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Her bloodwork is not indicative of PPID at this time so unless she tests high during the seasonal rise or positive on a TRH test (which is not reliable during the seasonal rise) there's no reason to put her back on Prascend (assuming that you did stop it as previously discussed). If you're concerned that she is PPID in addition to being IR you want to test sooner than later as you don't want to be 'chasing the rise'.
Putting her on Metformin should show a difference within 7 - 10 days if it's going to work but the only way to know that is to monitor bloodwork and to some extent comfort level. Ideally you would do bloodwork now and see where her insulin is at with diet changes alone. If it's still elevated and you have tightened up the diet as much as possible you would then add the Metformin in and retest in 7-10 days to see if the Metformin is working.
Again, unless she has elevated ACTH on a post-TRH stim test or during the seasonal rise or has symptoms without a positive ACTH result I would not be looking at putting her on Prascend now. Do you have current trim pictures and x-rays that you could post? Her trim could still be an issue at this point in addition to the insulin number.