How is hypotonic hyponatremia with dilute urine further assessed

How is hypotonic hyponatremia with dilute urine further assessed?

If the urine is maximally dilute, the pituitary-kidney axis is doing what it is supposed to be doing, and hypotonic hyponatremia is arising from electrolyte-free water ingestion in excess of what the kidneys can excrete. In the typical Western diet, approximately 800 mOsm of solute derived from ingested electrolytes and protein is consumed or generated and excreted in the urine on a daily basis. Assuming a maximal urine diluting capacity of 50 mOsm/kg H 2 O, such a solute load is sufficient to permit excretion of up to 16 L of urine (800 mOsm solute/50 mOsm/kg = 16 L). If electrolyte-free water intake exceeds this limit, the kidneys’ electrolyte-free water excretory capacity is overwhelmed, and hypotonic hyponatremia ensues. This condition, known as primary polydipsia, typically occurs among psychiatric patients. Inadequate solute intake impairs free water clearance. If solute intake decreases to 200 mOsm/day and intake of protein that would lead to urea production is reduced—as occurs with a “tea and toast diet” or beer potomania—the volume of maximally dilute urine the kidneys can excrete may be as little as 4 L. Water ingestion above this amount can more easily be achieved and will also result in hyponatremia.

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