A 74-year-old woman started hydrochlorothiazide 25 mg daily four weeks ago for hypertension. She reports dizziness and nausea. Serum sodium is 118 mEq/L, serum potassium 3.4 mEq/L, urine osmolality 450 mOsm/kg, and euvolemia is clinically apparent. The most likely diagnosis is:

• A SIADH unrelated to the drug
• B Thiazide-induced hyponatremia from impaired free water excretion ✓
• C Pseudohyponatremia due to hypertriglyceridemia
• D Primary polydipsia with dilute urine expected but masked by diuretic effect

Explanation

Elderly women within weeks of starting a thiazide are the classic demographic for thiazide hyponatremia. The drug impairs diluting capacity in the cortical collecting duct and stimulates vasopressin release through mild volume contraction, producing concentrated urine with low solute excretion despite hypo-osmolality. SIADH looks identical biochemically, but the temporal association with the thiazide makes it the primary suspect. Pseudohyponatremia shows normal measured osmolality, which is absent here.

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

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Written and medically reviewed by the StethoPrep medical team.