Renal · Year 2 · from Renal
Case 2: Gitelman Syndrome
Patient Presentation
A 19-year-old female presents with fatigue, muscle cramps, and episodes of muscle weakness. She has had these symptoms intermittently since childhood but they have worsened recently during summer heat.
History of Present Illness
- Recurrent muscle cramps since age 10
- Salt craving
- No vomiting or diarrhea
- No diuretic or laxative use
- Excessive thirst but normal urine output
- Mother had similar symptoms
Physical Examination
- Blood pressure: 102/68 mmHg (low-normal)
- Heart rate: 76 bpm
- No edema
- Mild tetany with Chvostek sign positive
- Normal neurologic examination otherwise
Workup
Laboratory Studies:
- Potassium: 2.8 mEq/L (low)
- Magnesium: 1.4 mg/dL (low)
- Bicarbonate: 30 mEq/L (high - metabolic alkalosis)
- Calcium: 9.2 mg/dL (normal)
- Urine calcium: Low (hypocalciuria - 24-hour urine calcium 40 mg/day)
- Urine potassium: 45 mEq/day (inappropriately high)
- Renin and aldosterone: Both elevated
Genetic Testing:
- SLC12A3 gene mutation identified (NCC transporter)
Diagnosis
Gitelman Syndrome
Discussion
This case demonstrates distal convoluted tubule function:
- NCC Transporter: The lecture describes the thiazide-sensitive sodium-chloride cotransporter (NCC) in the DCT. Gitelman syndrome results from loss-of-function mutations in NCC, mimicking chronic thiazide use.
- Hypocalciuria Mechanism: When NCC is blocked, less sodium enters DCT cells, lowering intracellular sodium. This enhances basolateral Na+/Ca2+ exchange (NCX), increasing calcium reabsorption - opposite to loop diuretics which cause hypercalciuria.
- Hypomagnesemia: The DCT is the major site of regulated magnesium reabsorption; dysfunction causes magnesium wasting.
- Hypokalemia and Alkalosis: Volume depletion from sodium wasting activates RAAS, increasing aldosterone-mediated potassium secretion and hydrogen ion secretion.
Treatment
- Oral potassium supplementation (60-80 mEq/day)
- Oral magnesium supplementation (essential - K won't normalize without Mg)
- Liberal salt intake
- Potassium-sparing diuretics (amiloride) may help
- Avoid situations causing additional potassium loss
Clinical Pearl
Gitelman syndrome mimics thiazide diuretic use while Bartter syndrome mimics loop diuretic use. The key differentiating feature is urine calcium: Gitelman causes hypocalciuria (thiazide effect) while Bartter causes hypercalciuria (loop diuretic effect).