Difficulty is often experienced when differentiating between primary polydipsia, mild cranial diabetes insipidus and nephrogenic diabetes insipidus. This is due to the fact that many polydipsic patients have a minor concentrating defect secondary to the solute loss from the renal interstitial medulla. Measurement of plasma vasopressin in response to osmotic stimulation, and the relationship of endogenous plasma vasopressin with urine osmolality after overnight dehydration, aids in the differentiation of these disorders.
