Etiologies of increased serum sodium include decreased fluid volume due to dehydration. There are also disease induced renal disorders such as acute and chronic glomerulonephritis with sodium and water retention, but there can be no significant change in serum sodium on clinical testing because of concomitant water retention. Endocrine disorders, such as primary or secondary aldosteronism, may cause hypernatremia; Cushing’s syndrome may have mildly elevated serum sodium, or long-term use of adrenocorticotropic hormones may cause hyperabsorption of renal tubular sodium, resulting in high serum sodium. Brain injury, which can cause hypernatremia, becomes traumatic uremia due to central disorders of osmotic pressure regulation, urine cannot be concentrated, fluid is lost, serum sodium is increased, plasma osmolality is elevated, and hypotonic urine occurs. This condition makes it difficult to normalize serum sodium even with massive hydration. Reflecting the increased serum sodium concentration, while the total amount of sodium in the organism can be increased, normal, or decreased is important for maintaining extracellular fluid volume, regulating acid-base balance, maintaining normal osmotic pressure and cellular physiological functions, and is involved in maintaining normal neuromuscular stress.