Nephrogenic diabetes insipidus (Fig. 5.19b) can be readily distinguished by the inappropriately high plasma vasopressin levels in relation to the low urine osmolality attained after dehydration. Occasionally, polydipsic patients have features of nephrogenic diabetes insipidus. In such cases, one week's treatment with desmopressin restores the responsiveness of the renal tubule to endogenous vasopressin is restored. Indeed, these patients may even develop hyponatraemia because of persistent inappropriate drinking in the presence of exogenous vasopressin. This observation has been recommended as the basis of a specific test for primary polydipsia.
