One of my boys had an ACTH of 1088 the first time he was tested in Aug. He went from 0 to 6mg in approx. a month's time. ACTH was retested and was at 84 in Sept so because the rise was still in full swing he was upped to 9mg to compensate for further rising and get it even lower. We achieved 46pg/ml the next round of testing, which was acceptable for him.
There are others here that have also had numbers in the 1000+ category so you are not alone. It is highly unlikely that 1mg of pergolide, in a large boy, during the seasonal rise period will bring the ACTH down into the normal ranges but it is possible. How a horse reacts to the meds is a very individual thing. The only way to know is to test about 2 weeks after he has been on the med to see where you stand. Getting the ACTH under good control is vital to managing the disease process over time as even with good control it is a progressive condition. The longer the ACTH stays uncontrolled the harder it becomes to get it controlled.
Have you had a chance to get King's case history done? I see that you have a file but it's empty. Having all that info will help us help you better.
Look in file #8 Pulling it Together. Check the folders Introducing New Feeds and Supplements and the Picky Eaters Checklist for ideas on how to get your boy to accept his changed rations:
Lavinia, Dante, George Too and Peanut
Jan 05, RI
EC Support team