I happen to be online so ill take a stab
Excessive peeing is called polyuria in vetspeak. It usually goes hand-in-hand w excessive drinking, aka polydipsia (sometimes you'll see shorthand as PU/PD)
Abnormally high sugar levels in the circulating bloodstream can cause PU/PD: the big sugar molecules in the blood are filtered out by the kidneys and pull water with them, resulting in increased urine production. Bc of the increased water loss in the bloodstream, the horse is stimulated to drink more (the polydipsia). That's why you see PU/PD together: its a simple osmotic mechanism that can be caused by any abnormally high substances in the bloodstream (high proteins, etc)
So excessive peeing/polyuria is seen commonly in IR as well as PPID bc of the abnormally high glucose in blood (hyperglycemia). There maybe be other reasons for the PU/PD in Cushings bc of altered hormones, etc, but ill have to let another chime in to clarify
FWIW, one of the first signs that my IR/PPID mare's insulin has spiked (insulin & glucose rise together) is a big jump in her drinking (she lives out so urination is hard to gauge). Ive adapted to using a muck tub filled daily so i can monitor her water consumption very closely.
Hope this helps,
Kerry in NY