Very low urinary sodium excretion is a significant clinical manifestation of hyponatremia. The normal value of blood sodium is 142 mmol/L (135-145 mmol/L), and blood sodium below 135 mmol/L is considered hyponatremia. Urinary sodium volume: 70-90 mmol/24h in normal adults, which is approximately 4.1-5.3 g of sodium chloride. if urinary Na+. Differential diagnosis of very low urinary sodium excretion: High urinary porphyrins in the urine: It is due to porphyria. Porphyria (porphyria) is a disorder of porphyrin metabolism characterized by increased excretion of porphyrins and porphyrin precursors in the urine and feces. Porphyria is a congenital disorder caused primarily by deficiencies in various enzymes related to heme synthesis and has a family history of occurrence. Increased urinary estrogens: Measurement of urinary estrogens: There are three main types of urinary estrogens, namely estrone, estradiol, and estriol. The normal value of estrogen in women of childbearing age varies at different stages of the menstrual cycle. During the first 7 days of the menstrual cycle, estrogen levels are very low, and then rise with the development of follicles, reaching a peak on day 13, called the peak of ovulation. It then suddenly decreases and then gradually rises again to reach a peak on day 21, which is called the luteal peak. It then decreases until the onset of menstruation. Functional uterine bleeding estrogen levels remain below normal. Uterine amenorrhea estrogen levels are mostly normal, but amenorrhea due to ovarian function defects or congenital undeveloped ovaries has low estrogen levels but no cyclical changes, and pituitary or subthalamic amenorrhea has generally lower estrogen levels. Persistent sodium excretion in urine: It belongs to the syndrome of abnormal secretion of antidiuretic hormone (SIADH) is a syndrome in which the secretion of antidiuretic hormone persists even when plasma osmolality and blood sodium are normal or low, resulting in a series of clinical manifestations such as reduced free water clearance, water retention, hyponatremia, and low osmotic blood pressure. children with SIADH, in addition to the manifestations of the primary disease, parallel the degree of hyponatremia, serum sodium When the blood sodium drops below 120 mmol/L, symptoms such as loss of appetite, nausea and vomiting may appear. When the urinary sodium content is high and the blood sodium is below 110 mmol/L, neuropsychiatric symptoms, even convulsions and coma until death may occur. Brain damage. Increased excretion of histamine in urine: Histamine is an active amine compound with the chemical formula C5H9N3 and a molecular weight of 111. As a chemical transmitter in the body, it can affect many cellular reactions, including allergies, inflammatory reactions, gastric acid secretion, etc. It can also affect neurotransmission in the brain, causing effects such as sleepiness. The metabolites after taking H1 receptor antagonists (i.e., antihistamines) are excreted in a few to several dozen hours, with the vast majority of the drug in its original form being excreted via the kidneys, with urinary excretion accounting for a very large portion of the excretion. Therefore, it leads to increased urinary excretion of histamine.