Hyperthyroidism in Cats: Symptoms, Diagnosis & Treatment Options

Hyperthyroidism in Cats: Symptoms, Diagnosis & Treatment Options

Middle-aged and senior cats may suffer from hyperthyroidism. Here, our vets in Bonita Springs explain the disease, symptoms, diagnosis and treatment options.

What is hyperthyroidism in cats?

When a cat's thyroid glands are overactive, hyperthyroidism can occur. This very common disorder is caused by an increase in production of thyroid hormones by the thyroid glands located in the neck. 

Thyroid hormones regulate many processes throughout the body and control the metabolic rate. When too much of this hormone is produced, clinical symptoms may be quite dramatic and make cats severely ill. 

Cats suffering from hyperthyroidism are likely to burn energy too quickly, which leads to weight loss despite eating more food and experiencing an increase in appetite. More symptoms are listed below.

What are the symptoms of hyperthyroidism in cats?

Usually seen in cats who are middle-aged and older. Most are older than 10 - between 12 and 13 years old - when the disease becomes an issue. Female and male cats are equally impacted.

Hallmark signs of hyperthyroidism include:

  • Increase in thirst
  • Increased irritability or restlessness
  • Increase in heart rate
  • Poor grooming habits
  • Typically a healthy or increased appetite

Some cats will also have mild to moderate diarrhea and/or vomiting, while others will seek cooler places to lounge and have a low tolerance for heat.

In advanced cases, some cats may pant when they are stressed (an unusual behavior for kitties). While we note that most cats have a good appetite and are restless, some may feel weaker, lethargic or have a lack of appetite. The key is to watch for significant changes in your cat, and have them addressed earlier rather than later.

These symptoms are usually subtle to start and gradually become more severe as the underlying disease gets worse. Other diseases can also complicate and mask these symptoms, so it’s important to see your vet early.

What causes hyperthyroidism?

For most kitties, benign (non-cancerous) changes in their bodies can trigger the condition. Both thyroid glands are most often involved and become enlarged (the clinical change is nodular hyperplasia, and it resembles a benign tumor).

Though we’re not sure what causes the change, it is much like hyperthyroidism in humans (clinically named toxic nodular goitre). Rarely, a cancerous (malignant) tumor called thyroid adenocarcinoma is the underlying cause of this disease.

What are long-term complications of hyperthyroidism?

Left untreated, hyperthyroidism can impact the function of the heart, changing the organ’s muscular wall and increasing heart rate. It can eventually lead to heart failure.

Another potential complication is high blood pressure (hypertension). Though we see this less often, it can result in damage to several organs including the brain, kidneys, heart and even the eyes. If your vet diagnoses your cat with hypertension in addition to hyperthyroidism, medication will be required to control blood pressure.

Hyperthyroidism and kidney disease often occur at the same time, as they are both commonly seen in older cats. When both these conditions are present, they need to be closely monitored and managed as managing hyperthyroidism may sometimes adversely affect kidney function.

How is hyperthyroidism diagnosed?

Diagnosis of hyperthyroidism in senior cats can be tricky. Your vet will complete a physical exam and palpate your cat’s neck area to look for an enlarged thyroid gland. At Southwest Florida Veterinary Specialists, our Bonita Springs vets offer I-131 Radioactive Iodine Therapy for cats with hyperthyroidism. Radioactive iodine is absorbed by abnormal thyroid tissue, while normal thyroid tissue and other organs are left undamaged. This normal thyroid tissue will regain function after therapy has eliminated the benign tumor, within a month of treatment. 

A battery of diagnostic tests will likely be needed to diagnose hyperthyroidism in your cat, as many other common diseases experienced by senior cats (intestinal cancer, chronic kidney failure, diabetes, inflammatory bowel disease and more) share clinical symptoms with hyperthyroidism.

A complete blood count (CBC) urinalysis and chemistry panel can help rule out kidney failure and diabetes.

A simple blood test showing elevated T4 levels in the bloodstream may be enough for a definitive diagnosis, though this is not true for 100% of cats due to concurrent illnesses or mild cases of hyperthyroidism, which can result in fluctuating levels of T4 or showing elevated T4 levels if another illness is influencing the result.

If possible, your vet may also check your cat’s blood pressure and perform an electrocardiogram, chest x-ray or ultrasound.

How will my vet treat my cat’s hyperthyroidism?

Your vet may choose one of several treatment options for your cat’s hyperthyroidism, based on your pet’s specific circumstances and the advantages and disadvantages of each option. They may include:

  • Radioactive iodine therapy (likely the safest and most effective treatment option)
  • Antithyroid medication, administered orally, to control the disease for either the short-term or long-term
  • Surgery to remove the thyroid gland
  • Dietary therapy

What is the prognosis for cats with hyperthyroidism?

Your kitty’s prognosis for hyperthyroidism will generally be good with appropriate therapy, administered early. In some cases, complications with other organs can worsen prognosis.


Curious about whether I-131 Radioactive Iodine Therapy could help your cat with hyperthyroidism? Discuss this potential treatment with your veterinarian, who can then refer you to us for followup. 

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