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For further information, see CMDT Part 21–02: Hyponatremia

Key Features

Essentials of Diagnosis

  • Must know volume status as well as serum and urine osmolality to determine etiology

  • Hyponatremia usually reflects excess water relative to sodium; total body sodium may be appropriate, low, or high

  • Hyponatremia in hospitalized patients is commonly caused by administration of hypotonic fluids

General Considerations

  • Most common electrolyte abnormality in hospitalized patients

  • Defined as a serum sodium concentration < 135 mEq/L (135 mmol/L)

  • Whether hyponatremia is symptomatic depends on both severity and acuity

  • Severity can be subclassified as

    • Mild (130–134 mEq/L)

    • Moderate (125–129 mEq/L)

    • Severe (< 125 mEq/L)

  • Acuity

    • Acute hyponatremia defined as present for< 48 hours

    • Chronic hyponatremia defined as present for > 48 hours (often diagnosed on routine electrolyte panels)

  • A diagnostic algorithm separates the causes of hyponatremia using serum osmolality, urine osmolality and volume status (Figure 21–1)

  • SIADH is a clinical diagnosis characterized by

    • Hyponatremia

    • Hypoosmolality (< 280 mOsm/kg)

    • Normal thyroid and adrenal function

    • Urine Na usually > 20 mEq/L

    • BUN may be < 10 mg/dL [3.6 mmol/L] and serum uric acid < 4 mg/dL [238 mcmol/L]

    • See Syndrome of Inappropriate Antidiuretic Hormone

Figure 21–1.

A diagnostic algorithm for the causes of hyponatremia using serum osmolality, urine osmolality, and urine sodium. SIADH, syndrome of inappropriate antidiuretic hormone.


  • Isotonic and Hypertonic Hypernatremia

    • Hyponatremia is typically associated with hypoosmolality with two exceptions: pseudohyponatremia and hypertonic hyponatremia.

  • Hypotonic Hyponatremia

    • ADH-independent causes

      • Psychogenic polydipsia

      • Beer potomania and "tea and toast" diet

      • Renal impairment

    • ADH-dependent causes

      • Failure to suppress ADH action may be appropriate (eg. hypovolemia), or inappropriate (eg. in the absence of hypovelemia), which is the syndrome of inappropriate ADH secretion (SIADH)

      • Hypovolemic hyponatremia

        • Occurs with renal or extrarenal volume loss (sodium and water) and subsequent hypotonic fluid replacement

      • Hypervolemic hyponatremia (eg. edematous states of cirrhosis or heart failure)

        • Decreased effective arterial blood volume occurs despite an overall increase in extracellular volume (edema)

        • ADH secretion results

      • SIADH (see Syndrome of Inappropriate Antidiuretic Hormone)

        • ADH is secreted in the absence of an appropriate physiologic stimuli such as a decreased effective circulating volume or hyperosmolality

      • Reset osmostat

        • Vasopressin release is regulated around a lower, or hypotonic, set point

      • Adrenal insufficiency and hypothyroidism

        • Cortisol normally provides negative feedback on ADH release

        • Cortisol deficiency can lead to uninhibited ADH and hyponatremia

        • Hyponatremia in myxedema coma may be due to appropriate ADH release from reduced cardiac output and concurrent adrenal insufficiency

      • Nausea, pain, and surgery

        • Nausea and pain are potent stimulators of ADH release

        • Severe hyponatremia can develop after elective surgery in healthy patients due to excessive use of hypotonic fluids

      • Exercise-associated hyponatremia

        • Hyponatremia during or after exercise, especially endurance events (eg. triathlons and marathons)

        • May be caused by excessive hypotonic fluid intake and ADH secretion ...

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