A 72-year-old woman on hydrochlorothiazide develops confusion. Labs: Na 118 mEq/L, K 3.1 mEq/L, serum osmolality 248 mOsm/kg, urine osmolality 520 mOsm/kg, urine sodium 68 mEq/L. She has no edema, normal BP, and is euvolemic clinically. The most likely cause is:

• A Primary polydipsia
• B Thiazide-induced SIADH-like syndrome ✓
• C Adrenal insufficiency
• D Hypothyroidism

Explanation

Thiazide diuretics are the most common cause of drug-induced euvolemic hyponatremia, producing a SIADH-like picture by impairing the renal diluting mechanism (ascending loop) while ADH secretion continues. Urine osmolality >100 mOsm/kg and urine Na >40 mEq/L in a euvolemic patient confirms the diagnosis. Primary polydipsia causes very dilute urine (UOsm <100 mOsm/kg). Adrenal insufficiency typically shows hyperkalemia and mild hypotension; hypothyroidism is a less common cause and typically presents with dry skin and bradycardia.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.