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Thiazide diuretics are a type of diuretic (a drug that increases urine flow). They act directly on the kidneys and promote diuresis (urine flow) by inhibiting the sodium/chloride cotransporter located in the distal convoluted tubule of a nephron (the functional unit of a kidney). Thiazides decrease sodium reabsorption which increases fluid loss in urine, which in turn decreases extracellular fluid and plasma volume. This reduces cardiac output (how hard the heart has to work to pump blood around the body) and lowers blood pressure. Thiazides also work in other ways to lower blood pressure. Thiazides also cause potassium loss while retaining calcium.

Thiazides are usually used for their blood pressure lowering effects because their diuretic activity is relatively weak compared to some other types of diuretic (such as loop diuretics).