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Hyperthyroidism common in older cats

Q: My friend’s cat and I were diagnosed with hyperthyroidism the same week, so I can’t help wanting to understand the disease we share. What causes hyperthyroidism in cats? How is it treated?

A: Hyperthyroidism, over-production of thyroid hormone by one or both thyroid glands in the cat’s neck, speeds up metabolism. Typical clinical signs include weight loss, a racing heart, vomiting and diarrhea.

The cause is uncertain, but several factors may play a role:

  • Advanced age. The average age at diagnosis is 13.
  • High iodine intake. Iodine levels in cat foods vary widely, with some foods containing up to 10 times the recommended amount of iodine.
  • Canned cat food. Risk increases with canned food that contains fish and with pop-top cans and cans lined with bisphenol-A-diglyciddyl ether.
  • Flame retardants. Including polybromated diphenyl ethers (PBDEs), which are chemically similar to thyroid hormone. PBDEs are used in furniture and carpeting, where cats spend a lot of time. They don’t metabolize PBDEs as efficiently as humans, so the chemicals reach high levels in their bodies.
  • Genetic predisposition. For example, Siamese and Himalayan cats are at decreased risk.

Fortunately, feline hyperthyroidism is easily treated. Options include:

  • Methimazole, a medication that suppresses thyroid function, may be given orally or rubbed on the inner ear flaps.
  • Hill’s y/d, a prescription food containing limited iodine, is an alternative to medication. It is available in dry and canned forms, and recipes are available to make treats.
  • Radioactive iodine therapy destroys the thyroid tissue responsible for excessive hormone production.
  • Surgery removes the affected thyroid gland(s).

Lee Pickett, V.M.D. practices companion animal medicine. Contact her at askdrlee@insurefigo.com.

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