Hyperthyroidism in Cats: Symptoms, Diagnosis & Treatment Options

Hyperthyroidism may become an issue for your middle-aged or older cat. In this post, our Greensboro vets define the disease, symptoms, diagnosis and treatment options.

What is hyperthyroidism in cats?

When a cat’s thyroid glands are overactive, the common disorder hyperthyroidism occurs. In cats, this condition is caused by a rise in production of thyroid hormones (created by the thyroid glands, which are located in the neck).

Many processes in the body are regulated by thyroid hormones, which also control your cat’s metabolic rate. When too many of these hormones are produced, clinical symptoms can be very dramatic and cats can become severely ill.

A cat suffering from hyperthyroidism will typically burn energy too fast, which leads to weight loss despite experiencing an increase in appetite and eating more food. We’ll describe more symptoms below.

What are symptoms of hyperthyroidism in cats?

Hyperthyroidism typically occurs in cats who are middle-aged or entering their senior years. Most are 10 years or older - between 12 and 13 years old - when the disease develops. Male and female cats are equally affected.

Notable symptoms of hyperthyroidism in cats include:

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

Sometimes, cats will also have mild to moderate diarrhea and/or vomiting, while others will seek cooler or more comfortable temperatures to lounge, as his or her tolerance for heat drops.

In advanced cases, some cats pant when they are stressed (not a typical behavior for our feline friends). While a good appetite and restless disposition may describe most cats, some may feel weaker, lethargic or experience a decrease in appetite. The takeaway: watch for significant changes in your cat, and take them to a vet so they can be addressed early on.

These signs are usually subtle to start but will gradually become more severe as the underlying disease worsens. Other conditions can also mask these symptoms and cause complications, so it’s important to see your vet as soon as possible.

What causes hyperthyroidism?

For most of our four-legged friends, benign (non-cancerous) changes in their bodies may trigger the condition. Typically, both thyroid glands become involved and grow to an enlarged state (in clinical terms, this change is called nodular hyperplasia, and resembles a benign tumor).

Though we are unsure of the root cause of the change, hyperthyroidism in cats behaves much the same way as the condition does in humans (clinically named toxic nodular goitre). Rarely, a malignant cancerous tumor called thyroid adenocarcinoma turns out to be the underlying culprit.

What are long-term complications of hyperthyroidism?

If left untreated, hyperthyroidism can eventually impact the heart’s function, altering the organ’s muscular wall and resulting in an uptick in heart rate. It can eventually cause heart failure.

High blood pressure or hypertension is another potential complication. Though this is seen less often, it can lead to organ damage in several areas such as the kidneys, brain, heart and eyes. If your cat is diagnosed with hypertension along with hyperthyroidism, medication will be needed to control high blood pressure.

Kidney disease can also become an issue as the two conditions are both commonly found in older cats. When both hyperthyroidism and kidney disease are present, your vet will need to closely monitor your cat. These conditions will need to be managed, as managing hyperthyroidism can sometimes adversely affect kidney function.

How is hyperthyroidism diagnosed?

In senior cats, diagnosis of hyperthyroidism can be tricky. Your vet will perform a physical exam and palpate your cat’s neck area to see if the thyroid gland is enlarged.

Your vet will also likely need to complete a battery of tests to diagnose hyperthyroidism in your cat, as senior cats also commonly experience other diseases such as chronic kidney failure, intestinal cancer, inflammatory bowel disease, diabetes and more. These share clinical symptoms with hyperthyroidism.

A chemistry panel and complete blood count (CBC) can help eliminate diabetes and kidney failure as causes for your pet’s symptoms.

Sometimes, a simple blood test showing elevated levels of T4 in the bloodstream may be sufficient to provide a definitive diagnosis, though you should keep in mind that this is not true for 100% of cats. This is because some will have mild cases of hyperthyroidism or concurrent illnesses, which may result in unstable levels of T4. They may also show elevated levels of T4 if another illness is a factor in the result.

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

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.