Blood urea nitrogen: A measure of the urea level in blood. Abbreviated BUN. Diseases that compromise the function of the kidney frequently lead to increased BUN levels. From Healthy Resources 10 Rheumatoid Arthritis SymptomsHigh-Risk Myelofibrosis: Treatment Options10 Ways to Take Control of Your Migraine Featured Centers What Are the Best PsA Treatments for You?Understanding Biologics 10 Things People With Depression Wish You Knew