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Dear Christopher Cat

What causes hyperthyroidism?

Christopher Responds

During old age, one or both of the cat’s thyroid glands may develop benign growths that produce excessive thyroid hormone. Rarely are the growths cancerous.

The excess thyroid hormone speeds up metabolism, causing the typical signs of hyperthyroidism: weight loss despite an excellent appetite, sometimes with diarrhea or vomiting.

No one knows what causes the disease, but veterinarians have some theories.

  • Some cats may be genetically predisposed, because the disease seems to occur less frequently in certain breeds, notably Siamese and Himalayan cats.

  • Cats that eat canned food are five times more likely to develop hyperthyroidism than cats that eat dry food.

Many pet food cans, particularly pop-top cans, are lined with bisphenol-A-diglyciddyl ether, which may adversely affect the thyroid gland.

In countries that don’t permit such use of this chemical, feline hyperthyroidism is uncommon.
Still, about 25 percent of hyperthyroid cats have never eaten canned food.

Another theory is that the amount of iodine, known to affect the thyroid gland, is more variable in canned food than dry food.

  • Indoor cats are predisposed to hyperthyroidism, and published reports have focused on flame retardants, such as polybromated diphenyl ether (PBDE), used in furniture, mattresses, carpet padding and electronic equipment.

PBDE is chemically similar to thyroid hormone, and its use over the past 30 years corresponds with an increased incidence of feline hyperthyroidism.

Indoor cats have very high PBDE blood levels; however, levels in hyperthyroid cats are the same as in healthy cats, so PBDE is an unlikely culprit.

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