Background: Patients who were surgically treated for Cushing's syndrome postoperatively surrender to "primary" adrenal insufficiency. However, the preoperative over-secretion of cortisol or the postoperative administration of excessive glucocorticoids can cause "secondary" adrenal insufficiency, in which the prevalence of hyponatremia is usually lower than that of primary adrenal insufficiency.
Case report: A 60-year-old woman with a past medical history of Cushing's syndrome developed hyponatremia with symptoms of acute glucocorticoid deficiency, such as prolonged general fatigue and anorexia, after upper respiratory tract infection. A decrease in the serum cortisol level and the lack of increase in the ACTH level, despite the increased demand for cortisol, enabled a diagnosis of "secondary" adrenal insufficiency. Although the initial fluid replacement therapy was not effective, co-administration of dexamethasone and sodium chloride quickly resolved her symptoms and ameliorated the refractory hyponatremia.
Conclusions: In this case, the hypothalamic-pituitary axis of the patient was thought to have become suppressed long after the surgical treatment for Cushing's syndrome. This case suggested a mechanism of refractory hyponatremia caused by secondary adrenal insufficiency, for which the administration of dexamethasone and sodium chloride exerted additional therapeutic efficacy.