Extremely low urinary sodium excretion is the prominent 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 hyponatremia. Urinary sodium: 70-90 mmol/24h in normal adults, which is about 4.1-5.3 g of sodium chloride. if urinary Na+. Diagnostic criteria for very low urinary sodium excretion: 1. Estimation of extracellular fluid volume status Hyponatremia in those with low volume is mainly caused by absolute or relative insufficiency of body fluids, low or falling blood pressure, poor skin elasticity, and laboratory tests that show a rise in blood urea nitrogen and a mild rise in creatinine support the diagnosis. history of gastrointestinal fluid loss, profuse sweating, and urinary sodium <10 mmol/L suggests that the loss of sodium via extrarenal; Urinary sodium >20mmol/L, history of diuretics or diabetes mellitus or hypoadrenocorticism can be identified as transrenal loss. Urinary potassium measurement is also important; high levels often suggest impaired Na reabsorption in the proximal tubule or medullary loops, or are caused by vomiting, diuretics, and so on; low levels suggest aldosterone hypoaldosteronism. If there is a lot of extracellular fluid and at the same time there is edema or third gap fluid accumulation, hyponatremia is mostly caused by cardiac, hepatic, renal and other causes of edema formation, such as no edema, blood pressure is normal, and there is no sign of body fluid, hyponatremia is mainly caused by excessive secretion of ADH, at this time, if the severe oliguria, blood urea nitrogen, creatinine increased significantly, urinary excretion of sodium is still >20mmol/L, for renal failure. If the urine osmolality is obviously reduced (<80mOsm/kgH2O), and accompanied by obvious polydipsia, the disease may be caused by polydipsia, the common cause of psychosis or taking some drugs that cause severe thirst (such as tricyclic antidepressants). 2, antidiuretic hormone secretion dysregulation syndrome (SIADH) clinical diagnostic criteria: persistent hyponatremia with the following four elements: ① no renal, cardiac, pulmonary, adrenal, pituitary dysfunction; ② low osmolality of extracellular fluid; ③ urine can not be diluted normally, given a fluid load (including injection of saline), because the water continues to be stored in the body, Na is still excreted from urine, hyponatremia continues to worsen; ④ restriction of water intake, the urine can not be normal, as water continued to be stored in the body, the Na is still excreted from the urine, hyponatremia (iv) Hyponatremia can be improved by restricting water intake. 3, in the diagnosis of this disease should pay attention to: ① blood uric acid level in the SIADH is usually low, if high, should be excluded from the effective amount of extracellular fluid caused by insufficient; ② blood potassium is usually normal, accompanied by low potassium is often caused by other reasons of hyponatremia, especially vomiting and hyperaldosteronism; high potassium should be aware of the presence of hypoaldosteronism; ③ HCO3-: usually normal, diuretic drugs can be high; aldosterone may also be high; HCO3-: usually normal, diuretics can lead to high; aldosterone can also be high; HCO3-: usually normal, diuretics can be high; aldosterone can be high. HCO3-: usually normal, caused by diuretics can be high; low aldosterone can be low; ④ blood urea nitrogen: mostly low. 4, clinically there are four subtypes of SIADH: ① sustained high level of ADH release, mostly caused by lung cancer, accounting for about 38% of SIADH; ② osmotic value readjustment, manifested as the regulation of ADH secretion is still normal, but the threshold is in the lower osmotic concentration, accounting for about 38%; ③ hypoosmolality has no inhibitory effect on ADH at all, accounting for about 16% of patients with this type of secretion is normal when osmolality is too high, but not when hypoosmolality. This type of patient has normal secretion when osmolarity is high, but cannot fall to zero level in hypotonic anemia; ④ The kidneys are hypersensitive to ADH, and this type has normal ADH level and secretion regulation, and there is no ADH-like substance present in the blood. Determination of hyponatremia is due to loss of sodium, excess water and normovolemic can be identified on the basis of total body water, total body sodium, total body fluid volume can be calculated on the basis of body weight and measured serum sodium values, but the patient's weight before the onset of the disease must be known.