Hypernatremic myopathy was rarely reported in the literature and its clinical features have never been well-described. We present a 22-year-old man who had adipsic hypernatremia manifested with progressive proximal muscle weakness and remarkably high creatine kinase level that has never been reported in the cases of hypernatremic myopathy. His initial presentations were similar to that of polymyositis without the evidence of central nervous system dysfunction and hypopituitarism. The serum level of sodium at the beginning of myopathy is the lowest known in the literature. All the clinical presentations in this patient resulted from a hypothalamic mixed germ cell tumor with sub-acute intra-tumoral hemorrhage.