Definition and normal values

The term hypernatremia - or hypersodemia - identifies an increase in sodium concentrations in the blood, above those that are considered normal values:

  • normal sodium or natriemia: 135-145 mmol / L *
  • hyponatremia: <136 mmol / L
  • hypersiemia:> 146mmol / L
    • mild hypersiemia <155 mEq / L
    • severe hypersiemia> 155 mEq / L

To understand ... the role of sodium in the body

Sodium is the main electrolyte of the extracellular fluid: 90% of the total body sodium is contained in the extracellular compartment, thanks to the action of the enzyme Na + - K + ATPase (which actively transports the sodium outside the cell).

Sodium is an important regulator of the osmolarity of plasma and extracellular fluid. When the sodium concentration exceeds the normal range (excess of sodium → hypernatremia) there is a more or less significant increase in the volume of blood and interstitial fluid, laying the foundations for creating edema and hypertension. At the same time, intracellular water is reduced and the cell "shrivels" (intracellular dehydration).

Furthermore, sodium is involved in the transmission of nerve impulses, in cellular exchange and in muscle contraction: according to this, we understand how a condition of hypernatremia can upset all these functions to which the organism must fulfill.

  • PLEASE NOTE: since sodium is freely distributed between plasma and interstitial fluid, the concentration of sodium in the blood is equal to that of extracellular fluids. In other words, if the sodium in the blood increases, the sodium concentration in the extracellular spaces is also raised.
  • Since the cell membrane is freely permeable to water, when the concentration of sodium increases in the extracellular component, then the water moves from the intracellular compartment to the extracellular compartment to restore the osmotic balance.
  • To limit the movement of water from the intra-extracellular compartment it is necessary to increase the volume to dilute the sodium, and at the same time increase the urinary loss of the mineral.
  • Ultimately, plasma sodium concentration is an indicator of the state of intracellular volume, whereby hyponatremia means cellular hyperhydration while hypersodium means cell dehydration .


Hypernatremia is a very frequent laboratory finding in clinical practice, although, fortunately, in most cases it does not lead to very high hypersodemia levels. The latter, in fact, are particularly dangerous and lethal in a good percentage of cases.

Hypernatremia, in general, is not caused by an excess of sodium, but rather by a relative deficit of body water which leads to a water impoverishment of the blood with concentration of the mineral. In some cases of hypernatremia the amount of sodium in the blood is even lower than normal, but the volume is reduced to the point of creating hypernatremia.

Under normal conditions, even a modest increase in sodemia above the base threshold causes the stimulus of thirst; the consequent water intake leads to the correction of sodium values.

Hypernatremia is more common among children and the sick (who depend on others for water supply), among the elderly (reduced effectiveness of the thirst mechanism), among subjects with an altered mental state and among those who do not mind to the daily intake of water by exaggerating with that of sodium. Hypernatremia, in general, is worsened by those diseases that cause a fluid loss, such as diarrhea or vomiting, and infections in general.

An increase in blood sodium levels can therefore occur due to:

  1. true (absolute) increase in sodium:
    • increased introduction of sodium with diet compared to water → hypervolemia
    • excessive renal sodium retention → hypervolemia
  2. False increase (relative) of sodium due to a decrease in body water:
    • insufficient intake of pure water with the diet (common + cause) → euvolemia or slight hypovolemia
    • water loss and hypotonic fluid (dehydration) → hypovolemia

In the first case, the absolute blood quantity of sodium increases (primitive hypersodemia), while in the second the blood sodium increases only in relative terms (it is quantitatively the same or even lower, but the volume and body water being lower, is more concentrated).


The classification of hypernatremia in three major classes allows the easier identification of the underlying causes, providing useful indications for therapeutic interventions:

  1. hypervolemic hypernatremia = increase in total body sodium and less increase in total body water: it is caused by an excessive intake of sodium compared to water
  2. euvolemic hypernatriemia = decrease in total body water: it is caused by a lack of intake or loss of pure water
  3. hypovolemic hypernatremia = greater loss of total body water compared to sodium loss: it is caused by the loss of hypotonic fluids
Modification of total body sodium and water in conditions of hypernatremia
Hypovolemic hypernatremiadecreased↓↓
Euvolemic hyponatremiaNormal (↓)-
Hypervolemic hyponatremiaIncreased↑↑


Hype: What is it? Characteristics, Causes and Associated Disorders of G.Bertelli