Copeptin as a diagnostic tool in a patient with adipsia and hypernatremia

Classic adipsic vasopressin deficiency features hypotonic polyuria and absent thirst despite hypernatremia. We report a 21-year-old patient with prior resected pilocytic astrocytoma who presented dehydration and hypernatremia of 158 mEq/L (158 mmol/L) (reference range, 135-145 mEq/L [SI: 135-145 mmol/L]). He denied thirst and had no polyuria or polydipsia. Plasma and urinary osmolalities were 317 mOsm/Kg (317 mmol/Kg) (reference range, 282-295 mOsm/Kg [282-295 mmol/Kg]) and 878 mOsm/Kg (878 mmol/Kg) (reference range, 200-1200 mOsm/Kg [200-1200 mmol/Kg]), respectively. After prescribed oral water intake, serum sodium normalized and urinary volume remained normal. To assess osmoregulation, plasma copeptin and thirst visual scale were assessed during 3% saline infusion. Despite increases in serum sodium, the patient reported no change in thirst, confirming adipsia. Copeptin value was 3.2 pmol/L at the end of the saline infusion (reference range, 3.7-43.3 pmol/L) and 4.1 pmol/L during hypovolemic hypernatremia, both low-normal values considering hypernatremia, suggesting impaired osmolality-driven vasopressin secretion. This case illustrates a partial defect in osmoregulatory vasopressin release with maintained renal response and highlights the diagnostic utility of osmotic-stimulated copeptin measurement for functional characterization of vasopressin dynamics in rare adipsic disorders.