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Isovolemic Hypoosmolar HyponatremiaAka: Euvolemic Hypoosmolar Hyponatremia

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  1. See Also
    1. Hypoosmolar Hyponatremia
  2. Definition
    1. Hyponatremia with low Serum Osmolality and normal ECV
  3. Causes
    1. Urine Sodium < 10
      1. Water Intoxication (Psychogenic polydipsia)
        1. Free water intake exceeds free water excretion
    2. Urine Sodium > 20-30
      1. Syndrome Inappropriate ADH Secretion (SIADH)
      2. Hypothyroidism
      3. Anterior hypopituitarism
        1. Selective glucocorticoid deficiency
        2. Incomplete ADH suppression
      4. Nausea, pain, emotional stress
      5. Potassium depletion and Diuretics
      6. Reset Osmostat
        1. Common in Pregnancy
        2. ADH released at lower levels of Serum Osmolality
        3. Serum Sodium establishes a new lower steady state
      7. Medications
        1. Amitriptyline (Elavil)
        2. Carbamazepine
        3. Clofibrate
        4. Cyclophosphamide
        5. Morphine Sulfate and other Narcotics
        6. Vincristine
        7. NSAIDs
        8. Barbiturates
        9. Thiazide Diuretics (e.g. Hydrochlorothiazide)
        10. Chlorpropamide
  4. Differential Diagnosis
    1. Often difficult to distinguish Iso- from Hypovolemic
    2. See Hypovolemic Hypoosmolar Hyponatremia
  5. Evaluation: Diagnostic testing to continue
    1. Chest XRay
    2. Brain CT
    3. Chem8
    4. Thyroid Stimulating Hormone (TSH)
    5. Free Urine Cortisol
  6. Management
    1. See Hyponatremia Management
    2. See SIADH Management
    3. Restrict water intake to 1 liter per day
      1. Calculate excess Total Body Water to be excreted
  7. References
    1. Kone in Tisher (1993) Nephrology, p. 87-100
    2. Levinsky in Wilson (1991) Harrison's IM, p. 281-84
    3. Rose (1989) Acid-Base and Electrolytes, p. 601-38

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