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

Key Features

Essentials of Diagnosis

  • Euvolemic hypotonic hyponatremia, particularly among hospitalized patients

  • Serum sodium concentration < 130 mEq/L (< 130 mmol/L)

  • Urine sodium > 20 mEq/L, urine osmolality usually > 300 mOsm/kg

  • Risk of osmotic demyelination syndrome from overly rapid correction of serum sodium

General Considerations

  • Nonphysiologic ADH secretion: water is inappropriately retained leading to relatively concentrated urine (Uosm is usually > 300 mOsm/kg)

  • Urinary solute handling remains preserved, thus, urine sodium is generally not low (UNa > 20 mEq/L), though it does depend on dietary ingestion of sodium

  • Hyponatremia after exercise, especially endurance events such as triathlons and marathons, is generally thought to be a combination of excessive hypotonic fluid intake and inappropriate ADH secretion

  • Specific universal recommendations for fluid replacement rates are not possible given the variability of sweat production, renal water excretion, and environmental conditions

Table 21–2.Common causes of syndrome of inappropriate ADH secretion (SIADH).

Clinical Findings

Symptoms and Signs

  • Frequently asymptomatic; symptoms depend on severity and acuity of the hyponatremia

  • Symptoms mainly relate to brain edema: nausea, malaise, headache, lethargy, disorientation

  • Most serious symptoms: seizures, coma, brainstem herniation, death

Differential Diagnosis

  • Severe hypothyroidism and glucocorticoid insufficiency can cause hyponatremia that cannot be differentiated from SIADH by urine or serum electrolytes alone

  • Hypovolemic or hypervolemic hyponatremia


  • Clinical in a euvolemic patient

  • Characterized by

    • Hypotonic hyponatremia

    • Absence of heart, kidney, or liver disease

    • Normal thyroid and adrenal function

    • Urine sodium usually > 20 mEq/L, urine osm usually > 300 mOsm/kg

  • In clinical practice, ADH levels are not measured

  • Low blood urea nitrogen (BUN) (< 5–10 mg/dL) and hypouricemia (< 4 mg/dL) may be present


  • Acute (< 24–48 hours, eg, postoperatively) hyponatremia

    • Generally, the 24-hour correction goal rate of 4–6 mEq/L (for chronic hyponatremia) is still recommended for acute hyponatremia, so after a brief period of rapid correction, no further correction may be needed until the next day

    • Monitor serum sodium every 2–8 hours, depending on severity, and hourly urinary output

    • High urinary output suggests ...

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