Several months ago, preliminary research suggested a link between hyperthyroidism in cats and a resin that was sprayed into the interior of pop-top cans used for cat food. I immediately took my cats off canned food but when I discussed this with my vet recently, she informed me that researchers are now looking at fish as a cause.
This is in the Short Takes section of the April 2005 issue of Cat Watch, Vol. 9, No. 4 in which they cite the journal Veterinary Medicine Vol. 99, Issue 11:
.... A[n] article in the ... journal examined possible causes of feline hyperthyroidism and elminated several possible culprits: exposure to fertilizers and herbicides, regular use of flea products and having a smoker in the house. But the jury is still out on heavy doses of fish, liver and giblets flavors of canned cat food, which the journal said "may have a significantly increased risk of hyperthyroidism."