A 72-year-old woman with CKD stage 3a (eGFR 48 mL/min) and type 2 diabetes presents with serum sodium of 128 mEq/L. She is on hydrochlorothiazide 25 mg daily. She is euvolaemic on examination. Serum osmolality is 262 mOsm/kg. Urine osmolality is 280 mOsm/kg. Urine sodium is 52 mEq/L. TSH and cortisol are normal. What is the MOST likely mechanism of her hyponatraemia?
- A SIADH
- B Hypovolaemic hyponatraemia
- C Thiazide-induced hyponatraemia ✓
- D Reset osmostat
Correct answer: C. Thiazide-induced hyponatraemia
Explanation
Thiazide diuretics cause hyponatraemia through urinary sodium and potassium loss combined with impaired free water excretion due to reduced diluting capacity. Unlike loop diuretics, thiazides do not impair urinary concentration, so ADH-mediated water retention can persist. The presentation with euvolaemia, concentrated urine, and high urine sodium fits thiazide-induced hyponatraemia. SIADH is a close differential but the thiazide history is the key discriminator; stopping the drug confirms the diagnosis.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.