What causes feline hyperthyroidism?

To answer the question I’ll refer to two well known, first class books. One source1 heads a section on the subject, “Hyperthyroidism (thyroid cancer). That is a firm statement. The authors states: “Hyperthyroidism in cat is almost always associated with a cancer“, either benign or malignant. The cancer causes increased thyroid hormone production. Second hand cigarette smoke may be important.

The other source2 is far less certain. The author says the disease is almost an epidemic in cats over 10. There has been a “dramatic increase in occurrence”.

The author lists these possible causes:

  • environmental factors – indoor, controlled environment
  • the lining of cat food cans
  • pet food ingredient: soy – soy affects the human thyroid gland.


  1. Cat Owner’s Home Veterinary Handbook
  2. Your Cat by Elizabeth Hodgkins DVM

Regarding the internet, Lisa A. Pierson, DVM of catinfo.org says, “I wish we knew the definitive answer to that question.” She, too, is unsure.

She refers to the coatings of cans with BPA (bisphenol A). She also refers to soy and fire retardents; PBDEs. These are in furniture. She also refers to iodine in cat food.

This post was written because it is one of the feline diseases listed as on the increase in the USA and DW a PoCer wanted some answers because her Bigfoot may have it.

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