Medicine & USMLE

Thiazide Diuretics

Cardio Drugs - Diuretics
  1. Loop Diuretics
  2. Thiazide Diuretics
  3. Spironolactone
  4. Mannitol


Thiazide diuretics remove excess fluid in the body by blocking the reabsorption of water, sodium and potassium in the kidney. As diuretics, thiazides cause fluid loss by increasing urine output. Thiazide diuretics are used to treat fluid overload states like edema, and are also used to treat hypertension. Side effects of the thiazide diuretics include hypokalemia and hyponatremia. Avoid giving thiazide diuretics to patients taking lithium, since thiazide diuretics can increase lithium levels in the body to toxic levels. Thiazide diuretics also cause hyperglycemia, hyperuricemia leading to gout, and photosensitivity.

Key Points

  • Key Drugs
    • End in “-thiazide”
      • Hydrochlorothiazide
      • Chlorothiazide
      • Methyclothiazide
    • Thiazide-like diuretics
      • Chlorthalidone
      • Indapamide
      • Metolazone
    • Mechanism (Saunders)
      • Remove excess fluid (Diuresis)
        • Act on the early distal convoluted tubule to block the reabsorption of sodium, potassium, and water
          • Water loves sodium, so if more sodium is excreted from the body, more water will also be excreted (diuresis)
        • May act directly on arterioles to cause vasodilation, decreasing blood pressure
        • Weaker diuretic and slower onset than loop diuretics, but thiazides are still fairly potent
        • Will lead to increased urination
          • Contraindicated in patient with renal impairment
            • You can anticipate that renal function will be measured through creatinine and BUN
            • The effectiveness of the thiazide diuretic greatly decreases with a creatinine clearance less than 30 mL/min
          • Watch for signs and symptoms of dehydration
          • Take in the morning to avoid nocturia
          • Closely monitor intake and output in the hospital setting. Report urine output less than 30 mL/hour.
          • Take daily weights
            • Best way to determine fluid retention
    • Clinical Use (Saunders, mictlan)
      • Hypertension
        • Hydrochlorothiazide is often combined with an antihypertensive to control high blood pressure
      • Edema
        • Due to heart failure, liver failure, or kidney failure
    • Side Effects and Adverse Reactions (Saunders, mictlan)
      • Fluid electrolyte imbalances
        • Hypokalemia
          • Can lead to muscle cramps and life-threatening heart arrhythmias
          • Potassium supplementation is often needed, or combination with potassium-sparing diuretic like spironolactone
          • The most serious drug reaction occurs with digoxin, as hypokalemia can cause digitalis toxicity
          • Closely monitor potassium levels and hold the drug if the patient’s potassium level is less than 3.5
        • Hyperglycemia
          • Use cautiously in patients with diabetes mellitus
          • Closely monitor blood glucose levels
        • Hyponatremia
          • Low sodium increases risk of lithium toxicity, so be extremely cautious if patient is taking lithium concurrently (generally lithium and thiazides are not taken together)
        • Hypercalcemia
          • In contrast to loop diuretics, which cause hypocalcemia
        • Hypomagnesemia
        • Hypochloremia
      • Increased lithium levels/toxicity
        • Diuresis causes a compensatory increase in reabsorption of cations at the proximal tubule, which increases lithium levels
      • Hyperuricemia
        • Patients are at an increased risk of gout
      • Photosensitivity
        • Related to sulfa drug structure
      • Sulfa drug (can cause allergy)
      • Orthostatic hypotension
        • Results from volume depletion induced by diuresis