A 42-year-old woman with recurrent calcium oxalate nephrolithiasis (3 stones in 2 years) and normal renal function is found to have hypercalciuria (urine calcium > 300 mg/day) on 24-hour urine collection. She has no parathyroid disease or sarcoidosis. What is the drug of choice for reducing stone recurrence?

Explanation

## Drug of Choice for Recurrent Calcium Oxalate Stones with Hypercalciuria **Key Point:** Thiazide diuretics are the first-line pharmacological agent for reducing recurrence in patients with idiopathic hypercalciuria and recurrent calcium oxalate stones. ### Mechanism of Action of Thiazides Thiazides reduce urinary calcium excretion through multiple mechanisms: 1. **Increased proximal tubule reabsorption** of sodium and calcium (volume depletion effect) 2. **Direct inhibition of distal convoluted tubule calcium reabsorption** is offset by enhanced proximal reabsorption 3. **Net effect:** 20–30% reduction in 24-hour urinary calcium 4. **Hypercalciuria correction:** Normalizes urine calcium to < 250 mg/day ### Evidence for Thiazides | Outcome | Thiazide | Loop Diuretic | Potassium Citrate | Allopurinol | |---------|---|---|---|---| | **Reduces urine calcium** | Yes (20–30%) | No (increases) | No direct effect | No | | **Reduces stone recurrence** | 50% reduction | Worsens | 30–40% (if low citrate) | Only for uric acid stones | | **First-line for hypercalciuria** | Yes | No | Alternative | No | | **Mechanism in hypercalciuria** | Blocks distal Ca reabsorption | Increases urinary Ca | Increases urine citrate | Reduces uric acid | | **Typical agent** | Hydrochlorothiazide 25 mg daily | — | Potassium citrate 20–40 mEq/day | — | **High-Yield:** Hydrochlorothiazide 25 mg once or twice daily is the standard regimen. Recurrence rates drop from ~50% (untreated) to ~10–20% with thiazide therapy over 5 years. ### Why NOT Loop Diuretics? **Warning:** Loop diuretics (furosemide) are CONTRAINDICATED in stone formers because they: - Increase urinary calcium excretion (opposite of desired effect) - Worsen hypercalciuria - Increase stone recurrence risk ### Role of Potassium Citrate Potassium citrate is indicated when: - Hypocitraturia is present (urine citrate < 320 mg/day) - Recurrent calcium oxalate or calcium phosphate stones - Renal tubular acidosis It works by increasing urinary citrate (a stone inhibitor), NOT by reducing calcium. In this patient with isolated hypercalciuria and normal citrate, thiazide is superior. ### Role of Allopurinol Allopurinol reduces uric acid stone formation and is indicated for: - Recurrent uric acid stones - Hyperuricemia or hyperuricosuria - NOT for calcium oxalate stones (unless concurrent hyperuricosuria) ### Clinical Pearl Thiazide therapy should be accompanied by: - Low sodium diet (< 2.3 g/day) - Adequate hydration (urine output > 2 L/day) - Moderate calcium intake (not restricted; paradoxically reduces oxalate absorption) - Monitoring: repeat 24-hour urine at 3–6 months to confirm calcium reduction **Mnemonic: CHOP** — Calcium reduction, Hypercalciuria, Oxalate stones, Prevention - **C**alcium: Thiazides reduce urinary calcium - **H**ypercalciuria: Idiopathic hypercalciuria is the indication - **O**xalate: Calcium oxalate stones are the target stone type - **P**revention: First-line for recurrence prevention