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What Are the Causes and Symptoms of Feline Hyperthyroidism?

Feline hyperthyroidism is a common health condition affecting middle-aged and older cats. While it can be worrisome to realize your feline friend is unwell, a solid understanding of this condition can help you recognize the symptoms early and seek intervention from a trusted veterinarian to manage the situation effectively.

What Is Feline Hyperthyroidism?

Feline hyperthyroidism is a common endocrine disorder that affects cats, particularly those in their middle to senior years. It is caused by an overactive thyroid gland in the neck, which produces an excess amount of thyroid hormones (T3 and T4). These hormones are crucial in regulating the cat’s metabolism, affecting various body functions.

With excessive production of thyroid hormones, the cat’s metabolism goes into overdrive, leading to a range of health issues and symptoms. If left untreated, hyperthyroidism can negatively impact the cat’s overall health and quality of life.

What Causes Feline Hyperthyroidism?

1. Age

  • Feline hyperthyroidism is most commonly observed in middle-aged to senior cats, typically over the age of 10 years.
  • The prevalence increases with age, with very few cases seen in cats under ten.

2. Non-Cancerous Growth (Adenoma)

  • Most cat hyperthyroidism cases involve a non-cancerous growth called an adenoma in one or both thyroid glands.
  • An adenoma leads to excessive thyroid hormone production, disrupting the body’s normal balance.

3. Genetic Predisposition

  • Some evidence suggests that certain cat breeds, such as Siamese and Himalayan, may have a higher predisposition to develop hyperthyroidism.
  • Genetic factors could contribute to the development of thyroid gland abnormalities.

4. Dietary and Environmental Factors

  • Some studies have linked hyperthyroidism to specific dietary and environmental factors, although the exact mechanisms are not fully understood.
  • Exposure to certain chemicals or environmental pollutants may be potential risk factors.

5. Hormonal Disruptions

  • Hormonal imbalances or disruptions in the cat’s endocrine system may influence the development of hyperthyroidism.
  • The interplay of various hormones could contribute to abnormal thyroid function.

6. Immune System Dysfunction

  • Dysfunction in the cat’s immune system might contribute to the development of hyperthyroidism.
  • The immune system may mistakenly attack the thyroid tissue, leading to abnormal growth.

7. Thyroid Adenocarcinoma (Cancerous Tumor)

  • In rare cases, feline hyperthyroidism can be caused by a cancerous tumor known as thyroid adenocarcinoma.
  • This aggressive tumor can lead to uncontrolled and excessive production of thyroid hormones.

8. Iodine Imbalance

  • Iodine is essential for thyroid hormone production.
  • Imbalances in iodine intake or utilization might influence the development of hyperthyroidism.

Despite extensive research, the exact cause of feline hyperthyroidism remains unclear and likely involves a combination of factors. Veterinarians and researchers continue to study the condition to gain further insights into its origins and better ways to diagnose and treat affected cats.

Symptoms of Hyperthyroidism in Cats

  • Weight Loss: Feline hyperthyroidism accelerates the cat’s metabolism, leading to increased energy expenditure and weight loss, despite an increased appetite. Excess thyroid hormones cause the body to burn calories rapidly, losing body fat and muscle mass over time.
  • Increased Appetite (Polyphagia): Hyperthyroid cats experience a significant increase in appetite due to the excessive stimulation of their metabolism. They may constantly seek food, act restless around meal times, or try to eat from other pets’ bowls.
  • Vomiting: The elevated thyroid hormone levels can irritate the digestive system, leading to occasional episodes of vomiting.
  • Increased Thirst (Polydipsia) and Urination (Polyuria): Excessive thyroid hormones influence the kidneys, causing them to filter more blood, leading to increased thirst (polydipsia). Consequently, cats may drink more water and produce larger volumes of urine (polyuria).
  • Hyperactivity and Restlessness: The heightened metabolic rate and increased circulating thyroid hormones result in hyperactivity and restlessness in affected cats. They may seem more energetic, exhibit pacing behavior, or be unable to relax.
  • Change in Behavior: Some cats with hyperthyroidism may show behavioral changes, such as increased irritability, anxiety, or agitation. They might become more vocal, demanding, or display signs of stress.
  • Rapid or Irregular Heartbeat (Tachycardia): The thyroid hormones influence the heart’s functioning, leading to an elevated heart rate (tachycardia). A veterinarian can detect this during a physical examination or by listening to the cat’s heartbeat.
  • Poor Coat Condition (Dermatologic Changes): Excessive thyroid hormones can negatively affect the skin and coat, leading to a dull, unkempt, or greasy appearance. Cats may experience dry, flaky skin, and some areas of the coat may appear thin or have hair loss.
  • Enlarged Thyroid Gland (Goiter): In some cases of hyperthyroidism, the thyroid gland may enlarge, forming a visible or palpable swelling in the neck region. This enlargement is a goiter, which can be felt during a physical examination.
  • Increased Heart Rate and Blood Pressure: In addition to a rapid heart rate, hyperthyroidism can lead to increased blood pressure. Elevated blood pressure can harm the cat’s organs, especially the eyes and kidneys.
  • Weakness and Lethargy (In Some Cases): While most cats with hyperthyroidism are hyperactive, some may experience periods of weakness and lethargy due to the impact of the thyroid hormones on their metabolism.

You must consult a qualified vet if you notice any of these symptoms. They can perform tests to confirm the diagnosis and talk to you about their hospital and treatment protocols.

Feline Hyperthyroidism Treatment

  • Radioactive Iodine Therapy: One of the most effective uses of radioactive iodine therapy for cats. This procedure involves a small dose of radioactive iodine, which destroys the overactive thyroid tissue without damaging surrounding structures. It’s a highly effective treatment that often cures hyperthyroidism permanently.
  • Anti-Thyroid Medication: Oral medication used to control the production of thyroid hormones. Requires regular administration throughout the cat’s life to manage the condition. Effective in controlling hyperthyroidism but not curative.
  • Surgical Thyroidectomy: A surgical option for surgically removing the affected thyroid gland(s). Reserved for cats who may not be suitable candidates for radioactive iodine therapy. Requires general anesthesia and skilled surgical intervention.
  • Prescription Diet: Some specialized diets are designed to limit iodine intake and reduce thyroid hormone production. It may be used as a part of the treatment plan or as a pre-treatment to stabilize the cat’s condition.
  • Medication Management Follow-Up: Regular monitoring of thyroid hormone levels is necessary for cats on medication therapy. Adjustments to the medication dosage may be required to maintain thyroid levels within the normal range.
  • Comprehensive Veterinary Care: Regular veterinary check-ups and monitoring of the cat’s overall health and response to treatment. Additional supportive care may be provided to manage any secondary health issues caused by hyperthyroidism.
  • Individualized Treatment Plan: The treatment approach may vary based on the cat’s age, health status, and other factors. The veterinarian will discuss the most suitable treatment option for each specific case.

From adopting a balanced diet to regular vet check-ups, maintaining your pet’s health should always be a priority. A comprehensive cat and dog wellness plan can be adjusted for your pet, equipping you with the right tools to proactively monitor your pet’s health. Remember, early detection is key in managing conditions like feline hyperthyroidism.


Feline hyperthyroidism can be scary, but it can be managed effectively with the right knowledge and veterinary partnership. Stay vigilant for changes in your cat’s behavior or physical health, and never hesitate to seek your trusted vet’s help. Their expert advice is crucial in treating hyperthyroidism and setting up a broader pet wellness plan to ensure your furry friend’s all-around health and longevity.