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What Is Hyperthyroidism in Cats and Should I Worry?

Hyperthyroidism, or overactive thyroid, is a condition that results from excessive production of thyroid hormone. It’s considered to be the most common hormone-related (endocrine) disease of middle-age and older cats. While virtually unrecognized by veterinarians 40 years ago, today hyperthyroidism is diagnosed in about 10 percent of cats over 10 years old.

If your favorite feline is approaching their senior years, you’ll want to know more about hyperthyroidism and its symptoms.

What is hyperthyroidism in cats?

A cat’s thyroid gland has two lobes, one on each side of the trachea (windpipe). This gland produces the hormones T4 (thyroxine) and T3 (triiodothyronine), which help regulate the body’s metabolic rate. If greater-than-normal amounts of these hormones are present, they rev up kitty’s metabolism, stressing nearly all of their organ systems including their heart and digestive tract.

The most common cause of hyperthyroidism is a benign (noncancerous) tumor, called an adenoma, that results in an increased number of cells in the thyroid and often noticeable enlargement of the gland. A very small percentage of hyperthyroidism is caused by adenocarcinoma (malignant cancer).

Weight loss is the most common sign

Cats with hyperthyroidism typically show a range of symptoms, which may be subtle at first but become more obvious as the condition progresses. Weight loss is seen in up to 98 percent of hyperthyroid cats, despite many of them having voracious appetites. Other signs may include drinking more than usual, urinating (peeing) more, hyperactivity (restlessness), crankiness or aggression, rapid heart rate and/or abnormally large thyroid glands. Overactive thyroid glands may also cause vomiting, diarrhea, panting and poor hair coat.

If you recognize any of these symptoms in your kitty, be sure to have them checked over by your veterinarian. The most common signs of hyperthyroidism — weight loss and increased appetite, thirst and urination — are also common signs of diabetes and kidney disease, which can be problems for older kitties, too. Your veterinarian is the best one to determine the underlying cause of your cat’s symptoms and will recommend appropriate treatment.

hyperthyroidism in cats

How is hyperthyroidism diagnosed?

An overactive thyroid is diagnosed through a physical exam and bloodwork. In fact, hyperthyroidism — and other common senior kitty conditions — may be detected early through twice yearly exams and testing that start when your feline friend is about 7 years old.

During the physical exam, your veterinarian will palpate, or feel, your cat’s neck to check for enlarged thyroid glands. Your cat’s heart rate and rhythm will definitely be evaluated and their blood pressure may also be checked, since high blood pressure can occur at the same time as hyperthyroidism and kidney disease. Your veterinarian will most likely recommend blood testing that includes a thyroid hormone level to detect increased levels of T4 in the blood.

Hyperthyroidism can be treated and even cured

An overactive thyroid, if left untreated, will cause your cat’s symptoms to progress and can lead to serious complications involving their heart and kidneys. The good news is that there are four options available for treating hyperthyroidism in cats:

  • Medication
  • An iodine-restricted diet
  • Surgery
  • Radioactive iodine therapy

The treatment options that your veterinarian recommends will depend on specific factors, such as the severity of your kitty’s hyperthyroidism, their age and physical status (specifically, their heart and kidney functions) and the availability of radioactive iodine therapy in your area. Each therapy has its advantages and disadvantages, which you’ll want to discuss with your veterinarian.

Most cats, even those in their teens, respond well to treatment. And when properly treated and managed, hyperthyroid kitties can live normal lives.

The information in this blog has been developed with our veterinarian and is designed to help educate pet parents. If you have questions or concerns about your pet's health or nutrition, please talk with your veterinarian.

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