A medical student compares 24-hour urinary studies of two patients, one on chronic hydrochlorothiazide and one on chronic furosemide, both eating a normal diet. Which pattern of urinary calcium is expected?

• A High calcium with both drugs
• B Low calcium with furosemide, high calcium with hydrochlorothiazide
• C Low calcium with both drugs
• D High calcium with furosemide, low calcium with hydrochlorothiazide ✓

Explanation

Furosemide inhibits NKCC2 in the thick ascending limb, abolishing the lumen-positive potential generated by potassium recycling and thereby reducing paracellular calcium and magnesium reabsorption, so urinary calcium rises. Hydrochlorothiazide acts distally where calcium is actively reabsorbed, plus its volume contraction enhances proximal calcium reabsorption, so urinary calcium falls. This divergence explains why loops are used in hypercalcemia and thiazides in stone disease.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.