Category: Nephrology

Hyponatremia: Primary polydipsia

Hyponatremia is excess of water relative to sodium and almost always due to increased ADH.

  • SIADH: inappropriate increase (Conrad: any lung, brain, drugs and cancer)
  • SADH: appropriate increase such as in hypovolemia or hypervolemia with decreased effective arterial volume

Evaluation of hyponatremia begins with the following laboratory datas

  1. Plasma osmolality: to determine if the patient really has hyponatremia
    • Pseudohyponatremia: will be high in patients with osmotic substances that draw water to dilute the concentration of sodium
  2. Urine osmolality: useful in limited circumstances  -> Uosm will be < 100
    • malnutrition(decrease solute intake)
    • primary polydipsia(increase water intake)
  3. Urinary sodium: used to determine the kidney’s concentrating capabilities(basically how the body is responding to this state, or did it cause it)
    • Normally should be low < 20

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