Dear Christopher Cat:  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?

Christopher Responds: Mom recognized the disorder in one of our feline family members who developed the typical clinical signs: weight loss, a racing heart, vomiting and diarrhea.

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

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

Fortunately, feline hyperthyroidism is easily treated. Options include:

  1. Methimazole, a medication that suppresses thyroid function, may be given orally or rubbed on the inner ear flaps.
  2. 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.
  3. Radioactive iodine therapy destroys the thyroid tissue responsible for excessive hormone production.
  4. Surgery removes the affected thyroid gland(s).