The normal value of urea nitrogen is 3.2 to 7.1 mmol/L, and the reference range varies slightly from hospital to hospital. There are several reasons for lower than normal values. For example, it is caused by insufficient protein intake and liver function damage. 1. Insufficient intake: it may be related to eating too little protein, pregnancy and so on. Patients with insufficient protein intake, coupled with abnormal creatinine production, a large number of consumption. So the low intake of protein food will lead to low urea nitrogen, but it is a normal physiological change, generally do not need special treatment. 2. The manifestation of liver function injury. This is because the liver is an important organ for synthesizing urea, and when the liver’s ability decreases, the urea in the patient’s body will naturally decrease. For this situation, patients need to do further liver function tests. Low urea nitrogen should be reviewed regularly, and if necessary, it is recommended to go to the hospital to improve the relevant examination to clarify the cause, so as to avoid delaying the condition.