Hi, Patti -  The beauty of leptin testing is two-fold: it can tell you whether or not your horse's appetite regulation is controlled;  but more importantly, it can help tell if an elevated insulin is being driven more by PPID, or by an IR condition at baseline.  If you are happy to treat your horse as IR, (currently essential in Fargo's case, given the crestiness and fat deposits)  then you could forgo the leptin.  However, if the sample is sent to Cornell, adding leptin adds very little cost to the whole package. You still need the ACTH, glucose and insulin.  Might as well ask for leptin as well. Cornell's cost for ACTH and insulin is $44.00;  the cost for ACTH, insulin and leptin is $66.00 (don't forget to ask for glucose as well, because it isn't included in those panels)  That is the lab cost; what your vet will charge will be quite a bit higher.

https://ahdc.vet.cornell.edu/test/list.aspx?Species=Equine&WebDisc=&TstTyp=&Test_Name=acth

 

Regarding metformin:  it can be useful for emergency insulin elevations, such as break-outs onto grass and so on.  However, it has been shown in horses that any clinical effect seems to disappear after 3 months of use, so as a long-term aid it isn't helpful.  Diet and exercise are the insulin busters; metformin is only a temporary band-aid at best. (you can read that as: "Not worth the money or pain of grinding 30 or more tablets twice daily")

Hope that helps.