A 35-year-old male patient was admitted with fatigue and muscle weakness. He had been on methimazole due to thyrotoxicosis for 2 weeks. Laboratory tests showed overt hyperthyroidism and hypokalemia. Potassium replacement was started with an initial diagnosis of thyrotoxic hypokalemic periodic paralysis. Later on, despite the euthyroid condition and potassium chloride treatment, hypokalemia persisted. Further investigations revealed hyperreninemic hyperaldosteronism. The patient was considered to have Gitelman's syndrome (GS) and all genetic analysis was done. A c. 1145C>T, p.Thr382Met homozygote missense mutation located on solute carrier family 12, member gene 3, exon 9 was detected and GS was confirmed.