I think in her case, the muscle loss is just due to the fact that she was in a really intense work-out program for a horse her age (3-4 1+hr rides/week, over really hilly terrain, all gaits) and that she typically builds muscle in a more bulky way...and then now she's not being ridden at all really so the bulky muscle is flattening out which seems weird to my eye...if that makes sense?
I'm not too worried about the muscle-loss itself since she's really returning to what she looked like (muscle-wise) 4 years ago when I bought her out of an 11 year retirement.
That was actually what made me wonder! "Good" thin or bad thin? I know my eye prefers a chubby horse so I'm probably biased towards any thin = "PANIC". Haha
She is insulin resistant. At least, my vet and I have chosen to treat her like she is IR given her history, fat pads in the summer, etc. We determined that the least expensive option (I'm a college student with very limited funds), as long as she doesn't show a need to be tested (ie, no signs of cushings, etc), we would just assume that she's IR and manage her as such.
Re: her ribs - that's something I forgot to clarify. She's one of those really wide barreled horses that can look somewhat ribby even when the rest of her says she's obese. Since the ribs aren't constantly visible, I was unsure. Her backbone still has a good layer of chub, her neck is still somewhat cresty, her butt is still really filled out, it's just those ribs that say she's thin.
Here's a picture, a la last week:
That's a good point about a loose molar - would there be any other signs to look for, if that were the case?