How loop diuretics are tested on Step 1

Expect questions on the nephron segment and transporter, the direction of calcium and potassium changes, the acid–base result, or a toxicity such as hearing loss or gout.

workspace_premium Loops lose calcium; thiazides keep it

Loop diuretics increase urinary calcium; thiazides decrease it. That single contrast answers a large share of diuretic questions — loops treat hypercalcemia, thiazides prevent calcium stones.

Mechanism in the thick ascending limb

The thick ascending limb is impermeable to water. NKCC2 brings Na⁺, K⁺, and 2 Cl⁻ into the cell; K⁺ leaks back into the lumen through ROMK, creating a lumen-positive voltage that pushes Ca²⁺ and Mg²⁺ between cells.

NKCC2blockedThickascendinglimbNa⁺, Cl⁻stayIn the lumenNo K⁺recyclingLumen-positivevoltage lostCa²⁺,Mg²⁺ lostNoparacellulardriveGradientlostMedullarywashout
Loop diuretic mechanism. Loop diuretics are secreted into the tubule by organic anion transporters in the proximal tubule; they must reach the lumen to work.
ParameterEffectMechanism
Urine Na⁺, Cl⁻↑↑NKCC2 blocked
Serum K⁺↓More Na⁺ delivered to collecting duct → ↑ K⁺ secretion; ↑ aldosterone
Serum Ca²⁺↓ (urine Ca²⁺ ↑)Loss of lumen-positive potential
Serum Mg²⁺↓Same paracellular mechanism
Acid–baseMetabolic alkalosisContraction alkalosis; ↑ H⁺ secretion in collecting duct
Uric acid↑Volume contraction increases proximal reabsorption

Furosemide also increases renal prostaglandin synthesis; NSAIDs blunt its effect.

Clinical uses

Loop diuretics are chosen when a large, fast diuresis is needed or when GFR is low.

UseRationale
Acute pulmonary edemaRapid natriuresis; IV furosemide also causes early venodilation
Edema from heart failure, cirrhosis, nephrotic syndromePotent; works at low GFR (unlike thiazides)
HypercalcemiaWith IV saline, increases calcium excretion
HyperkalemiaIncreases potassium excretion
Hypertension with CKD or heart failureThiazides lose efficacy when GFR is low

In cirrhosis, spironolactone is the backbone and furosemide is added (often in a 100:40 ratio).

Adverse effects

Most toxicity follows from potent sodium and water loss; ototoxicity reflects NKCC1 in the inner ear.

Loop diuretic toxicityOtotoxicityInner-ear NKCC1;worse withaminoglycosidesElectrolytes↓ K⁺, ↓ Mg²⁺, ↓Ca²⁺VolumeDehydration,hypotension,AKIMetabolicAlkalosis,hyperuricemia,goutSulfaallergyExceptethacrynic acid
Loop diuretic toxicity. Hypokalemia increases the risk of digoxin toxicity.
Loop diureticsThick ascending limb, NKCC2Most potentUrine Ca²⁺ ↑ → serum Ca²⁺ ↓Work when GFR is lowOtotoxicThiazidesDistal convoluted tubule, NCCModerate potencyUrine Ca²⁺ ↓ → serum Ca²⁺ ↑Weak when GFR < 30Hyperglycemia, hyperlipidemiavs
Loop vs thiazide. Both cause hypokalemia, metabolic alkalosis, and hyperuricemia.

Clinical application

Combine drug knowledge with electrolyte physiology.

functions Worked example — the electrolyte panel

A man with heart failure is treated with high-dose IV furosemide. Three days later he has muscle cramps. Which set of lab results is expected?

  1. NKCC2 blockade delivers more sodium to the collecting duct, where it is exchanged for K⁺ and H⁺, so potassium falls and pH rises.
  2. Volume contraction activates aldosterone, amplifying K⁺ and H⁺ loss.
  3. Loss of the lumen-positive potential reduces paracellular Ca²⁺ and Mg²⁺ reabsorption.

Answer. Hypokalemia, hypomagnesemia, hypocalcemia, and metabolic alkalosis — replete K⁺ and Mg²⁺ (hypomagnesemia makes hypokalemia refractory).