Which type of hyponatremia is characterized by hyponatremia with no signs of dehydration or volume retention?

• A. Hypovolemic
• B. Euvolemic
• C. Hypervolemic
• D. Hypertonic

Answer: (B)

Euvolemic hyponatremia means there’s low serum sodium without a change in effective circulating blood volume—patients don’t appear dehydrated, nor do they show signs of fluid overload. In this state, total body water is increased enough to dilute sodium, but there isn’t edema or dehydration on exam. A classic scenario is SIADH, where ADH is inappropriately elevated, causing water retention with continued salt loss; the urine is often inappropriately concentrated and sodium excretion continues, yet the patient remains euvolemic. This contrasts with hypovolemic hyponatremia, which presents with dehydration signs and reduced circulating volume, and hypervolemic hyponatremia, which shows edema and fluid overload. Hypertonic hyponatremia, caused by high plasma osmolality (for example, severe hyperglycemia), is a separate category. So when hyponatremia occurs without dehydration or volume retention, the correct classification is euvolemic hyponatremia.