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).

Once hyperthyroidism is treated, blood flow to the kidneys decreases to normal, and kidney function returns to what it was before the hyperthyroidism developed. 

Treatment of hyperthyroidism does not damage the kidneys. However, it can unmask underlying but previously undetected kidney disease or make existing [chronic kidney disease (CKD) appear worse. Approximately 15 to 35 percent of hyperthyroid cats eventually develop CKD, which shouldn’t be surprising since each disease is common in elderly cats.

Your friend can begin managing the hyperthyroidism with medication or a diet change, and then evaluate his kidney function and modify his treatment if necessary.

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