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Feline Hyperthyroidism

This excess thyroid hormone has a number of effects on your cat’s body. Symptoms commonly seen in cats with hyperthyroidism include:

  • An increased appetite (sometimes described as a voracious appetite)
  • Weight loss (often despite an increase in appetite)
  • Increased thirst
  • Increase urination
  • Vomiting
  • Diarrhea
  • Restlessness/hyperactivity

Besides these symptoms, several other complications can occur in cats suffering from hyperthyroidism. Heart disease can occur as a result of the toxic effect of the circulating thyroid hormones on the heart. High blood pressure (hypertension) is another potential complication.

Kidney disease is also commonly diagnosed concurrent with hyperthyroidism in cats. Cats suffering from both diseases may need treatment for both and the diagnosis of kidney disease in a cat with hyperthyroidism can affect the cat’s prognosis.

There are several options for treatment of cats with hyperthyroidism.

  • Radio-iodine treatment, or I131 treatment, uses radioactive iodine to kill the diseased tissue in the thyroid gland. Most cats undergoing I131 treatment are cured of the disease. However, these cats must be monitored for hypothyroidism after treatment.
  • Surgical removal of the diseased thyroid gland is another potential treatment. Like I131 treatment, surgical treatment is curative but these cats also must be monitored afterward for hypothyroidism.
  • Medical treatment with methimazole is probably the most common treatment choice. This medication can be administered by mouth or can be formulated into a transdermal gel which can be applied to your cat’s ear. Methimazole is effective in controlling the symptoms of hyperthyroidism. However, it does not cure the disease and, if this treatment option is elected, your cat will need to receive the medication for the rest of his life.
  • Feeding a diet restricted in iodine is a newer alternative for treatment of feline hyperthyroidism. Like methimazole treatment, this alternative is not curative and your cat will require lifelong treatment.

According to Dr. Ellen Behrend, who presented some new facts and knowledge about feline hyperthyroidism at the 2013 American Animal Hospital Association conference, cats undergoing curative alternatives for hyperthyroidism (I131 or surgical treatments) tend to have longer survival times than those cats undergoing medical or dietary therapy alone. This finding is particularly important for cats that are diagnosed with hyperthyroidism at a younger age.

Another finding that Dr. Behrend reported is that compensatory hypothyroidism is more common in treated cats than previously believed and treated cats need to be monitored accordingly. She also mentioned that correcting compensatory cases of hypothyroidism where applicable can improve kidney function and help resolve some cases of kidney disease, in turn giving these cats a higher quality of life and potentially prolonging their lives.

Another potentially more disturbing finding reported by Dr. Behrend is the possibility that sarcomas, an aggressive form of cancer, may be responsible for more cases of feline hyperthyroidism than previously reported. This finding was reported in one study and needs further validation and exploration. At this point, the significance of the finding is questionable and we’ll have to wait to see whether further research supports the findings in this study. Hyperthyroidism caused by sarcoma of the thyroid gland could be significantly more difficult to treat than that resulting from other causes and this finding raises serious concerns about survival rates for these cats.

Have you had a cat that suffered from hyperthyroidism? How did you elect to treat the disease? We invite you to share your experiences.

Dr. Lorie Huston

Image: Alexey U / via Shutterstock

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