To follow up on what Westtexas said, I'd suggest talking to your vet about routine blood work for your geriatric cat(s). We get something our vet calls the "Senior 7" as part of our older cat's semi-annual exam. It won't catch everything that can befall an aging cat, but it keeps track of some major things (like thyroid and kidney function). Obviously, it's an additional expense, and your vet may only suggest having it done once a year. I sure wish I'd used it when my now-deceased cat was getting up in years. We would have caught some issues earlier and provided her with a better quality of life as she aged. Like your cat, she lost weight that I didn't detect at first because she was a very furry medium haired cat. And once I did, I just chalked it up to old age. She was hyperthyroid as it turns out.