thiazide diuretics
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Updated 2026-03-16
Contents
thiazide and thiazide-like diuretics
Inhibitors of the sodium-chloride cotransporter (NCC) in the distal convoluted tubule. Used as first-line antihypertensives and as “booster” agents in sequential nephron blockade for acute heart failure.
mechanisms
- Primary site: Distal convoluted tubule (DCT).
- Effect: Blocks NCC → inhibits ~5% of filtered sodium reabsorption.
- Secondary effect: Increases calcium reabsorption (useful in nephrolithiasis; contrasts with loop diuretics which are calciuric).
- Diuretic braking: Chronic loop diuretic use causes DCT hypertrophy; thiazides are the specific “antidote” to this mechanism of resistance.
comparison of agents
| Agent | Type | Half-life | Clinical roles |
|---|---|---|---|
| Chlortalidone | Thiazide-like | 40–60 h | Preferred for hypertension; potent; effective in advanced CKD (CLICK trial) |
| Indapamide | Thiazide-like | 14–24 h | Hypertension; glucose-neutral; preferred in elderly |
| Hydrochlorothiazide | Thiazide | 6–15 h | Most common; often in fixed-dose combinations; less potent than chlortalidone |
| Metolazone | Thiazide-like | 14–24 h | Decongestion booster; maintains efficacy at low GFR; very potent synergism with loops |
sequential nephron blockade in heart failure
Thiazides (especially metolazone) are used to break established diuretic resistance — see diuretic therapy for the full protocol.
- Synergy: Blocking the DCT prevents the “rebound” sodium reabsorption that occurs when loop diuretics reach the distal tubule.
- CLOROTIC (2023): Adding hydrochlorothiazide to IV furosemide in ADHF improved weight loss but increased AKI and electrolyte disturbances. No mortality benefit.
- Dosing: Metolazone 2.5–5 mg PO daily or BID (in extreme cases). Give 30 minutes before the loop diuretic to “prime” the tubule.
adverse effects & monitoring
| Complication | Mechanism | Management |
|---|---|---|
| Hyponatraemia | Impaired free water excretion (diluting segment block) | Stop if Na⁺ < 125 mmol/L; fluid restrict. More common than with loop diuretics |
| Hypokalaemia | Increased distal Na⁺ delivery → K⁺ wasting | Proactive KCl repletion; target K⁺ > 4.0 |
| Hypercalcaemia | Increased proximal and distal Ca²⁺ reabsorption | Avoid in pre-existing hypercalcaemia |
| Hyperuricaemia | Competitive inhibition of urate secretion | May trigger gout; monitor in at-risk patients |
| Hyperglycaemia | Reduced insulin secretion (hypokalaemia-mediated) | Monitor in DM; indapamide is the most “metabolically neutral” |
safety in CKD
- Historical myth: “Thiazides don’t work if GFR < 30.”
- Evidence: The CLICK trial (2021) demonstrated that chlortalidone effectively lowers BP even in stage 4 CKD (eGFR 15–30). Metolazone is also well-known to remain effective in low-GFR states.
- Monitoring: AKI risk is higher in CKD when combined with loop diuretics; monitor creatinine and volume status closely.