Which statement correctly describes the relationship between glomerular filtration rate and thiazide efficacy?

• A Thiazides retain full efficacy until the GFR falls below 10 mL/min
• B Thiazide efficacy is independent of renal function because they act on the distal nephron
• C Thiazides become more potent as GFR falls because of reduced protein binding
• D Thiazides lose effectiveness once GFR falls below approximately 30 mL/min ✓

Explanation

Thiazides act on the distal convoluted tubule and require delivery of filtered sodium to their site of action. When GFR falls below roughly 30 mL/min, reduced filtered load and increased proximal reabsorption leave too little sodium reaching the distal tubule, so natriuretic response largely disappears. This is why loop diuretics, whose site lies upstream and handles a far larger sodium fraction, remain effective in advanced renal failure.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.

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