There are no medical requirements to hold a US ATP. But you need a first class medical to get an airline job here...
For diagnostic screening we use 12 hours fasting, and 99mg/dL is the upper limit for normal.
100-125 is borderline, >125 is considered diabetic.
Our Aviation Medical examiners don't actually even test blood sugar directly, they test a urine sample. The urine test will only show positive if you have very high blood sugar.
Technically you could probably get a medical in the US if your fasting blood sugar is less than 125 mg/dL. Pre-diabtes may not disqualify you unless it gets worse.
If you are below 100, you would completely legal in the US...won't show up on the ME test and you would not have a diabetes diagnosis. 100-125 you would probably be required to report that to the AME but you would probably still get certified.
But if you meet diagnostic criteria for diabetes you would definitely have to report that, and would probably not get a first class medical.
But that does not mean you are healthy or that the condition will not get worse over time.
Diet and exercise are EXTREMELY effective in reversing marginally elevated blood sugar and preventing full-blown diabetes...look into that.