What Are the Causes of Hyperthyroidism in Cats?

Hyperthyroidism occurs due to enlarged thyroid glands.
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The two thyroid glands located on either side of your cat's larynx have a big job. They secrete hormones that manage your cat's organs and tell them how hard to work. Too much hormone causes havoc in the body, as the organs work overtime to process the excess.


Hyperthyroidism occurs through no fault of yours or your cat. Father Time is mostly to blame, with a sprinkling of bad luck. Older cats commonly suffer from hyperthyroidism, due to enlarged glands. In most cases this enlargement is caused by benign tumors, while some rare instances involve malignant ones. At this point in time no other causes have been identified, nor do we fully understand why some cats develop these tumors while others don't.


A frustrating aspect of hyperthyroidism is the fact that, because just about all his organs are affected, any symptoms he displays are vague and seemingly unrelated. Your cat's appetite may be off the charts, but he's losing weight. He's suddenly crankier than usual, or he's draining his water bowl in a day. You may notice an increase in hyperactivity, or that he's hitting the litter box more often than normal. The excess thyroid hormone in his system is sending his body into overdrive, which is a danger to his organs if left untreated.

Secondary Medical Issues

The mere presence of too much thyroid hormone isn't necessarily a danger to your cat in and of itself, but the secondary issues it causes to his organs are. Hyperthyroidism causes your cat's organs to work harder than they should, eventually causing damage and additional problems. If left untreated, your cat's heart and kidneys could suffer irreparable damage. These secondary medical problems usually resolve once the hyperthyroidism is treated, but some issues may stick around for the rest of your cat's life.

Diagnosis and Treatment

Older cats, like people, are prone to various aches, pains and the occasional kidney infection, so diagnosing hyperthyroidism sometimes requires a multitude of tests to check thyroid hormone levels and rule out other conditions. Once properly diagnosed, your cat has three options for treatment: surgery, medications or an injection of radioactive iodine. Both surgery and the iodine injection remove and destroy the tumors in the glands, essentially curing the cat. Medication helps correct the issue, but is a lifelong treatment option and offers unwanted side effects.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.

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