Agreed with the above link! It sounds like your guy is Cushings and potentially IR. Go to the link above and there is information on the best way to do testing and which lab is best to use. In short you will want to test thyroid, ACTH, insulin, glucose, and leptin and send it to Cornell's lab. The blood once pulled has to be spun down ASAP and put on ice and shipped that way. All this info can be found in the above link also. Good luck!
I included the test above but to join the group first create a yahoo account then go and join the group. It has invaluable information, shipping discount to send the bloodwork to Cornell, and a plethora of other information.
It is! Unfortunately many vets aren't up to date on Cushings and IR protocol and research so that is why the literature is there. Theses disease are tough and you become your horses's number 1 advocate during it but the information on that page is invaluable!