—Kathryn, via e-mail
A.I would say the answer to this is twofold: good medical management and quality husbandry.
Your horse should be on the proper therapeutic dose of pergolide (brand name Prascend). Dosing should be according to an amount where you see an improvement in symptoms and endocrine testing.
As for the care of your horse during this stressful time of heat, keeping him cool and hydrated are the most important factors. Horses with PPID tend to have decreased or delayed shedding, so frequent body clipping—possibly as often as every two to four weeks during the summer months—can be beneficial.
Also, keeping your horse stabled in a well-ventilated barn with fans and/or fan/misters is important, as anhidrosis (inability to sweat) or excessive sweating can also be signs of PPID.
I believe the most important consideration for your horse is access to free choice water. With this disease they tend to drink more water than a normal horse, and the added heat will increase the urge to drink.