A 55-year-old man has serum sodium of 122 mEq/L with symptoms of confusion and nausea. He has no oedema, BP is normal, urine osmolality is 480 mOsm/kg, serum osmolality is 254 mOsm/kg, and urine sodium is 62 mEq/L. TSH and morning cortisol are normal. What is the MOST likely diagnosis?

• A Psychogenic polydipsia
• B Hypothyroidism-induced hyponatraemia
• C Adrenal insufficiency
• D SIADH (syndrome of inappropriate ADH secretion) ✓

Explanation

SIADH is characterised by hyponatraemia with inappropriately concentrated urine (Uosm >100, typically >300 mOsm/kg), normovolaemia, normal thyroid and adrenal function, urine sodium >20-40 mEq/L (high due to intact pressure natriuresis), and low serum osmolality. Psychogenic polydipsia causes maximally dilute urine (Uosm <100). TSH and cortisol are stated to be normal, excluding hypothyroidism and Addison's disease. This pattern is classic SIADH.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.