Hydrochlorothiazide Drug Review: PTCB Exam Guide
Study hydrochlorothiazide for the PTCB exam. Covers HCTZ, thiazide diuretics, mechanism, electrolyte effects, dosing, and key pharmacy tech exam tips.
Hydrochlorothiazide Overview
Hydrochlorothiazide, commonly abbreviated HCTZ, is one of the most prescribed diuretics and antihypertensive medications in the United States. It is a cornerstone drug for the PTCB certification exam, testing your knowledge of diuretic classes, electrolyte effects, and combination products.
Brand Name and Classification
Hydrochlorothiazide has been available generically for decades. Brand names include Microzide and HydroDIURIL (discontinued but still referenced). HCTZ belongs to the thiazide diuretic class. Other thiazides include chlorthalidone and indapamide. HCTZ is also found in many combination products including Hyzaar (losartan/HCTZ), Diovan HCT (valsartan/HCTZ), and Zestoretic (lisinopril/HCTZ).
Mechanism of Action
HCTZ inhibits sodium reabsorption in the distal convoluted tubule of the kidney. By blocking the sodium-chloride cotransporter, it increases excretion of sodium, chloride, and water, reducing blood volume and blood pressure. The site of action in the distal tubule distinguishes thiazides from loop diuretics (which act in the loop of Henle) and potassium-sparing diuretics (which act in the collecting duct).
Common Indications
- Hypertension: First-line therapy, often combined with other antihypertensives
- Edema: Mild to moderate fluid retention
- Nephrolithiasis prevention: Reduces calcium excretion, preventing calcium kidney stones
Dosing Information
HCTZ is available in 12.5 mg and 25 mg capsules and tablets. The typical dose for hypertension is 12.5 to 25 mg once daily. Higher doses (up to 50 mg) are rarely used due to increased side effects without proportional blood pressure reduction.
Electrolyte Effects
The electrolyte effects of thiazide diuretics are among the most heavily tested PTCB topics:
- Hypokalemia: Low potassium is the most significant concern; patients may need potassium supplementation
- Hyponatremia: Low sodium, especially in elderly patients
- Hypercalcemia: Thiazides increase calcium reabsorption (opposite of loop diuretics)
- Hyperuricemia: Elevated uric acid, which can trigger gout
- Hyperglycemia: May worsen blood glucose control in diabetic patients
Drug Interactions
- Lithium: Thiazides decrease lithium clearance, increasing toxicity risk
- Digoxin: Hypokalemia from HCTZ increases digoxin toxicity risk
- NSAIDs: May reduce the antihypertensive and diuretic effect
- ACE inhibitors and ARBs: Commonly combined but monitor for hypotension and hyperkalemia
PTCB Exam Tips
Key facts: HCTZ is a thiazide diuretic acting on the distal tubule, the major concern is hypokalemia, thiazides cause hypercalcemia (loop diuretics cause hypocalcemia), and it is found in many combination products. Know the electrolyte mnemonic: thiazides waste potassium, sodium, and magnesium while retaining calcium.
Practice diuretic questions at PTCBprep and review at our flashcards.
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