What is the cause of isotonic hyponatremia?

Isotonic hyponatremia is primarily caused by an increase in the osmotic agents such as proteins or lipids in the blood that can dilute sodium concentration without affecting the water balance significantly. This condition is often seen in scenarios such as hyperproteinemia (e.g., multiple myeloma) or hyperlipidemia, where high levels of protein or fat in the blood lead to a pseudo-hyponatremia. In these cases, the sodium level appears low due to the increased plasma volume occupied by these substances, rather than a true loss of sodium or a decrease in blood sodium concentration relative to fluid volume.

Excessive intravenous fluids or fluid overload generally lead to dilutional hyponatremia, which is different from isotonic hyponatremia as it results from an excess of water not associated with an increase in osmotic particles. Hypertension is unrelated to the classification of hyponatremia and does not influence sodium levels directly in this context. Thus, the correct response highlights the role of elevated proteins or fats in causing isotonic hyponatremia by altering plasma composition rather than expanding extracellular fluid volume alone.