Anna, 
I would also urge you to read the sidebar selections in the ECIR website that was linked in your welcome letter. It will help you understand better what is Cushings, what is Insulin Resistance, when the two overlap and what symptoms are unique to each issue. 
The information in the website is based on best veterinary practices as the result of what valid studies are currently available for both Cushings (PPID) and IR.  The definitions, physiological symptoms , changes. etc. are science based from research here and in other countries . If you have a better understanding of what may be happening with your horse, you can make better decisions for him and be able to judge better if you have professionals you are working with that are up to date in recommendations. 
https://www.ecirhorse.org/

Dawn Wagstaff and Tipperary   

Saline, MI  2003

Tipperary Case History

Juniper Case history: https://ecir.groups.io/g/CaseHistory/files/Dawn%20and%20Juniper/Case%20history%20Juniper.pdf .