blood Pressure (Hypertension): Increased cardiac workload can lead to persistently high blood pressure, risking retinal detachment and sudden blindness.
Heart Disease: The heart muscle may thicken (hypertrophy) to compensate for increased workload, eventually leading to heart enlargement and, without intervention, heart failure.
Kidney Disease: Chronic kidney disease may be masked by elevated thyroid hormone levels. Some cats are diagnosed with both conditions, which can influence treatment decisions and prognosis.
Other Conditions to Differentiate From: Renal failure, diabetes mellitus, liver disorders, and certain cancers (particularly intestinal lymphoma) can mimic or coexist with hyperthyroidism.

How Veterinarians Diagnose Hyperthyroidism in Cats

Accurate diagnosis involves a combination of clinical assessment and laboratory testing:

Physical Exam: Palpation may reveal an enlarged thyroid gland (goiter or thyroid nodule).
Blood Tests: Measurement of total T4 (TT4) is the primary screening test; most hyperthyroid cats have elevated TT4 levels.
Additional Hormone Testing: For borderline or early-stage cases, veterinarians may test free T4 (fT4) and TSH levels, or use a T3 suppression test for greater accuracy.
Thyroid Scans (Scintigraphy): May be used in complex, ambiguous, or pre-treatment cases.
Exclusion of Other Diseases: Since other conditions may cause similar symptoms, your veterinarian may also run kidney function tests (BUN, creatinine), liver enzyme panels, glucose levels, and general health screens.

Main Treatment Options for Feline Hyperthyroidism

Treatment aims to restore normal hormone levels and minimize side effects from excess thyroid hormone. Main modalities include:

1. Medical Therapy with Antithyroid Drugs

Methimazole: The most commonly prescribed oral or transdermal medication; blocks hormone production but does not cure the underlying disease.
Felimazole: FDA-approved methimazole in tablet form. Alternative is Felanorm, a liquid oral solution.
Lifetime Daily Administration: Requires regular bloodwork to monitor hormone levels and detect potential side effects (e.g., vomiting, facial itching, liver or blood cell abnormalities).
Pros: Non-invasive, allows reversible control.
Cons: Daily dosing needed, possible gastrointestinal and hematologic side effects.

2. Radioactive Iodine Therapy (I-131)

Definitive Treatment: Involves a single injection of radioactive iodine, which selectively destroys abnormal thyroid tissue.
High Cure Rate: Over 95% of treated cats are cured after one treatment.
Minimal Side Effects: Low risk of hypothyroidism (underactive thyroid post-treatment), which can usually be managed if it occurs.
Limited Availability: Requires specialized hospital facilities with isolation for a few days post-treatment due to radiation safety regulations.
Expensive Upfront Cost: Higher initial cost than medication but may be less expensive long-term.

3. Surgical Removal of Affected Thyroid Gland (Thyroidectomy)

Curative in Many Cases: Particularly effective if only one gland is affected.
Risks: Potential complications include damage to adjacent parathyroid glands (important for calcium regulation), anesthetic risks especially for senior cats, and post-op hypothyroidism.
Less Common Today: Given the availability of less invasive therapies, surgery is not a first-line option except in selected cases.

4. Dietary Management

Iodine-Restricted Prescription Diet: Specially formulated diets (e.g., Hillâs y/dâ¢) can control hyperthyroidism in some cats by restricting dietary iodine intake, which is essential for thyroid hormone synthesis.
Must Be Fed Exclusively: Any additional treats, table scraps, or standard kibble will negate effectiveness.
Used For: Cats unable to tolerate medication or where other therapies are not viable.
Not a cure: Controls hormone production only while the diet is strictly maintained.

Prognosis for Hyperthyroid Cats

Cats receiving appropriate therapy can live for years with a good quality of life. The most important factors affecting prognosis are the presence of complicating diseases, such as kidney disease or heart failure, and how consistently the treatment plan is followed.

Points for Cat Owners

Early diagnosis and treatment offer the best chance for long-term wellbeing.
Monitor for any new symptoms or side effects after beginning treatment, especially changes in appetite, vomiting, facial itching, or signs of lethargy.
Routine monitoring with your veterinarian is essential.

Frequently Asked Questions (FAQs)

Q: Are certain breeds more prone to hyperthyroidism?

A: Hyperthyroidism appears to affect all breeds of cats equally. There is currently no strong evidence to suggest breed predisposition.

Q: Why does my hyperthyroid cat lose weight but eat more?

A: Excess thyroid hormone causes the metabolic rate to increase, meaning your cat burns energy and calories faster than normal. Despite increased appetite and food intake, they can’t keep up with their bodyâs “overdrive.”

Q: Is hyperthyroidism fatal if untreated?

A: Yes. Left untreated, hyperthyroidism is nearly 100% fatal, due to severe heart disease, high blood pressure, or multi-organ failure. Treatment greatly improves prognosis and longevity.

Q: Can I stop medication if my cat feels better?

A: No. Medications control, but do not cure, the disease. Discontinuing medication will cause symptoms and harmful metabolic effects to recur.

Q: Should I be concerned about radioactive iodine treatment?

A: Radioactive iodine is highly effective and safe. Treated cats must sometimes stay at the hospital for a few days after injection due to regulatory requirements, but most resume normal life afterward. Rarely, hypothyroidism may occur post-treatment, but this can be easily managed.

Summary Table: Hyperthyroidism in Cats

Treatment	Pros	Cons	Cure/Control
Antithyroid Drugs	Non-invasive, reversible control, affordable short-term	Daily dosing, possible side effects, routine monitoring required	Control only (not a cure)
Radioactive Iodine	Definitive cure, minimal side effects, high success rate	High upfront cost, limited hospital availability, short hospitalization	Cure
Surgery	Can be curative if only one gland affected	Anesthetic risks, possible post-op complications	Cure in select cases
Iodine-Restricted Diet	No medication needed, good option for some cats	Must be strictly exclusive, not a cure, may affect social feeding	Control only

Early Detection and Partnering With Your Veterinarian

Monitor your senior cat for behavior or weight changes and bring them for regular veterinary checkups, especially after the age of seven. Hyperthyroidism is common but highly manageable with proper care and early intervention. With the right treatment plan, most affected cats can return to a comfortable, happy life.

References

Author

medha deb

Medha Deb is an editor with a master's degree in Applied Linguistics from the University of Hyderabad. She believes that her qualification has helped her develop a deep understanding of language and its application in various contexts.

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