Sodium metabolism
Hypernatremia
Hyponatremia
WikEM overview on hyponatremia: https://www.wikem.org/wiki/Hyponatremia
Quick hits on hyponatremia: http://lifeinthefastlane.com/ccc/hyponatraemia/
Another good hyponatremia overview (with some links to cases): http://lifeinthefastlane.com/investigations/hyponatraemia/