History and clinical findings: A 26-year-old woman presented with fatigue, muscle cramps and weakness. Since the age of 8 years she had moderate hypokalemia of unknown origin that was confirmed on multiple occasions. There was no family history of disease.
Investigations: Laboratory tests showed moderate to severe hypokalemia with a serum potassium concentration of 2.7 to 3.0 mmol/l, hypomagnesemia, metabolic alkalosis and pronounced stimulation of the renin-angiotensin-aldosterone system. Despite normal serum calcium levels, urinary calcium excretion was below the detection threshold. Increased natriuresis was observed after administration of furosemide, but not after administration of hydrochlorothiazide. This finding pointed to the presence of a non-functional thiazide-sensitive sodium/chloride cotransporter in the distal convoluted tubule, characteristic for Gitelman's syndrome. Genetic analysis confirmed the diagnosis of Gitelman's syndrome and documented two heterozygous mutations in the gene encoding the sodium/chloride cotransporter.
Treatment and course: The patient was treated with 160 mmol potassium and 30 mmol magnesium supplementation per day. Serum potassium was normalized and magnesium serum levels increased. Weakness and fatigue improved markedly.
Conclusion: Gitelman's syndrome is an important differential diagnosis in the evaluation of the normotensive patient with hypokalemia.