input: [It might be...] if sudden, if accompanied by diarrhea, or the gerbils sides have sunken in suddenly, treat as above under diarrhea (namely, see the vet). If the weight loss has been more gradual, first check your gerbils teeth. Gerbils should have two nice, fairly long top teeth and two very short lower teeth. The mouth should be able to close fully. If your gerbil is not chewing cardboard voraciously and not eating well, teeth could be the problem. Try a soft diet for a couple of weeks, then check the teeth again. If they still don't look right... you know, see the vet.. In an older animal, extreme loss of weight usually signals a tumor. [My Gerbil...] 
output: looks thin; her head looks more pointy, and I can feel her backbone