Thiazide Diuretics

Loop diuretics get the dramatic results, but thiazides are prescribed far more often for hypertension — and their electrolyte traps catch students off guard on the NCLEX.

Thiazide diuretics (hydrochlorothiazide, chlorthalidone) block sodium-chloride reabsorption in the distal convoluted tubule — a segment that handles only about 5-10% of filtered sodium. That's why their diuretic effect is mild to moderate compared to loop diuretics. Their real clinical value is sustained blood pressure reduction: they lower peripheral vascular resistance over weeks, making them first-line monotherapy for primary hypertension per JNC guidelines. Because they waste potassium and spare calcium, thiazides cause hypokalemia and hypercalcemia — the opposite electrolyte pattern of loop diuretics. They also raise uric acid levels, worsening gout, and can elevate blood glucose, making them a concern in diabetic patients. Thiazides lose effectiveness when GFR drops below approximately 30 mL/min (except metolazone), so they are generally ineffective in significant renal impairment. Onset is 1-2 hours oral; duration is 6-12 hours for HCTZ, up to 48-72 hours for chlorthalidone.

Key Distinctions

Thiazides cause hypercalcemia; loop diuretics cause hypocalcemia — this is the single most tested electrolyte flip between the two classes. Students confuse thiazide site of action (distal convoluted tubule) with loop site (ascending loop of Henle). Thiazides are first-line for hypertension, not for acute fluid overload — that's loop diuretic territory.

Clinical Pearl

Think "thiazides keep calcium" — they increase calcium reabsorption. Loops lose calcium. This one distinction answers half the diuretic comparison questions on the NCLEX.