# Diuretics

> Diuretics for NEET-PG: loop, thiazide, potassium-sparing and carbonic anhydrase inhibitors — sites of action, electrolyte effects and exam points.

- Canonical URL: https://prepelephant.com/topics/neet-pg/pharmacology/diuretics
- Exam / course: NEET-PG · Subject: Pharmacology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Diuretics", PrepElephant, https://prepelephant.com/topics/neet-pg/pharmacology/diuretics

## Direct answer

Diuretics are classified by their site of action in the nephron: loop diuretics block the Na-K-2Cl cotransporter in the thick ascending limb, thiazides block the Na-Cl symporter in the distal convoluted tubule, potassium-sparing agents act in the late distal tubule and collecting duct, and acetazolamide inhibits carbonic anhydrase in the proximal tubule. NEET-PG tests the site of action together with the signature electrolyte profile of each class — above all the opposite handling of calcium by loops (lose calcium) and thiazides (retain calcium).

## What you must remember

- **Loop diuretics** (furosemide, bumetanide, torsemide): most powerful natriuresis; cause hypokalaemia, metabolic alkalosis, hypomagnesaemia, hyperuricaemia, ototoxicity and hypocalcaemia through urinary calcium loss; first choice in pulmonary oedema, resistant oedema and renal impairment.
- **Thiazides** (hydrochlorothiazide, chlorthalidone, indapamide): cornerstone antihypertensives; cause hypercalcaemia, hypokalaemia, hyponatraemia, hyperuricaemia, hyperglycaemia and dyslipidaemia; used for calcium-oxalate stone prophylaxis in hypercalciuric patients; lose efficacy in advanced renal failure, metolazone being the exception.
- **Aldosterone antagonist** spironolactone: used in heart failure with reduced ejection fraction, ascites and Conn syndrome; causes hyperkalaemia and metabolic acidosis; endocrine effects include gynaecomastia and menstrual irregularity; eplerenone is the selective analogue with fewer antiandrogenic effects.
- **ENaC blockers** amiloride and triamterene: potassium-sparing, useful alongside thiazides to offset potassium loss; amiloride also reduces lithium-induced nephrogenic diabetes insipidus.
- **Acetazolamide**: carbonic anhydrase inhibitor producing bicarbonaturia, alkaline urine and hyperchloraemic metabolic acidosis; used in glaucoma and altitude sickness; avoid in cirrhosis and sulfa-allergic patients.
- **Osmotic diuretic mannitol**: increases tubular fluid osmolarity, used for cerebral oedema; contraindicated in anuria and pulmonary oedema.
- **Aquaretics** (tolvaptan): vasopressin V2 antagonists that promote free-water excretion in hyponatraemia.

## Common confusion

Loop versus thiazide is the classic trap, and calcium settles it: loops abolish the lumen-positive potential in the thick ascending limb, so calcium is lost in urine and serum calcium falls, whereas thiazides enhance distal calcium reabsorption, so serum calcium rises — the reason thiazides prevent calcium stones while loops cannot. Potassium handling is the second differentiator: both classes waste potassium, so the exam point is which agent to add for a hypokalaemic patient — a potassium-sparing diuretic. Loops also retain efficacy in renal failure, where thiazides fail — metolazone excepted.

## Exam-focused takeaway

NEET-PG questions on diuretics are site-of-action and electrolyte-matching stems: identify the cotransporter blocked, predict the potassium, calcium, urate and acid-base consequences, and choose the class for a named scenario — acute pulmonary oedema, hypertension, hypercalcaemia of malignancy (loop), recurrent calcium stones (thiazide), ascites or Conn syndrome (spironolactone), and glaucoma or altitude sickness (acetazolamide). Adverse-effect vignettes featuring ototoxicity, gynaecomastia or metabolic acidosis are common. Master each class as site, electrolyte fingerprint and best indication — that chain answers almost every question on this reliably repeated topic.

## Frequently asked questions

### Where do loop and thiazide diuretics act?

Loops block the Na-K-2Cl cotransporter in the thick ascending limb; thiazides block the Na-Cl symporter in the distal convoluted tubule.

### Why do loops lower and thiazides raise serum calcium?

Loops abolish the lumen-positive transepithelial potential that drives paracellular calcium reabsorption, causing calciuria, while thiazides stimulate distal calcium reabsorption, retaining calcium.

### Which diuretic is preferred in acute pulmonary oedema?

Intravenous furosemide, for its rapid venodilation and powerful natriuresis; loops also remain effective in renal impairment where thiazides fail.

### What are the adverse effects of spironolactone?

Hyperkalaemia and metabolic acidosis, plus antiandrogenic effects such as gynaecomastia, breast tenderness and menstrual irregularity; eplerenone causes less gynaecomastia.

### Why is acetazolamide used in glaucoma and altitude sickness?

By inhibiting carbonic anhydrase it reduces aqueous humour formation in the eye and produces a metabolic acidosis that stimulates ventilation at altitude, alongside its bicarbonaturia.

### Which diuretic is safe to combine with a loop in resistant oedema?

Metolazone, a thiazide-like agent that retains efficacy in renal impairment, is classically added to a loop for sequential nephron blockade under close electrolyte monitoring.