Hypernatremia due to increased water retention EXCEPT

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Hypernatremia reflects a relative deficit of free water or an excess of sodium. When it is due to increased water retention, sodium and water are both retained more than normal, often with conditions that promote sodium reabsorption (like aldosterone excess), which can push serum sodium upward if water intake doesn’t match.

Diabetes insipidus is the exception because it causes hypernatremia through free-water loss, not water retention. In DI, the kidneys cannot concentrate urine due to lack of ADH action (central DI) or renal insensitivity (nephrogenic DI), leading to large volumes of dilute urine and ongoing free-water loss. This raises plasma osmolality and sodium concentration despite not retaining water. The other scenarios involve mechanisms where water is retained relative to sodium or where sodium handling promotes hypernatremia in the context of water retention.