Question

Low serum sodium with normal serum osmolality is a feature of

• A. Primary polydipsia
• B. Diuretic use
• C. Hyperlipidemia
• D. Hypoaldosteronism

Hint:

Think about which option is a lab artifact rather than a true body water or sodium problem.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We are told that the serum sodium is low but the serum osmolality, measured directly in the lab, is normal. We need to find which condition matches this exact pattern.

Step 2: Key Formula or Approach:
Hyponatremia is grouped by the serum osmolality that goes with it. There are three groups: low osmolality (true or dilutional hyponatremia), high osmolality (an osmotically active solute like glucose pulls water out of cells and dilutes sodium), and normal osmolality, which is called pseudohyponatremia.
Pseudohyponatremia happens when the non water part of plasma, such as excess lipid or protein, takes up space in the sample. The lab method that measures sodium per unit volume of whole plasma then reads a falsely low sodium value, even though the sodium concentration in the plasma water itself, and the directly measured osmolality, are both normal.

Step 3: Detailed Explanation:
Primary polydipsia causes true hyponatremia from drinking too much water. Excess free water lowers both the sodium and the osmolality together, so the osmolality is low, not normal.
Diuretic use, especially thiazides, causes sodium and water loss with impaired dilution, giving low osmolality hyponatremia as well.
Hypoaldosteronism causes salt wasting with low aldosterone, so sodium is lost in urine and osmolality again falls along with sodium.
Hyperlipidemia is different. A high triglyceride or cholesterol level fills a large fraction of the plasma volume with lipid, which carries no sodium. The lab method dilutes and measures sodium against total plasma volume, so it reports a lower than real sodium value. Since this is a lab artifact and not a true loss of body sodium or water, the directly measured plasma osmolality stays normal. This is the classic pattern of pseudohyponatremia.

Step 4: Final Answer:
Low serum sodium with normal serum osmolality is the definition of pseudohyponatremia, caused by hyperlipidemia (or hyperproteinemia). The other three options all produce a low measured osmolality along with the low sodium, so they do not fit the question. Note that some printed answer keys for this paper mark primary polydipsia as correct, but that is inconsistent with the standard osmolality based classification of hyponatremia, so hyperlipidemia is the medically correct choice here.