It's embarassing to admit that up until a couple of months ago, I had no idea what BPA was, nor why it might be important to cat parents. The fact is that BPA lining in pet food cans may be a contributing factor for hyperthyroidism and other diseases and conditions in cats.
My Joey was only eight years old when he was diagnosed as hyperthyroid - a disease which is more common in older cats. I had been feeding him various brands of canned cat foods since adopting him and his brother, Jaspurr at 6.5 weeks. BPA is a hard, clear plastic, which is used "as a protective lining on the inside of metal-based food and beverage cans," according to the FDA. It has been approved for that purpose since the 1960s. However, studies done over the years have linked BPA to a number of health concerns in humans, and at least as a contributing factor to hyperthyroidism in cats.
It is too late for me to undo whatever damage has been done to my cats by feeding them canned food contaminated by BPA. However, going forward, I am using only foods in cans the manufacturers declare to be free of BPA. This last is the hard part, as there are hundreds of various cat food brands and formulas. So far, among those who are free of BPA lining in the cats appear to be the 3 1/2 oz. and 5 oz cans. The larger cans are questionable. As Susan Thixton has said, the only way to know for sure is to call the manufacturer.