What causes hypertonic hyponatremia?

Hypertonic hyponatremia occurs when there is a significant increase in osmotically-active substances in the blood that dilute sodium levels, leading to a lower concentration of sodium relative to the serum osmolality. Elevated glucose levels serve as an osmotically active substance; when glucose levels are high, such as in uncontrolled diabetes mellitus, it draws water into the bloodstream, which can cause the plasma sodium concentration to appear lower than it actually is.

This phenomenon occurs because the increase in glucose raises serum osmolality and can effectively shift water from the intracellular space to the extracellular space in an attempt to equalize osmotic pressure. As a result, even though the total sodium may be in the normal range, the relative concentration of sodium decreases in the context of this increased osmolality, leading to hyponatremia.

Other potential causes, such as excessive fluid intake or drugs acting as diuretics, would not result in hypertonic hyponatremia but rather in isotonic or hypotonic hyponatremia. Elevated proteins could be a factor contributing to certain conditions like multiple myeloma, but they do not typically lead to hypertonic hyponatremia like elevated glucose does.