Introduction. Gitelman syndrome (GS) is a rare inherited disorder. Mutations in SLC12A3 gene that encode tubular Na+Cl-cotransporter (NCCT) cause hypokalemic metabolic alkalosis, salt loss, hypomagnesaemia and hypocalciuria. The symptoms include weakness, vertigo, hypotension, tetany, paresthesia and nausea. Diagnostic criteria are a normal urine concentrating ability, normal glomerular filtration rate (GFR), hypomagnesaemia (<0,65mmol/l), hypokalemia (<3,6mmol/l) and hypocalciuria (<0,1mmol/mmol creatinine). Previously, the diagnosis was made by exclusion. Today, genetic analysis can ensure diagnosis. Thiazide diuretics (TD) abuse with similar abnormalities can make the differential diagnosis difficult. Causal therapy of GS does not exist. The substitution of potassium and magnesium are therapeutic strategies. Case presentation. A 41-year-old obese woman presented at the emergency department with recurrent episodes of hypokalemia with concomitant weakness, muscle cramps, polyuria and collapse. The results of laboratory testing of blood and urine led to the suspected diagnosis of GS. In the follow-up examinations, the results were inconsistent. Therefore, a transient thiazide diuretics abuse was assumed. Discussion. This case demonstrates the difficulties in making the diagnosis of GS on the basis of only clinical and laboratory tests, without the use of genetic analysis. The differentiation between GS and thiazide diuretic abuse is especially difficult.