Definitely try stopping the estradiol. I can't explain how it could make her worse but it isn't helping. I'm wondering if there is prolactin overproduction? Prolactinoma could be her issue although it's never been reported in a horse. The ovarian hormones and involvement of IR can fit with that. I don't know of anyone currently looking at prolactin who could test her. You could do a trial of Evitex (follow product directions for loading dose). https://emeraldvalleyequine.com/product/evitex/ If they don't ship to Canada, this one does and has a "recipe" for making your own although I can't vouch for its effectiveness: https://www.earthsongranch.com/chasteberry-for-cushings-in-horses-make-your-own-organic-tea/
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Eleanor in PA

www.drkellon.com 
EC Owner 2001