Diuretics lower the blood pressure at least initially by inducing sodium and fluid loss. (See "Use of thiazide diuretics in patients with primary (essential) hypertension".) When used in patients with primary hypertension and relatively normal renal function, the thiazide diuretics, particularly chlorthalidone [1,2], have a longer antihypertensive effect than the loop diuretics .
The mechanism is probably related to the short duration of action of the loop diuretics furosemide and bumetanide (less than six hours) since the initial fluid loss can be counteracted by activation of the renin-angiotensin-aldosterone system, leading to sodium retention during the period when the diuretic effect has worn off . Longer-acting loop diuretics are available; torsemide, for example, has a duration of action up to 12 hours. It is not known if the antihypertensive effect of torsemide is greater than that of furosemide or bumetanide.
The two classes of diuretics also have differing effects on calcium balance:
●The thiazides tend to promote calcium retention
●The loop diuretics enhance urinary calcium loss