Endocrine Abstracts

Introduction: Acquired isolated ACTH deficiency with normal secretion of the remaining pituitary hormones is a rare condition in the absence of exposure to exogenous glucocorticoids. The aetiology is assumed to be autoimmune in most adult cases with traumatic brain injury and radiation exposure being rare causes. Pituitary transcription factor mutations may be identified in congenital and childhood onset cases. Hyponatraemia is typically associated with primary adrenal insufficiency due to a lack of aldosterone, but is rarely encountered in secondary adrenal insufficiency.

Case report: We report three patients (two females and one male, age 58–82 years) presenting with moderate to severe hyponatraemia (114–127 mmol/l) as the initial and principal manifestation leading to the diagnosis of secondary adrenal insufficiency due to isolated ACTH deficiency (serum cortisol 20–113 nmol/l, ACTH 5–20 ng/l). All patients reported longstanding (1–10 years) fatigue and weakness retrospectively. Relapsing hyponatraemia had been previously documented in one patient. The further workup did not reveal deficiencies of other pituitary hormones and sellar MRI scans were normal in two patients and showed a partially empty sella in the remaining. Hydrocortisone replacement resulted in rapid resolution of hyponatraemia and clinical improvement in every patient.

Conclusion: Secondary adrenal insufficiency due to isolated ACTH deficiency, although rare, must be included in the differential diagnosis of hyponatraemia. Enhanced vasopressin release due to a lack of cortisol mediated negative feedback suppression is the likely pathogenetic mechanism leading to euvolemic hyponatraemia. Associated autoimmune diseases may point to the underlying mechanism of corticotroph dysfunction but the etiology may remain obscure despite extensive workup. Hydrocortisone replacement results in rapid clinical and biochemical improvement and should promptly be installed.