Plasma vasopressin responses in postpartum hypopituitarism: impaired response to osmotic stimuli

The neurohypophyseal function was assessed in a group of 15 patients with postpartum hypopituitarism by measuring plasma arginine-vasopressin concentrations during 5% hypertonic saline infusion. None of the patients had symptoms of diabetes insipidus and all patients were on adequate cortisone and thyroxine replacement therapy before testing. The mean basal plasma vasopressin value in the patients (0.6 +/- 0.1 pmol/l) was significantly lower than that in the normal subjects (2.9 +/- 0.3 pmol/l; p < 0.01), whereas the mean serum sodium, plasma osmolality, plasma renin activity and serum aldosterone values were similar in the two groups. During the osmolar load (5% hypertonic saline), the patients revealed varying degrees of arginine-vasopressin responses to the increase in plasma osmolality. Three patients showed normal arginine-vasopressin responses, 10 had subnormal responses, and 2 had no response. During the dehydration test, the patients revealed significantly lower maximum urine osmolalities (p < 0.0025) with significantly higher concurrent mean plasma osmolality (p < 0.0025) than the controls. None of the patients showed overt polyuria at the time of the study. The results indicate the impaired osmoregulation of arginine-vasopressin secretion to an osmolar stimuli in patients with postpartum hypopituitarism, suggesting neurohypophyseal damage. In patients with Sheehan's syndrome, partial diabetes insipidus seems to be much more frequent than previously believed.