Thyroid problems in cats

Senior cats can develop many health problems, just like people.  The symptoms are often subtle, as cats hide illness well.  Weight loss is one thing that should make you take your cat to the vet.  Remember that your cat is tiny, so weight loss of half a pound is a lot for some cats.  Drinking a lot of water on a regular basis is also a warning sign.  Both symptoms can be a sign of diabetes, kidney failure, or thyroid problems.  Of these three illnesses, thyroid is one of the easiest to treat.  My girl Scully has hyper-thyroid, and all that I need to do to treat it is to give her half of a small pill twice a day.  It's easy, I break it into 3 pieces and mix it in with her canned food.  The pieces are so small that often she doesn't notice them.  Once she took the medicine, she began gaining weight back.
So if you notice either or both of these symptoms, take your cat to the vet.  Initially it may cost 200.00 or 300.00, but once they know what's wrong, you can get your baby healthier and keep them healthier.  For me it's worth it to keep my babies with me as long as possible.
After the initial diagnosis, your vet will probably want to see them every 6 months or so to take blood and see if the medication is still working well.  If you notice changes before this time, though, it can't hurt to take them in and get them re-checked.

All 3 of my babies are in this photo. Scully is the dark circle at the top, Buddy is the gray girl on the pink blanket, and Max is the playful boy in the pink bed.

Scully sleeping in the window perch