An ACTH test may work on this horse as she appears to have been Cushingoid for some time now. It would be wonderful if dietary management would work, but you simply don't have the time at this juncture. Cushingoid horses can be very painful without rotation. The pain seems to originate from deep flexor tendon pull and is often not evident on the sole, especially if the horse has good hoof mass. Horses utilize volatile fatty acids along with glucose for energy and have lower glucose values than humans or many other domestic animals.
Cattle are even lower.
It isn't a very helpful test in the equine. A short course of antibiotics may also be of benefit to lower the Streptococcal organisms in the cecum. I use Naxcel, two bottles, to give eight days of medication. Ampicillin can also be used and I have used 4 grams twice daily for 6 to 10 days. Do not allow the horse out of the stall when on antiinflammatories.