SIADH
USMLE Step 1Difficulty: mediumLast updated:
A 67-year-old man with a 50-pack-year smoking history is evaluated for 1 week of confusion and fatigue. A chest CT done for weight loss shows a central hilar mass. He takes no medications. Blood pressure is 134/82 mm Hg without orthostatic change; mucous membranes are moist, jugular venous pressure is normal, and there is no peripheral edema. Serum sodium is 123 mEq/L, glucose 96 mg/dL, and BUN 9 mg/dL. Thyroid function and morning cortisol are normal.
Which of the following sets of findings (plasma osmolality in mOsm/kg, urine osmolality in mOsm/kg, urine sodium in mEq/L) is most likely?
Answer and explanation
Correct answer: C. Plasma osmolality 258, urine osmolality 460, urine sodium 58
A smoker with a central lung mass and euvolemic hyponatremia, after hypothyroidism and adrenal insufficiency are excluded, most likely has the syndrome of inappropriate ADH secretion (SIADH). Inappropriate ADH makes the collecting duct reabsorb free water despite low plasma tonicity, so the urine is inappropriately concentrated. Because volume is normal or slightly expanded, the kidney does not conserve sodium. The source describes SIADH as hypo-osmolality with a less-than-maximally-dilute urine (inappropriately high urine osmolality, > 100 mOsm/kg), urine sodium usually > 30 mEq/L, and generally low serum uric acid. A urine sodium < 10–20 mEq/L would instead point to effective hypovolemia (renal sodium conservation), and a maximally dilute urine (< 100 mOsm/kg) points to excess water intake.
Each option explained
- A. Plasma osmolality 258, urine osmolality 70, urine sodium 10Incorrect. Maximally dilute urine with low urine sodium suggests primary polydipsia or low solute intake, where ADH is appropriately suppressed.
- B. Plasma osmolality 258, urine osmolality 460, urine sodium 8Incorrect. Concentrated urine with urine sodium < 10 mEq/L indicates renal sodium conservation from hypovolemia or edematous states, not euvolemic SIADH.
- C. Plasma osmolality 258, urine osmolality 460, urine sodium 58Correct. Hypotonic plasma, inappropriately concentrated urine, and urine sodium > 30 mEq/L are characteristic of SIADH.
- D. Plasma osmolality 296, urine osmolality 460, urine sodium 58Incorrect. SIADH causes hypotonic hyponatremia; a normal plasma osmolality suggests pseudohyponatremia or isotonic causes.
- E. Plasma osmolality 296, urine osmolality 80, urine sodium 15Incorrect. A normal plasma osmolality with dilute urine does not fit true (hypotonic) hyponatremia from ADH excess.
Key point
SIADH: euvolemic hyponatremia with low plasma osmolality, urine osmolality > 100 mOsm/kg, and urine sodium usually > 30 mEq/L.
From the source
“It is defined as less-than-maximally-dilute urine in the presence of plasma hypo-osmolality (hyponatremia) without volume depletion or overload ... Urine sodium concentration is usually > 30 mEq/L (30 mmol/L)”