Hi Kari:
Your horse could be Insulin Resistance and NOT be Cushings so please keep that in mind. Do you know what tests your vet is running? You need to ask for an ACTH test as well as a T4 and T3 tests.
I also moderate the Yahoo Cushings/IR (Insulin Resistance) site and I highly recommend that you join our group. Below is part of the Newbie primer that we send out to members, which defines IR.
BIG question -- is your horse displaying any symptoms such as inappropriate sweating? Are his feet at all sore? Another HUGE question - what are you feeding him right now? I cannot stress enough how important diet is right now to your horse. If he is IR, he can develop laminitis very, very quickly. He needs to be on a low starch, sugar free diet right now.
INSULIN RESISTANCE: A condition where the cells of the body do not respond to insulin like they should. As a result, the pancreas needs to produce more insulin than normal to get glucose and protein into the cells. This is similar to early type 2 diabetes in people – the type that does not require insulin. People with insulin resistance commonly also have an elevated blood sugar (blood glucose) level, which we don’t see in horses except in very severe cases. Insulin resistant horses will have abnormally elevated insulin though. Insulin resistance may be seen as a result of the high cortisol production in classical Cushings Disease. Might also be found in association with hypothyroidism.
Ponies and “easy keeper” breeds of horses appear to be genetically programmed to be insulin resistant. In that case, it probably should not be considered a disease or disorder per se but does mean you have to be careful with their diet.
Here is the link to the group:
http://pets.groups.yahoo.com/group/EquineCushings/
Liz R.