Hyperthyroidism has become an oft-diagnosed sickness in felines in recent years, unfortunately.  New research studies appear to implicate an environmental source, according to the September 2007 Healthy Pet Net newsletter.

Polybrominated diphenyl ethers, also known as PBDE's, are chemicals used only in recent years as a flame retardant. It has have been proven to cause injury to the feline nervous system, while also interfering with the production of various hormones.
Scientists are worried that these chemicals may also lead to similar symptoms in humans as well. Go to the Cat Blog for more information,
Although they have now been eliminated from current production, many products which contain them, such as fabrics, furniture, foams, some plastics and other items, nonetheless still have them, sadly enouph.
The evidence appears to indicate that if your cat or kitten is suffering from hyperthyroidism, it may be because of an object in your huse – one that could be removed, and perhaps decrease the severity of the illness.