Pharmacology · patient case question
Which adverse effect is most characteristic of hydrochlorothiazide?
Patient case: Controlled Hypertension, and a Moment of Lightheadedness
Patient: 63-year-old male
Chief Complaint: Scheduled restoration of tooth #30.
Background: Hypertension, well controlled for six years. Attends his physician regularly. Ate breakfast this morning.
Medications: Lisinopril 20 mg daily, hydrochlorothiazide 25 mg daily
Vital Signs: Pre-operative BP 132/80 mmHg · HR 74 bpm · RR 16 · T 98.4°F
Findings: Ten minutes into the procedure, reclined in the chair, he reports feeling lightheaded. He is conscious, speaking normally, mildly pale, skin slightly moist. No chest pain, no shortness of breath, no weakness or slurred speech.
Show answer and explanation
Correct answer: A. Hypokalaemia
- A. CORRECT. HYPOKALAEMIA is the signature adverse effect of thiazide diuretics. By blocking the sodium-chloride symporter in the DISTAL CONVOLUTED TUBULE, more sodium reaches the collecting duct, where it is exchanged for potassium — which is then lost in the urine. Thiazides also cause hyponatraemia, hyperuricaemia (gout), hyperglycaemia, hypercalcaemia and photosensitivity, and dry mouth is a common patient complaint.
- B. Ototoxicity belongs to the LOOP diuretics — furosemide in particular, especially given rapidly by intravenous route or with an aminoglycoside.
- C. Dry cough belongs to the ACE inhibitors, through bradykinin accumulation.
- D. Hyperkalaemia is the opposite, and belongs to the potassium-SPARING diuretics (spironolactone, amiloride, triamterene) and to ACE inhibitors and ARBs.
Note: ⚑ RECURRING SOURCE ERROR ACROSS THIS SERIES: hydrochlorothiazide is a THIAZIDE acting on the distal convoluted tubule, NOT a loop diuretic. Loop diuretics (furosemide, bumetanide) act on the thick ascending limb of the loop of Henle. Both waste potassium, but they are different classes with different potencies and different toxicities.