Hi, Jody - Not Dr. Kellon, but I can give you an informed opinion. Given that he has laminitis, there are three likely causes: IR alone; IR driven by Cushing's; or something from left field, like Lyme disease (not common in WA, but apparently not impossible, either).  Since he is hurting, I would continue with the metformin (if he were mine), just in case it is helping. Anecdotally, metformin hasn't seemed to drop hyperinsulinemic horses down to normal, and the effects of metformin seem to wane after 3 months or so. If you get the standard insulin, glucose, leptin and ACTH, you should be well on the way to a diagnosis even with metformin on board.