Which of the following is typical of SIADH?

• A. Fluid overload with hyponatremia and urine osmolality greater than 100 mOsm/L
• B. Hypernatremia with polyuria
• C. Hypotension with dehydration
• D. Low urine osmolality with hyponatremia

Answer: (A)

SIADH involves the body retaining water due to inappropriately high ADH secretion, which dilutes the serum and leads to hyponatremia while the kidneys keep reabsorbing water. Because of this, you see a relatively fluid-overload or euvolemic state with a urine that remains concentrated. A urine osmolality above 100 mOsm/L in the setting of hyponatremia reflects this inappropriately concentrated urine, signaling SIADH.

So the description of fluid overload with hyponatremia and urine osmolality greater than 100 mOsm/L matches SIADH. The other scenarios point to different problems: hypernatremia with polyuria suggests diabetes insipidus; hypotension with dehydration indicates hypovolemic states; hyponatremia with very dilute urine would imply the kidneys are excreting free water appropriately, not SIADH.