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Source: Chessman KH, Haney J. Disorders of sodium and water homeostasis. In: DiPiro JT, Talbert RL, Yee GC, Matzke GR, Wells BG, Posey LM, eds. Pharmacotherapy: A Pathophysiologic Approach. 10th ed. New York, NY: McGraw-Hill; 2017. Accessed April 18, 2017.


  • Serum sodium <135 mEq/L [<135 mmol/L])


  • Result of excess extracellular fluid (ECF) relative to sodium because of impaired water excretion.

    • Drug-induced (eg, diuretics, psychotropics)

    • Ingestion of excessive quantities of fluids.

    • Administration of hypotonic fluids to hospitalized patients.

  • Syndrome of inappropriate antidiuretic hormone (SIADH)

    • Associated with oncologic disease, especially small cell lung cancer.


  • Homeostasis: Physiologic processes that maintain a relatively stable equilibrium between interdependent elements.

    • Mechanisms for regulating blood volume and plasma osmolality involve control of sodium and water balance.

      • Homeostatic mechanisms for controlling blood volume focused on controlling sodium balance.

      • Plasma osmolality largely determined by serum sodium concentration and controlled by water balance.

      • Sixty percent of total body water distributed intracellularly; 40% contained in extracellular space.

        • Sodium, chloride, and bicarbonate comprise >90% of total osmolality of ECF.

        • Intracellular osmolality depends primarily on concentration of potassium.

  • Arginine vasopressin (AVP), commonly known as antidiuretic hormone, released from posterior pituitary when plasma osmolality increases by 1–2% or more.

  • Classified as isotonic, hypertonic, or hypotonic depending on serum osmolality (Figure 1).

    • Hypertonic hyponatremia associated with increased serum osmolality, most commonly due to hyperglycemia.

    • Hypotonic hyponatremia most common with many potential causes.

      • Hypovolemic hypotonic hyponatremia associated with loss of ECF volume and sodium, with loss of more sodium than water.

        • Relatively common in patients taking thiazide diuretics and typically develops within 2 weeks of initiation of therapy (urine sodium >20 mEq/L [>20 mmol/L]), Extrarenal sodium loss with diarrhea (urine sodium <20 mEq/L [<20 mmol/L]).

      • Euvolemic hypotonic hyponatremia associated with normal or slightly decreased ECF sodium content and increased total body water and ECF volume.

        • Most commonly result of SIADH release.

      • Hypervolemic hypotonic hyponatremia associated with increase in ECF volume in conditions with impaired renal sodium and water excretion, such as:

        • Cirrhosis.

        • Congestive heart failure (CHF)

        • Nephrotic syndrome.


Diagnostic algorithm for the evaluation of hyponatremia. (HF, heart failure; EABV, effective arterial blood volume; SIADH, syndrome of inappropriate antidiuretic hormone; UNa, urine sodium concentration [values in mEq/L are numerically equivalent to mmol/L]; UOsm, urine osmolality [values in mOsm/kg are numerically equivalent to mmol/kg].) Reprinted with permission from Wells BG, DiPiro JT, Schwinghammer TL, et al. Pharmacotherapy Handbook. 10th ed. New York, NY: McGraw-Hill; 2017.


  • Most common electrolyte abnormality in clinical practice.

  • Nursing home incidence 2-fold higher than in similar aged-community dwelling individuals.


  • Athletes should consider oral fluids that contain electrolytes.

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