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+. Etiological classification of very low urinary sodium excretion: (a) Sodium loss hyponatremia Sodium loss is accompanied by water loss, but salt loss is greater than water loss through water intake or compensation of the body, so sodium loss hyponatremia is hypotonic, including hypotonic dehydration i.e. hyponatremia with reduced extracellular fluid volume. It is commonly seen in vomiting, diarrhea, gastrointestinal drainage resulting in massive loss of gastrointestinal digestive fluid, massive sweating, massive exudate from severe burns, pumping of pleural fluid and ascites, renal dysfunction, hyperalgesia, abnormal ADH secretion syndrome, diabetic acidosis, massive application of diuretics, etc. (ii) Dilutional hyponatremia refers to excessive water retention in the body, excessive overall water volume, the total amount of sodium in the body remains unchanged or slightly increased, and hyponatremia is manifested due to blood dilution, which is also hypotonic. This is also hyponatremia. It is common in patients with psychogenic thirst, who drink a lot of water and the kidneys are unable to excrete it all; brain diseases, malignant tumors, lung lesions, and stress stimuli such as surgery and trauma, which cause an abnormal increase in ADH secretion; and hypothyroidism. Patients with dilutional hyponatremia may have a slight increase in blood volume, so urine sodium is mostly not reduced, often >20 mmol / L plasma osmolality may drop from the normal 285 mmol / L to about 240 mmol / L, and serum sodium is often 130 ~ 140 mmol / L or lower. (iii) Hyponatremic increased total sodium This primary factor is sodium retention, while water retention > sodium retention, and leads to a decrease in blood sodium, also known as expansive hyponatremia. It is commonly seen in congestive heart failure, cirrhosis of the liver in the decompensated phase, nephrotic syndrome and acute and chronic renal failure. This type of hyponatremia is mostly progressive and can often maintain a new equilibrium at a certain hypotonic state. Patients often have a combination of hypokalemia, hypoproteinemia, low urine output, urinary sodium often <20 mmol/L, high urinary potassium, and high urinary relative density. (iv) Asymptomatic hyponatremia is mainly seen in chronic wasting diseases such as severe tuberculosis. The mechanism is not well understood, so some people call it idiopathic hyponatremia. The name of asymptomatic hyponatremia is inappropriate because many early or slowly progressing cases of hyponatremia are asymptomatic. (E) Pseudo-hyponatremia When hyperlipidemia or hyperproteinemia results in the presence of a large amount of hyperosmotic substances such as hyperglycemia and mannitol in the blood. This is called pseudo-hyponatremia. In general, when total serum lipid reaches 60 g/L or total serum protein reaches 140 g/L, the blood sodium concentration decreases by about 5%. (vi) Cerebral salt depletion syndrome caused by injury to the inferior optic thalamus or brainstem, resulting in impaired neural regulation of the kidneys, osmotic diuresis in the distal renal tubules, and increased sodium, chloride, and potassium in the excreted urine, and decreased in the blood. The causes of clinical hyponatremia are sometimes single, but often compound, and need to be fully understood and considered when analyzing the etiology and pathogenesis of hyponatremia. The treatment of hyponatremia and dilutional hyponatremia is referred to "hypotonic dehydration", "overhydration and water intoxication". The main treatment for wasting hyponatremia is to treat the primary cause.