The aetiology of hypotonic polyuria, after excluding solute diuresis, is one of primary polydipsia, central, or nephrogenic diabetes insipidus. Theoretically, these disorders should be relatively easily distinguished based on history and the results of an indirect water deprivation test. Practically, however, there is a significant overlap in diagnostic evaluation, potentially leading to an erroneous diagnosis and deleterious management plan. The ability to measure a stimulated copeptin level, either with hypertonic saline or arginine infusion, has led to greater diagnostic accuracy.