What are the causes of hypernatremia? It is not rocket science. Either you lost water, or gained sodium (which is less likely, because where did all that extra sodium come from). If the water has been lost, it was either lost through the kidneys, or somewhere else (meaning gut, sweat, etc). If it was lost therough the kidneys, it was either diabetes insipudus, or some kind or diuresis, or hyperaldosteronism. If it was diabetes insipidus, it was either nephrogenic (eg. due to some sort of renal disease or drugs) or it was central (i.e. something has been interfering destructively with the posterior pituitary). Of the weirder causes of diabates insipidus, one needs to remember hypokalemia, hypercalcemia, multiple myeloma and medullary sponge kidney (because everybody always remembers lithium).