An elderly hypertensive woman on hydrochlorothiazide develops hyponatremia with serum Na+ of 122 mEq/L over three weeks. Which feature most reliably distinguishes thiazide-induced hyponatremia from loop diuretic-associated hyponatremia?

• A Loop agents act in the distal convoluted tubule where dilution occurs
• B Thiazides cause obligatory solute diuresis that drags water out
• C Thiazides impair maximal urinary dilution while preserving medullary concentrating ability ✓
• D Thiazides inhibit ADH secretion leading to water loss

Explanation

Thiazides act on the distal convoluted tubule, the segment responsible for diluting urine, so they blunt free water excretion while leaving the countercurrent mechanism intact, allowing ADH-mediated water retention to continue. Loops impair both dilution and concentration, so the kidney cannot retain water maximally, making hyponatremia far less likely. Thiazide hyponatremia typically appears within weeks in elderly women. ADH secretion is not inhibited by thiazides; if anything ADH effects persist unopposed.

Reference: Katzung Basic and Clinical Pharmacology, 16th ed.

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