Thiazide diuretics may not be effective in patients with which condition?

Thiazide diuretics function by inhibiting sodium reabsorption in the distal convoluted tubule of the nephron, promoting diuresis and effectively reducing blood pressure. However, their efficacy is significantly diminished in patients with advanced renal impairment.

In cases of advanced renal impairment, the filtration rate through the kidneys is substantially reduced. Since thiazides rely on adequate kidney function to achieve their action, they are less effective in producing the desired diuretic effect when the glomerular filtration rate (GFR) falls below approximately 30 mL/min. This diminishes the ability of thiazides to exert their full therapeutic effects, resulting in inadequate control of fluid balance and hypertension in these patients.

In contrast, thiazide diuretics can still be beneficial in managing hypertension and heart failure in patients with normal or mildly impaired renal function, as kidneys can still adequately respond to the diuretic. They are also not contraindicated in liver disease, where their effects may still be realized; thus the most significant limitation to their use is indeed in advanced renal impairment.