Diuretics by nephron segment: loop, thiazide and more
Each diuretic class blocks one transporter in one nephron segment, and that transporter predicts the whole lab pattern. Map the four stops, then split loops from thiazides on calcium.
Chapters (9)
Map each diuretic class to its nephron segment and transporter, and predict its characteristic electrolyte and acid-base effects.
The short version
- Acetazolamide acts in the proximal tubule on carbonic anhydrase and causes a high chloride metabolic acidosis with a normal anion gap.
- Loop diuretics block NKCC2 in the thick ascending limb; thiazides block NCC in the distal convoluted tubule; both cause hypokalemia and metabolic alkalosis.
- Loops lose calcium and can harm hearing; thiazides keep calcium, so serum calcium can rise, and they can lower sodium.
- Potassium-sparing drugs act in the collecting duct (ENaC blockers, aldosterone receptor blockers) and cause high potassium and a high chloride metabolic acidosis, like type 4 RTA.
- The transporter predicts the labs: learn the transporter first.
Diuretics by nephron segment: why one patient's potassium rose
A 55-year-old man takes a blood pressure tablet and a diuretic. His potassium is 5.9 and his bicarbonate is 18 mmol/L. Most diuretics drain potassium, so which kind raises it?
The answer comes from mapping diuretics by nephron segment: each class blocks a specific transporter, and the transporter predicts the signature electrolyte and acid-base effects. In the previous lesson, aldosterone and ADH worked on the distal nephron. Here, drugs are placed on the same map.
The diuretic map: four stops, four transporters
| Segment | Target | Drug class |
|---|---|---|
| Proximal tubule | Carbonic anhydrase | Acetazolamide |
| Thick ascending limb | Na⁺–K⁺–2Cl⁻ cotransporter (NKCC2) | Loop diuretics, e.g. furosemide |
| Distal convoluted tubule | Na⁺–Cl⁻ cotransporter (NCC) | Thiazides |
| Collecting duct | Sodium channel (ENaC) | Amiloride, triamterene |
| Collecting duct | Aldosterone receptor | Spironolactone, eplerenone |
Acetazolamide: proximal tubule and high chloride acidosis
Acetazolamide inhibits carbonic anhydrase, so the proximal tubule reclaims less bicarbonate. Bicarbonate is lost in urine and chloride rises in blood.[1][3]
The signature is a high chloride metabolic acidosis with a normal anion gap.
Loop diuretics versus thiazides: the calcium split
Blocking NKCC2 stops sodium, potassium and chloride entering the cell. That also removes the positive voltage in the lumen that drives calcium and magnesium through the paracellular path, so calcium and magnesium are lost in urine.[2][3]
More sodium reaches the collecting duct, so potassium and hydrogen ions are lost there. The loop signature is hypokalemia, metabolic alkalosis, urinary calcium loss and, with high doses, hearing loss. Hearing loss is more likely with aminoglycosides or reduced kidney function.[3]
Thiazides act one step later, at NCC. They share the same potassium and acid loss, so they also cause hypokalemia and metabolic alkalosis. Calcium goes the other way: urinary calcium falls, so serum calcium can rise. Thiazides can also cause hyponatremia, through volume loss, ADH and thirst.[2][3]
| Feature | Loop (NKCC2) | Thiazide (NCC) |
|---|---|---|
| Potassium | Low | Low |
| Acid–base | Metabolic alkalosis | Metabolic alkalosis |
| Urinary calcium | Lost | Falls; serum calcium can rise |
| Other | Hearing loss at high doses | Hyponatremia |
Potassium-sparing diuretics: collecting duct, high potassium
Block ENaC, or block the aldosterone receptor, and the lumen stays less negative. Less potassium and less hydrogen are pushed out.[1][3]
The signature is high potassium and a high chloride metabolic acidosis, like type 4 RTA. Risk rises with kidney disease and with ACE inhibitors. That fits the opening patient: a potassium-sparing diuretic with an ACE inhibitor.[3]
Worked example: predict the labs from the drug
A woman with leg swelling starts furosemide. Where does it act? The thick ascending limb, so NKCC2.
- Site: thick ascending limb, NKCC2 (loop diuretic)
- Prediction: low potassium, high bicarbonate, high urine calcium, maybe ringing in the ears
- Her labs: potassium 3.2 mEq/L, bicarbonate 31 mmol/L, urine calcium high
- Every prediction follows from the transporter
Practice question
Original · USMLE styleA 66-year-old woman is evaluated three weeks after starting a new daily medication for hypertension. She has no hearing complaints, edema or other symptoms. Laboratory studies show:
| Serum sodium | 128 mEq/L |
|---|---|
| Serum potassium | 3.0 mEq/L |
| Serum chloride | 86 mEq/L |
| Serum bicarbonate | 34 mEq/L |
| Serum calcium | 11.1 mg/dL |
| Serum parathyroid hormone | suppressed |
| Urine calcium excretion (24 h) | low |
Common questions
Which diuretic causes hyperkalemia?
Potassium-sparing diuretics: ENaC blockers (amiloride, triamterene) and aldosterone receptor blockers (spironolactone, eplerenone). They cause high potassium and a high chloride metabolic acidosis, like type 4 RTA.
Where do loop diuretics and thiazides act?
Loop diuretics block NKCC2 in the thick ascending limb. Thiazides block NCC in the distal convoluted tubule.
How do loop diuretics and thiazides differ on calcium?
Loops lose calcium in urine; thiazides keep it, so urinary calcium falls and serum calcium can rise.
Which diuretic causes a normal anion gap metabolic acidosis?
Acetazolamide. It inhibits carbonic anhydrase in the proximal tubule, so bicarbonate is lost in urine and chloride rises in blood.
Which diuretics cause hypokalemia and metabolic alkalosis?
Both loop diuretics and thiazides. More sodium reaches the collecting duct, so potassium and hydrogen ions are lost there.
Transcript
Read the full narration, by chapter672 words
0:00A diuretic and an unexpected potassium
A 55-year-old man takes a blood pressure tablet and a diuretic. His potassium is 5.9, and his bicarbonate is 18 millimoles per litre. Most diuretics drain potassium, so which kind of diuretic raises it? Today: map each diuretic class to its nephron segment and transporter, and predict its signature electrolyte and acid-base effects. Last time, aldosterone and ADH worked on the distal nephron. Now we put drugs on the same map.
0:31The map: four stops, four drug classes
The map has four stops. Proximal tubule: carbonic anhydrase, blocked by acetazolamide. Thick ascending limb: the sodium, potassium, two chloride cotransporter, NKCC two, blocked by loop diuretics like furosemide. Distal convoluted tubule: the sodium chloride cotransporter, NCC, blocked by thiazides. Collecting duct: the sodium channel EN a C, blocked by amiloride and triamterene, and the aldosterone receptor, blocked by spironolactone and eplerenone. The rule: every upstream block sends more sodium downstream, and the collecting duct trades that sodium for potassium and acid.
1:17Acetazolamide: the proximal tubule
Let's go down the nephron. Acetazolamide first. It inhibits carbonic anhydrase, so the proximal tubule reclaims less bicarbonate. Bicarbonate is lost in urine and chloride rises in blood. The signature: high chloride metabolic acidosis, with a normal anion gap.
1:38Loop diuretics: the thick ascending limb
Next, loop diuretics. Blocking NKCC two stops sodium, potassium and chloride entering the cell. That also removes the positive voltage in the lumen that drives calcium and magnesium through the paracellular path. So calcium and magnesium are lost in urine. More sodium reaches the collecting duct, so potassium and hydrogen ions are lost there. The signature: hypokalemia, metabolic alkalosis, urinary calcium loss and, with high doses, hearing loss. Hearing loss is more likely with aminoglycosides or reduced kidney function.
2:16Thiazides: the distal convoluted tubule
Thiazides act one step later, at NCC. They share the same potassium and acid loss: hypokalemia and metabolic alkalosis. But calcium goes the other way. Urinary calcium falls, so serum calcium can rise. They can also cause hyponatremia, through volume loss, ADH and thirst. Loops lose calcium, thiazides keep it.
2:41Potassium-sparing drugs: the collecting duct
Last stop: the potassium-sparing drugs. Block EN a C, or block the aldosterone receptor, and the lumen stays less negative. Less potassium and less hydrogen are pushed out. The signature: high potassium and a high chloride metabolic acidosis, like type four RTA. Risk rises with kidney disease and with ACE inhibitors. That fits our opening patient: a potassium-sparing diuretic with an ACE inhibitor.
3:12Worked example: predict the labs from the drug
Let's predict one. A woman with leg swelling starts furosemide. Where does it act? The thick ascending limb, so NKCC two. Predict: low potassium, high bicarbonate, a high urine calcium, maybe ringing in the ears. Her labs: potassium 3.2, bicarbonate 31, and a urine calcium that is high. Every prediction follows from the transporter, so learn the transporter first.
3:42Practice question
Your turn. A 66-year-old woman started a new daily tablet for hypertension three weeks ago. Sodium is 128, potassium 3.0, chloride 86, bicarbonate 34. Calcium is 11.1 milligrams per decilitre, parathyroid hormone is suppressed, and urine calcium is low. Which drug and site best explain these findings? A, acetazolamide, proximal tubule carbonic anhydrase. B, furosemide, thick ascending limb NKCC two. C, hydrochlorothiazide, distal convoluted tubule NCC. D, amiloride, collecting duct sodium channel. E, spironolactone, collecting duct aldosterone receptor. Pause here and decide. A is out: acetazolamide causes a high chloride acidosis, not an alkalosis. B is out: loops make potassium and acid loss too, but they waste calcium, so urine calcium would be high, not low. D is out: amiloride would raise potassium and cause acidosis. E is out for the same reason: spironolactone gives high potassium and acidosis. So the answer is C: low potassium, alkalosis, low sodium, and calcium kept in the body, which is thiazide at NCC.
5:14High-yield recap and next lesson
Three things to keep. One: acetazolamide acts in the proximal tubule and causes a high chloride acidosis. Potassium-sparing drugs in the collecting duct cause high potassium and acidosis. Two: loops block NKCC two and thiazides block NCC, and both cause hypokalemia and alkalosis. Three: loops lose calcium and can harm hearing, thiazides keep calcium and lower sodium. Next up: the renin angiotensin aldosterone system, from renin release to aldosterone.
Sources
Numbers in the text, e.g. [1], point here- Emerging Targets of Diuretic Therapy. Cheng et al., Clin Pharmacol Ther 2017. PMC5546989. pmc.ncbi.nlm.nih.gov/articles/PMC5546989
- Thiazide-Associated Hypercalcemia: Incidence and Association With Primary Hyperparathyroidism. Griebeler et al., J Clin Endocrinol Metab 2016. PMC4803175. pmc.ncbi.nlm.nih.gov/articles/PMC4803175
- Medical Pharmacology, Chapter 26: Renal Pharmacology (diuretics). pharmacology2000.com. pharmacology2000.com/Renal/Renal10.htm
Example lab values are illustrative.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. All video diagrams are original (code-drawn); no third-party artwork is used in the video. Thumbnail: kidney/nephron illustration from Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (recoloured, outlined, resized).
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-REN-029.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01GFR and Starling forces at the glomerulus5:25
- EP 02Afferent vs efferent arteriole: GFR, RPF and filtration fraction5:16
- EP 03Renal clearance: inulin, creatinine and PAH5:20
- EP 04Glucose transport maximum, splay and SGLT24:55
- EP 05Countercurrent multiplier: how the kidney concentrates urine5:02
- EP 06Aldosterone and ADH in the distal nephron5:20
- EP 07Diuretics by nephron segment: loop, thiazide and more5:53
- EP 08RAAS explained: renin, angiotensin II and aldosterone5:23