Hyperthyroidism is a common disease in cats that most often occurs in older pets. It's usually caused by an overproduction of thyroid hormones by the thyroid glands.

Cats have two thyroid glands in their necks. They play an important role in regulating the metabolic rate, which is the number of calories the body needs to carry out basic functions, according to VCA Hospitals.

Sometimes one or both of the thyroid glands can enlarge, explains Washington State University College of Veterinary Medicine. When that happens, the glands can produce too much thyroxine, a thyroid hormone that increases metabolism in the body. Many organs can be affected, including the heart.

What causes hyperthyroidism?

Researchers aren't sure what causes feline hyperthyroidism, according to the Cornell University College of Veterinary Medicine, but dietary factors and chemicals in food or the environment may be contributing factors.

In a new study published in Environmental Science & Technology, researchers linked widely used household flame retardants to the disease.

Researchers recruited 78 household cats 7 years old and older — some diagnosed with hyperthyroidism and some not — to wear silicone tags which absorb organic compounds. The cats wore the tags for seven days before researchers removed them to record what they found in their home environments. Researchers discovered higher levels of the flame retardant TDCIPP from those cats with hyperthyroidism.

According to the study, "higher TDCIPP exposures were associated with air freshener use, houses built since 2005 and cats that prefer to nap on upholstered furniture."

Symptoms of hyperthyroidism

cat eating Hyperthyroid cats typically have a hearty appetite. (Photo: g215/Shutterstock)

The most common signs of hyperthyroidism in cats are weight loss and an increased appetite, reports WebMD. Nearly all (95% to 98%) hyperthyroid cats lose weight and most (67% to 81%) have a strong appetite.

Other symptoms of feline hyperthyroidism can include:

  • Excessive thirst
  • Increased urination
  • Restlessness or hyperactivity
  • Unkempt fur
  • Increased shedding
  • Diarrhea
  • Vomiting
  • Breathing issues
  • Aggression
  • Vocalizing, especially at night
  • Panting

How is hyperthyroidism diagnosed?

If your veterinarian suspects your cat may have hyperthyroidism, she will do an exam and specifically palpate your cat's neck to see if the thyroids are enlarged. Your vet will likely order a blood test and a test to examine thyroid hormone levels.

Because hyperthyroidism can affect your cat's other organs and overall health, your vet may also order other tests such as a urinalysis.

Hyperthyroidism treatment

woman giving cat a pill Although medication is effective, it has to be given twice daily and it's no fun to administer. (Photo: Dora Zett/Shutterstock)

There are several ways to treat feline hyperthyroidism, with benefits and disadvantages to each method.

Medication — Anti-thyroid drugs help slow the production and release of thyroid hormones from the thyroid glands. They are typically very effective but must be given for the lifetime of the cat. They also can have side effects including vomiting, fever, anemia and lethargy, according to Cornell. Pills typically must be given twice a day, although some medicine is available as a skin gel. Blood tests must be given regularly to make sure the treatment is effective.

Surgery — If only one of the thyroid glands is affected, surgical removal is an option. Surgery is likely a long-term, permanent cure, although it has risks because it's a serious procedure that requires anesthesia, which can often be dangerous for older cats, says WebMD.

Radioactive iodine therapy — Considered the safest and most effective treatment, this involves injecting your cat with radioactive iodine. The iodine is absorbed by the thyroid gland and the radiation destroys abnormal tissue. The treatment has no side effects, doesn't require anesthesia and most often cures hypothyroidism, but is only performed by certain facilities, reports Cornell.

Diet — Not a mainstream treatment, but according to Cornell some research suggests that limiting iodine in the diet of hyperthyroid cats may be an option for treating the disease, particularly when other treatment options aren't possible. Limiting dietary iodine is controversial due to the possible health problems from long-term restrictions and the fear that the diet could backfire and make the condition worse. Research is ongoing.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

Why hyperthyroidism in cats is such a big deal
Hyperthyroidism is a common disease in cats that can affect the entire body.