High blood urea nitrogen may be related to renal function damage, pre-renal oliguria, protein breakdown or excessive intake. 1. Renal function damage: such as primary glomerulonephritis, pyelonephritis, interstitial nephritis, renal tumor, polycystic kidney and other renal function damage, glomerular filtration rate is severely reduced, which may lead to high blood urea nitrogen. 2. Pre-renal oliguria: such as severe dehydration, large amount of ascites, cardiac and circulatory failure, hepatic and renal syndrome, etc., causing blood volume insufficiency, reduced renal blood flow and insufficient perfusion leading to pre-renal oliguria, that is, the reduction of urine caused by conditions unrelated to the kidneys themselves, which may lead to the situation of high blood urea nitrogen. 3. Protein decomposition or excessive intake: mainly related to high protein diet, can also be seen in acute severe diseases such as hyperthermia, upper gastrointestinal hemorrhage, massive burns, after major surgery, severe trauma, and hyperthyroidism, resulting in protein decomposition or excessive intake, may lead to high blood urea nitrogen. The causes of high blood urea nitrogen are many, it is recommended to exclude high protein diet and other physiological factors, timely cooperation with the doctor to improve the relevant examination, a clear diagnosis, and follow the doctor’s instructions for treatment.