When ADH Is Off But Sodium Still Falls: Water Intoxication, Exercise Hyponatremia, and the Low Solute Trap

Every episode so far has been about ADH being on when it shouldn't be. Today we flip that entirely.This episode covers hyponatremia where ADH is suppressed, the urine is maximally dilute, and the kidneys are doing exactly what they're supposed to — but the patient is still hyponatremic. Either the water intake is overwhelming the kidneys' capacity, or there isn't enough solute to carry the water out.We work through four presentations: acute water intoxication from a college drinking contest, where the critical teaching point is that rapid correction is safe in acute hyponatremia (unlike chronic); exercise-associated hyponatremia in a marathon runner, including field treatment with hypertonic saline and why normal saline makes things worse; tea-and-toast hyponatremia, with the solute math that explains why a patient eating almost nothing can develop hyponatremia even with maximally dilute urine and normal ADH; and primary polydipsia, where the diagnosis is pure volume overwhelming a compromised excretory system.A three-question framework at the close ties the group together: is intake overwhelming capacity, is solute too low, or is excretory capacity compromised?Final episode of the hyponatremia series next: cirrhosis and dialysis — the special populations where the standard algorithm breaks down.