Hypertonic dehydration, also known as primary dehydration or hypernatremia with reduced extracellular fluid, is characterized by more water loss than sodium loss, a serum sodium concentration of 150 mmol/L, and a plasma osmolality of 310 mOsm/L. When there is more water deficiency than sodium deficiency, the osmolality of extracellular fluid increases, antidiuretic hormone secretion is increased, renal tubular reabsorption of water is increased, and urine output is decreased. Aldosterone secretion increases, and sodium and water reabsorption increases to maintain blood volume. If water deficiency continues, the osmolarity of extracellular fluid increases further, intracellular fluid moves to the extracellular level, and eventually the degree of intracellular water deficiency exceeds that of extracellular fluid deficiency, which can eventually lead to brain cell dehydration and cause brain dysfunction. The diagnosis can be made based on the medical history and clinical manifestations. Laboratory findings: (1) high urine specific gravity; (2) elevated serum sodium mostly above 150 mmol/L; (3) mildly elevated red blood cell count, hemoglobin, and hematocrit. According to the different symptoms, hypertonic dehydration is generally divided into three degrees: Mild dehydration: except for thirst, there are no other symptoms. The water deficit is 2% to 4% of body weight. Moderate dehydration: there is extreme thirst with weakness, low urine and high urine specific gravity. Dry lips and tongue, poor skin elasticity, sunken eyes, and often irritability. Water deficiency is 4% to 6% of body weight. Severe water deficiency: In addition to the above symptoms, symptoms of brain dysfunction such as mania, hallucinations, delirium, and even coma appear. Water deficiency is more than 6% of body weight.