Hydrochlorothiazide
Brand names: HCTZ, Microzide
Class: 💦 Diuretics
On board day, hydrochlorothiazide is the most-prescribed thiazide and one of three first-line antihypertensives alongside ACE/ARB and CCB. It blocks the Na-Cl cotransporter in the distal convoluted tubule, lowering blood pressure with modest natriuresis. Electrolyte effects to memorize: low sodium, low potassium, low magnesium, high calcium, high uric acid, high glucose. The classic AANP vignette is the older woman with new hyponatremia after starting HCTZ. AHA now favors chlorthalidone for outcomes, but HCTZ remains broadly used and combined with lisinopril, losartan, or amlodipine in fixed-dose tablets. Loses efficacy below CrCl 30 — switch to a loop diuretic in advanced CKD.
✅ Indications
HTN (first-line), edema, nephrogenic diabetes insipidus.
⚙️ Mechanism of Action
Thiazide diuretic — DCT Na-Cl symporter.
📏 Dosing
12.5–50 mg PO daily (BP effect plateaus at 25 mg).
🚫 Contraindications
Anuria, severe sulfa allergy (caution), severe hepatic/renal impairment.
⚠️ Adverse Effects
Hypokalemia, hyponatremia, hypercalcemia, HYPERuricemia (gout), hyperglycemia, hyperlipidemia (6 H's).
🔬 Monitoring
BMP 1–2 weeks after start, then periodically. BP. Uric acid if gout hx.
💎 Board Pearls
- 🧂 The "6 Hs" pearl: hypoKalemia, hypoNatremia, hyperCalcemia, hyperUricemia, hyperGlycemia, hyperLipidemia.
- 📉 BP effect plateaus at 25 mg — higher doses just add side effects.
- 💡 Loss of effectiveness when eGFR <30 → switch to loop.
Practice Questions
A 72-year-old woman with hypertension started hydrochlorothiazide (Microzide) 25 mg daily approximately 3 weeks ago. She now presents with a red, hot, exquisitely tender right first metatarsophalangeal joint. She has no prior history of arthritis and no recent trauma. Serum uric acid is 9.8 mg/dL. Which thiazide-related mechanism most likely precipitated this presentation?
Related Drugs in This Class
- Chlorthalidone — Hygroton, Thalitone
- Furosemide — Lasix
- Spironolactone — Aldactone
Sources
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