As ADH is secreted, water is normally absorbed, with the subsequent elevation of urine osmolality. Following desmopressin administration, only a small additional rise in osmolality is observed. In diabetes insipidus, the urine fails to concentrate to twice the plasma osmolality due to lack of ADH, hence plasma osmolality rises. The urine concentrates adequately only after administration of desmopressin. Desmopressin therefore has no effect on patients with nephrogenic diabetes insipidus where the renal tubules are unresponsive to ADH. Diabetes insipidus may be masked in untreated hypopituitarism, sometimes being revealed only after hormone replacement, particularly with cortisol.
