A serum copeptin cutoff value greater than 3452 pg/mL is associated with poor glycemic control (HbA1c >7%) in adults with type 2 diabetes, with 90% sensitivity and 95% specificity, and a cutoff greater than 370 pg/mL is associated with diabetic kidney disease, with 95% sensitivity and 90% specificity.

High blood sugar overstimulates the brain's stress sensors, causing the body to release too much of a hormone that tells the kidneys to hold onto water and constrict blood vessels. This increases pressure inside the kidney filters, damaging them over time and causing protein to leak into urine. At the same time, the excess hormone worsens insulin resistance and reduces insulin production, making blood sugar even harder to control.