I have a 14 year old indoor tabby that has slowly been losing weight this year (and is currently around 9lb, down from 10 or 11 lb), and has reduced her eating a lot. She doesn't respond very much to her favorite foods, and when she does eat it is only a little amount at a time. EG if we feed her 1.5 oz of wet food, it can take up to an entire day before she finishes it (with us putting it in front of her and taking it away multiple times during the day), and even then we have to take the food to her, rather than placing it where she has normally eaten..
We have taken her to our local vet multiple times and have had multiple blood work tests (including thyroid) and an abdominal ultrasound done, but no problems have been indicated. The vet has indicated that the next steps could be exploratory surgery and/or prescription diet.
She is lethargic and has taken to staying inside enclosed places. We've tried introducing kitten (and other) food, but of course strange food is not desirable to her.
I'm running out of ideas of what could be wrong (and treatable), and am fearing that we'll lose her within the next few months. So I'm looking for suggestions on how to best fatten up a cat.
Update. Sadly the cause of her not eating was likely due to a large mass that had grown next to her stomach and was likely putting pressure on it. This was missed in the ultrasound, but may have been seen in an x-ray. Thus no amount of dietary change, or appetite stimulant medicine would ever have helped. Things took a change for the worst this week and we had to say goodbye to her.