Diuretics — Loop, Thiazide, Potassium-Sparing & Osmotic

Also known as Diuretics · Loop diuretics · Furosemide · Bumetanide · Torasemide · Thiazide · Hydrochlorothiazide · Chlorthalidone · Metolazone · Potassium-sparing · Spironolactone · Eplerenone · Amiloride · Mannitol · Acetazolamide · Sequential nephron blockade · Diuretic resistance · DOSE trial

Diuretics — by site of action in the nephron. LOOP (furosemide, bumetanide, torasemide — inhibit the Na-K-2Cl [NKCC2] cotransporter in the thick ascending limb — the MOST POTENT class, blocking up to a quarter of filtered Na+). THIAZIDE (hydrochlorothiazide, chlorthalidone, indapamide, metolazone — inhibit the Na-Cl [NCC] cotransporter in the distal convoluted tubule — a smaller fraction of filtered Na+). POTASSIUM-SPARING (spironolactone/eplerenone — aldosterone/mineralocorticoid-receptor antagonists; amiloride/triamterene — ENaC blockers — cortical collecting duct). OSMOTIC (mannitol — filtered, not reabsorbed — osmotic water pull throughout the nephron; weight-based IV boluses reduce ICP). CARBONIC ANHYDRASE INHIBITOR (acetazolamide — proximal tubule — HCO3 loss → metabolic acidosis)

medium31 referencesUpdated 17 Aug 202633 min readSource-verified · Sept 2026

Checked against its sources on 3 Sept 2026

Practise this topic

8 MCQs

The one-line answer

Diuretics are classified by their site of action in the nephron — and so is their potency. The loop (furosemide, bumetanide, torasemide) blocks the Na-K-2Cl cotransporter in the thick ascending limb — a transporter that reabsorbs up to 25% of filtered salt — making loop most potent class; the thiazide blocks the Na-Cl cotransporter in the distal convoluted tubule; the potassium-sparing drugs block the cortical collecting duct; acetazolamide inhibits carbonic anhydrase in the proximal tubule; and mannitol acts osmotically throughout. In ICU, the loop dominates — an initial IV furosemide dose of 2.5× the home oral dose (given as twice-daily boluses) is an appropriate initial strategy in acute decompensated heart failure (DOSE trial), with sequential nephron blockade (adding metolazone or IV chlorothiazide) for diuretic resistance, and frequent K+, Mg2+, Na+ and creatinine monitoring during active diuresis. Calcium contrast: loops cause HYPOcalcaemia; thiazides cause HYPERcalcaemia.^[6]^[1]^[25]

Meet the patient

A 72-year-old man with ischaemic cardiomyopathy and an LVEF of 25% arrives at 2 am with acute pulmonary oedema after missing his medications for three days. He is sitting bolt upright, SaO2 82% on 15 L non-rebreather, BP 178/100, HR 122, and you can hear crackles from the doorway. The nurse asks for the furosemide dose. The question that decides the next hour is the same question that decides every diuretic decision in the ICU: where in the nephron does this drug act, and how much filtered sodium does it block?^[6]

You give 80 mg IV furosemide. Almost immediately, before any urine flows, his breathing is easier — that is the prostaglandin-mediated venodilation, not the diuresis. By time first litre of urine is in the bag, he is asking for a pillow. That is the loop blocking a Na-K-2Cl cotransporter that normally reabsorbs up to 25% of the filtered salt. Hold those two questions — site and potency — and the rest of the pharmacology lines up behind them.^[6]

References31Show

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