A Pitfall in the Differential Diagnosis of Hyponatremia

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We report a case of a 75-year-old female, with a known history of Hashimoto’s disease, who was admitted with anorexia, nausea and vomiting. Laboratory data revealed hyponatremia and hypothyroidism. Despite thyroid hormone replacement with synthetic thyroxine, the patient had persistent hyponatremia. Further investigations revealed secondary adrenal insufficiency but otherwise normal pituitary function, based on a rapid adrenocorticotrophic hormone (ACTH) test as part of a combined anterior pituitary stimulation test. She was diagnosed with isolated ACTH deficiency (IAD) with concomitant Hashimoto’s disease. Adrenal insufficiency should be considered in patients with hypothyroidism and persistent hyponatremia. In patients with Hashimoto’s disease who are found to have concomitant hypoadrenalism, IAD should also be suspected after primary adrenal insufficiency is ruled out.