The causes of high blood urea nitrogen are physiologic and pathologic. Physiological causes are commonly found in eating a large amount of high-protein foods; pathological causes are commonly found in hyperthyroidism, gastrointestinal bleeding, dehydration, renal insufficiency, etc. 1. Physiologic causes are more commonly found in large quantities of high protein foods. 2. Pathologic causes (1) Hyperthyroidism, severe burns and other causes of catabolic exuberance, when it can make too much urea production. (2) Gastrointestinal bleeding, blood decomposition in the gastrointestinal tract leads to excessive urea production. (3) Insufficient renal blood flow due to severe blood loss, dehydration, etc., causing elevated urea. (4) Renal insufficiency, resulting in decreased excretion of urea. At this time, in addition to elevated urea, patients are often accompanied by creatinine, cystatin C elevation. Once the blood urea nitrogen is elevated, it is recommended to go to the regular hospital in time and follow the doctor’s instructions for standardized treatment.