A patient with hyponatremia, low serum osmolality, and high urine osmolality. What is the most likely diagnosis?

• A. SIADH; treat with fluid restriction
• B. Hyperglycemia
• C. Hypovolemia
• D. Dehydration

Answer: (A)

When you see hyponatremia with a low serum osmolality and a urine that is inappropriately concentrated (high urine osmolality), think about water balance problems driven by excess ADH. This pattern indicates SIADH: the body holds onto free water because ADH is being released inappropriately, which dilutes the serum and lowers its osmolality, while the kidneys keep concentrating the urine. The patient is typically euvolemic, and urine sodium is usually elevated.

Hyperglycemia would raise serum osmolality, not lower it, so it doesn’t fit. Hypovolemia or dehydration can cause hyponatremia too, but they usually present with signs of volume loss and different urine findings, and the overall picture here points to SIADH.

First-line management is fluid restriction to reduce free-water intake and allow the serum sodium to rise gradually.