Hyperthyroidism in Cats: Symptoms, Diagnosis & Treatment Options

Hyperthyroidism can become an issue for middle-aged and senior cats. In this post, our South Charlotte vets explain the disease, symptoms, diagnosis and treatment options.


What is hyperthyroidism in cats?

Hyperthyroidism happens when a cat’s thyroid glands is overactive. It’s a very common disorder caused by an increase in production of thyroid hormones by the thyroid glands, which are located in the neck.

Thyroid hormones are used to regulate many processes in the body and to control the metabolic rate, and when too much of the hormone is produced, clinical symptoms can be quite dramatic and make cats severely sick.

Cats suffering from hyperthyroidism tend to burn energy too quickly, which results in weight loss despite eating more food and experiencing an increase in appetite. We’ll discuss more symptoms 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 Sharon Lakes Animal Hospital, our Charlotte vets are trained in internal medicine and have access to a variety of diagnostic tools and treatment methods.

A battery of 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.

Note: The advice provided in this post is intended for informational purposes and does not constitute medical advice regarding pets. For an accurate diagnosis of your pet's condition, please make an appointment with your vet.

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