Understanding Hyperthyroidism in Cats:

Symptoms, Diagnosis, and Treatment

Hyperthyroidism is one of the most common endocrine disorders affecting middle-aged and senior cats. It occurs when the thyroid glands produce excessive amounts of thyroid hormone, leading to an increased metabolic rate and a range of clinical signs. Without treatment, hyperthyroidism can lead to serious complications, including high blood pressure, heart disease, kidney dysfunction, retinal damage, and diabetes mellitus.

Understanding the symptoms, diagnosis, and treatment options is crucial for ensuring your feline companion’s long-term health and well-being.

Visual sample of where your cats thyroid is and the structural differences between an enlarged and normal thyroid.

Symptoms of Hyperthyroidism

Cats with hyperthyroidism often exhibit a variety of symptoms, including:

• Increased appetite – Owners often notice increased appetite.

• Weight loss – This is often the most noticeable sign, even in cats with a ravenous appetite.

• Increased activity and restlessness – Cats may seem hyperactive, vocal, or irritable.

• Increased thirst and urination – Excessive drinking and more frequent urination are common.

• Vomiting and diarrhea – Gastrointestinal upset can occur in some cases.

• Unkempt or matted fur – Poor coat condition, unkempt coat.

Diagnosing Hyperthyroidism

If your cat is exhibiting any of these symptoms, please call to make an appointment so we can do a thorough physical exam, check blood work, and urinalysis. A blood test that includes the measurement of thyroid hormone level (T4) is the most common method used to confirm hyperthyroidism.

Treatment Options for Hyperthyroidism

There are four main treatment options for hyperthyroidism in cats, and the best choice depends on your cat’s health status, lifestyle, and your preferences as a pet owner.

1. Radioactive Iodine Therapy (I-131) – Considered the gold standard for treating hyperthyroidism, this treatment has a >95% success rate in curing the condition. It involves a single injection of radioactive iodine that selectively destroys the overactive thyroid tissue without the need for lifelong medication. This treatment is available at specialty veterinary facilities, such as Advanced Veterinary Medical Imaging in Tustin, CA.

   
   

2. Medication (Methimazole) – The most commonly used treatment is a drug called Methimazole, which lowers thyroid hormone production. It is available as a pill (which many veterinary clinics carry) or in special-order formulations like an oral liquid or transdermal topical gel that is applied to the ear flaps. Cats starting on Methimazole require a blood recheck after one month to assess response to treatment and monitor kidney function. In some cases, treating hyperthyroidism can reveal underlying kidney dysfunction. To be clear, this occurs in cats that already had kidney dysfunction. Once the hyperthyroidism is treated, the kidney values on blood work appear worse.

   
   

3. Surgical Thyroidectomy – In some cases, surgical removal of the affected thyroid gland is an option. However, this procedure carries risks associated with anesthesia and potential complications affecting the parathyroid glands.

   
   

4. Prescription Diet (Hill’s y/d) – A strict iodine-restricted diet can help manage the disease by limiting thyroid hormone production. However, this option requires feeding only Hill's y/d for the rest of the cat’s life. Any deviation from the diet or refusal of the cat to eat the food will disrupt treatment. This method of management is not typically recommended.

   
   
   
   

Next Steps and Support

Hyperthyroidism is a manageable condition, and early treatment can help prevent severe complications. If your cat has been diagnosed with hyperthyroidism, discussing treatment options with your veterinarian is crucial to determine the best approach for your pet.

If you have any questions or would like more information about treatment options like Methimazole or I-131 therapy, your veterinarian will be happy to assist you. With proper management, cats with hyperthyroidism can continue to live happy, healthy lives.

If you are currently looking for a veterinarian near you, I practice at Alicia Pet Care Center in Mission Viejo, CA. There are 10 veterinarians here who all regularly handle cats with hyperthyroidism. Please feel free to give us a call at 949-768-1313 book an appointment online.