Another cause which results in drinking and urinating often, is PPID: also known as Cushings disease. Your veterinarian can do a ACTH test to see if your horse has PPID.
Cushings disease is more common with horses that are 15 years or older, but there are also cases of younger horses. So don’t exclude it immediately.
It can also help to let your veterinarian do a urine analysis besides the blood test.
This enables your veterinarian to see if your horse can concentrate his urine enough.
There’s a rare condition where horses can’t concentrate their urine enough, so they almost immediately pee out the water again, which causes them to drink continually.
It doesn’t happen often that horses have this condition. I happened to have a patient like that this year, but the chances of your horse having this condition are very small.
Sadly, the number one cause that can cause your horse to drink or urinate too much, is out of stress or boredom.
If you have excluded the other causes, this is the last one left. So that means you have to search for ways to stop the boredom or stress, so your horse will drink less.