In hyponatremia, plasma osmolality is typically:

• A. Low plasma osmolality
• B. Normal plasma osmolality
• C. High plasma osmolality
• D. No change in osmolality

Answer: (A)

Plasma osmolality is typically low in hyponatremia because this condition most often represents hypotonic hyponatremia — more water relative to solutes. Sodium is the main contributor to plasma osmolality, so when the sodium concentration drops due to excess free water (dilution) or impaired water excretion, the overall osmolar content of plasma falls.

There are exceptions to keep in mind: if hyponatremia occurs with high levels of osmotic agents like glucose or mannitol, the osmolality is high despite the low sodium. Conversely, some cases show a normal osmolality (pseudohyponatremia) due to laboratory artifacts from high lipids or proteins. But the typical scenario is a low plasma osmolality.