Difficulty: Intermediate | Prerequisites: Basic renal physiology, fluid and electrolyte balance
Diuretics are the backbone of antihypertensive therapy and heart failure management. They lower blood volume by increasing urine output, but each class does it at a different part of the nephron, which determines whether potassium goes up or down. Knowing which electrolyte shifts to expect (and what to monitor) is the single most testable concept in this section.
Diuretic
A drug that increases urine production, thereby reducing blood volume and lowering blood pressure. In simple terms, it makes you urinate more so there is less fluid in the bloodstream.
Loop diuretic
A diuretic that acts on the ascending loop of Henle to block sodium, potassium, and chloride reabsorption. Think of it as the strongest class of diuretic, sometimes called "high-ceiling" diuretics because of their potency.
Thiazide diuretic
A diuretic that acts on the distal convoluted tubule to inhibit sodium and chloride reabsorption. In simple terms, a milder diuretic than loops, commonly used as first-line treatment for hypertension.
Potassium-sparing diuretic
A diuretic that promotes sodium and water excretion while retaining potassium. Think of it as the opposite of loops and thiazides when it comes to potassium: these hold onto it instead of wasting it.
Osmotic diuretic
A drug that is filtered by the glomerulus but not reabsorbed, drawing water into the tubules by osmosis. In simple terms, it pulls water out of swollen tissues (like the brain) and into the urine.
Hypokalemia
Serum potassium below 3.5 mEq/L. Think of it as "too little potassium" in the blood, a common and dangerous side effect of loop and thiazide diuretics.
Hyperkalemia
Serum potassium above 5.0 mEq/L. In simple terms, "too much potassium" in the blood, the key risk with potassium-sparing diuretics.
Ototoxicity
Drug-induced damage to the inner ear causing hearing loss or tinnitus. This is the toxicity that loop diuretics can cause with prolonged use or high doses.
Therapeutic drug level
The blood concentration range at which a drug is effective without being toxic. For digoxin (which interacts with diuretics), this range is 0.5 to 2.0 ng/mL.
Normal serum potassium
The normal range is 3.5 to 5.0 mEq/L. This is the single most important lab value to know for any diuretic question.
Tags: furosemide, Lasix, bumetanide, Bumex, torsemide, Demadex, loop of Henle, high-ceiling diuretic, hypokalemia, ototoxicity
Block reabsorption of sodium, potassium, and chloride in the ascending loop of Henle
Produce a large volume of dilute urine, making them the most potent diuretics available
Reduce blood volume, which lowers blood pressure and relieves fluid overload
Furosemide (Lasix) – the most commonly tested
Bumetanide (Bumex)
Torsemide (Demadex)
Hypokalemia (the big one): serum K+ drops below 3.5 mEq/L
Hyponatremia, hypocalcemia, hypomagnesemia
Dehydration and hypotension
Ototoxicity: hearing loss, tinnitus – risk increases with high doses, rapid IV push, and concurrent use of aminoglycosides
Hyperglycemia
Hyperuricemia (can trigger gout)
Muscle weakness, leg cramps
Fatigue, lethargy
Cardiac dysrhythmias (irregular heartbeat)
Decreased bowel motility, constipation
Weak, thready pulse
ECG changes: flattened T wave, ST depression, U wave
Loop diuretics can cause ototoxicity (damage to hearing) when used at high dosages or for prolonged periods. This is reversible if caught early but can become permanent. Risk is higher when combined with other ototoxic drugs such as aminoglycoside antibiotics.
Normal serum potassium: 3.5 to 5.0 mEq/L
Loop diuretics lower potassium by blocking its reabsorption
Patients often need potassium supplementation or potassium-rich foods: bananas, oranges, potatoes, spinach, avocados, tomatoes
Furosemide causes potassium loss. Low potassium increases the risk of digoxin toxicity. If a patient is on both drugs, potassium levels must be monitored closely. Hypokalemia sensitises the heart to digoxin's effects, making dysrhythmias more likely.
Monitor intake and output (I&O)
Monitor daily weight (a gain of 1 kg in 24 hours suggests fluid retention)
Check serum electrolytes, especially potassium, sodium, magnesium
Monitor blood pressure and heart rate
Administer in the morning or early afternoon to avoid nocturia
IV furosemide should be given slowly (no faster than 20 mg/min) to reduce ototoxicity risk
Assess for orthostatic hypotension: teach patients to rise slowly
Monitor renal function (BUN, creatinine)
Assess hearing if patient reports tinnitus or hearing changes
Tags: hydrochlorothiazide, HCTZ, chlorthalidone, metolazone, distal convoluted tubule, hypokalemia, first-line hypertension
Inhibit sodium and chloride reabsorption in the distal convoluted tubule
Produce a moderate diuresis (less potent than loop diuretics)
Lower blood pressure through reduced blood volume and, over time, decreased peripheral vascular resistance
Hydrochlorothiazide (HCTZ) – the most commonly prescribed and tested
Chlorthalidone
Metolazone
Hypokalemia (same risk as loop diuretics, same mechanism of potassium loss)
Hyponatremia
Hypercalcemia (note: thiazides increase calcium reabsorption, the opposite of loop diuretics)
Hyperglycemia
Hyperuricemia
Hyperlipidemia
Photosensitivity
Elevated BUN and creatinine
Decreased urine output
Oedema
Fatigue, nausea
Thiazides are less effective in patients with significantly impaired renal function (GFR below 30 mL/min), and loop diuretics are preferred in those cases.
The same interaction as loop diuretics: thiazides lower potassium, and low potassium increases the risk of digoxin toxicity. Monitor potassium levels closely when a patient takes both.
Normal serum potassium: 3.5 to 5.0 mEq/L
Thiazides lower potassium, so supplementation or dietary adjustment is often needed
Potassium-rich foods: bananas, oranges, cantaloupe, potatoes, sweet potatoes, spinach, beans, avocados, tomatoes, dried apricots
Monitor serum electrolytes (potassium, sodium, calcium)
Monitor blood glucose (especially in diabetic patients)
Monitor uric acid levels
Check blood pressure regularly
Administer in the morning to prevent nocturia
Teach patients about photosensitivity: wear sunscreen and protective clothing
Monitor renal function
Weigh daily and track I&O
Encourage potassium-rich diet unless potassium supplement is prescribed
Tags: spironolactone, Aldactone, triamterene, Dyrenium, amiloride, Midamor, aldosterone antagonist, hyperkalemia, gynecomastia
Act on the collecting duct to block sodium reabsorption while retaining potassium
Spironolactone specifically blocks aldosterone receptors (aldosterone antagonist)
Triamterene and amiloride block sodium channels directly, independent of aldosterone
Produce mild diuresis; often used in combination with loop or thiazide diuretics to offset potassium loss
Spironolactone (Aldactone) – the most commonly tested in this class
Triamterene (Dyrenium)
Amiloride (Midamor)
Hyperkalemia (the defining risk): serum K+ above 5.0 mEq/L
GI disturbances: nausea, vomiting, diarrhoea
Spironolactone-specific side effects: gynecomastia (breast enlargement in males), menstrual irregularities, deepening of voice, impotence
Muscle weakness, fatigue
Bradycardia, cardiac dysrhythmias
Nausea, diarrhoea, abdominal cramping
Paraesthesias (tingling, numbness)
ECG changes: peaked T waves, widened QRS, eventual cardiac arrest if severe
Because potassium-sparing diuretics retain potassium, patients should limit high-potassium foods:
Bananas, oranges, cantaloupe
Potatoes, tomatoes, spinach
Salt substitutes (these often contain potassium chloride)
Dried fruits, avocados
This is the opposite advice from loop and thiazide diuretics, where patients are encouraged to eat these foods.
Spironolactone is contraindicated in pregnancy. It crosses the placenta and has anti-androgen effects that can cause feminisation of a male foetus.
ACE inhibitors (e.g. lisinopril, enalapril) – both raise potassium, compounding the risk of hyperkalemia
ARBs (e.g. losartan, valsartan) – same concern
Potassium supplements – additive hyperkalemia risk
NSAIDs – reduce renal blood flow and can worsen hyperkalemia
Serum potassium level (hold if K+ is elevated or above 5.0 mEq/L)
Renal function (BUN, creatinine)
Blood pressure
Current medication list for interactions (ACE inhibitors, ARBs, potassium supplements)
Monitor serum potassium closely and frequently
Teach patients to avoid potassium-rich foods and salt substitutes
Monitor for signs of hyperkalemia
Administer with food to reduce GI upset
Weigh daily, track I&O
Assess for gynecomastia and other endocrine side effects in patients on spironolactone
Educate about the importance of follow-up lab work
Tags: mannitol, Osmitrol, osmotic diuresis, increased intracranial pressure, ICP, intraocular pressure
Mannitol is filtered by the glomerulus but not reabsorbed by the tubules
It creates an osmotic gradient that pulls water from tissues (including brain and eye) into the vascular space and then into the urine
Does not act on a specific ion transporter the way other diuretics do
Reduce increased intracranial pressure (ICP), such as in traumatic brain injury or cerebral oedema
Reduce intraocular pressure (before eye surgery or in acute glaucoma)
Promote excretion of toxic substances in drug overdose
Prevent or treat acute renal failure by maintaining urine flow
Fluid and electrolyte imbalances
Dehydration
Hypotension or rebound hypertension
Pulmonary oedema (if given too rapidly or to a patient who cannot handle the fluid shift)
Headache, nausea, vomiting
Administer IV only (never given orally)
Use an in-line filter for IV administration (mannitol can crystallise)
Monitor serum electrolytes and osmolality
Monitor I&O strictly; an indwelling catheter is often needed
Monitor neurological status (level of consciousness, pupil response) when used for ICP
Check for signs of fluid overload: crackles, dyspnoea, jugular vein distension
Weigh daily
Inspect the IV solution before administration – if crystals are present, warm the solution to dissolve them before use
Students often assume all diuretics cause hypokalemia. They do not. Potassium-sparing diuretics retain potassium and carry the opposite risk: hyperkalemia.
Students sometimes confuse the calcium effects. Loop diuretics waste calcium (hypocalcemia), while thiazides retain calcium (hypercalcemia). A memory aid: "loops lose calcium."
Many students think potassium-sparing diuretics are safe to combine with ACE inhibitors without concern. Both raise potassium, and the combination significantly increases the risk of life-threatening hyperkalemia.
Ototoxicity is sometimes attributed to thiazides. It is loop diuretics that carry this risk, particularly furosemide at high doses or with rapid IV administration.
⚠️ Know the normal potassium range (3.5 to 5.0 mEq/L) cold. Nearly every diuretic question hinges on whether potassium goes up or down.
⚠️ The furosemide-digoxin interaction is a classic exam question. Low potassium from furosemide increases digoxin toxicity risk.
⚠️ Be able to differentiate hypokalemia signs (muscle weakness, dysrhythmias, flattened T wave) from hyperkalemia signs (peaked T waves, bradycardia, widened QRS).
⚠️ Know when to administer diuretics (morning) and why (to prevent nocturia).
⚠️ Spironolactone's endocrine side effects (gynecomastia) and pregnancy contraindication are frequently tested.
⚠️ For mannitol, remember: IV only, use a filter, and monitor for pulmonary oedema.
True or False: Loop diuretics and thiazide diuretics both cause hypokalemia.
True. Both classes waste potassium.
Fill in the blank: The normal serum potassium range is ______ to ______ mEq/L.
3.5 to 5.0 mEq/L.
True or False: Potassium-sparing diuretics should be combined freely with ACE inhibitors.
False. Both raise potassium, increasing the risk of hyperkalemia.
Fill in the blank: Loop diuretics can cause ______ toxicity with prolonged use or high doses.
Ototoxicity.
True or False: Mannitol can be given orally or intravenously.
False. Mannitol is administered IV only.
Q: A patient on furosemide and digoxin has a serum potassium of 3.0 mEq/L. What is the nurse's priority concern?
A: The priority concern is digoxin toxicity. Hypokalemia (K+ below 3.5) increases the heart's sensitivity to digoxin, raising the risk of fatal dysrhythmias. The nurse should hold digoxin, notify the provider, and expect an order for potassium replacement.
Q: A patient asks why they should take hydrochlorothiazide in the morning. What is the best response?
A: Taking it in the morning allows the diuretic effect to occur during waking hours, preventing nocturia (frequent nighttime urination) that would disrupt sleep.
Q: Which diuretic class is most likely to cause gynecomastia, and why?
A: Spironolactone (a potassium-sparing diuretic) can cause gynecomastia because it is an aldosterone antagonist with anti-androgen effects. It blocks androgen receptors and can increase oestrogen activity.
Q: A patient on spironolactone reports using a salt substitute at home. What should the nurse teach?
A: The nurse should instruct the patient to stop using salt substitutes. Most salt substitutes contain potassium chloride, which combined with spironolactone's potassium-sparing effect could cause dangerous hyperkalemia.
Q: What assessments should the nurse perform before administering IV mannitol?
A: Inspect the solution for crystals (warm to dissolve if present), ensure an in-line filter is in place, assess baseline neurological status (if used for ICP), check vital signs, and verify that the patient has a urinary catheter for strict I&O monitoring.
Q: A patient taking furosemide reports ringing in the ears. What is the nurse's best action?
A: Tinnitus is an early sign of ototoxicity. The nurse should hold the next dose, notify the provider, and document the finding. The provider may reduce the dose or switch to a different diuretic.
This material connects directly to cardiac glycosides (digoxin), because the potassium changes caused by diuretics are the main driver of digoxin toxicity risk. It also ties into ACE inhibitors and ARBs, which are often prescribed alongside diuretics and share potassium-related interactions. Understanding the autonomic nervous system and fluid/electrolyte balance from earlier coursework is essential here.
Diuretics, loop diuretics, thiazide diuretics, potassium-sparing diuretics, osmotic diuretics, furosemide, Lasix, bumetanide, Bumex, torsemide, Demadex, hydrochlorothiazide, HCTZ, chlorthalidone, metolazone, spironolactone, Aldactone, triamterene, Dyrenium, amiloride, Midamor, mannitol, Osmitrol, hypokalemia, hyperkalemia, ototoxicity, digoxin interaction, potassium levels, electrolyte imbalance, antihypertensive agents, nursing pharmacology, cardiovascular drugs, renal pharmacology, nephron, loop of Henle, distal convoluted tubule, collecting duct, aldosterone antagonist, fluid overload, heart failure diuretics, blood pressure medications