Misinterpretation of serum cortisol in a patient with hyponatraemia

Primary adrenal insufficiency or Addison's disease is caused by bilateral adrenocortical destruction and is a relatively common endocrinopathy, with a prevalence of about 40-60 per million adults.1 Adrenal insufficiency is an important consideration in any critically ill patient, as failure to make the diagnosis may have fatal consequences. In contrast, early diagnosis and treatment with appropriate corticosteroid replacement restores health and a normal life expectancy. We present a case in which an unusual presentation of Addison's disease caused diagnostic confusion. Although the possibility of adrenal insufficiency was eventually considered, incorrect interpretation of laboratory investigations led to diagnostic delay. A 93 year old man without a significant medical history and taking no regular drugs had become unwell on holiday, 18 months before his admission to our department. At that time, he complained of weight loss, nausea, and vomiting. He was admitted to a local hospital and underwent an upper gastrointestinal endoscopy. This revealed oesophagitis with Barrett's mucosa, and treatment with a proton pump inhibitor was started. He again become unwell two months later and was admitted to hospital with vomiting, drowsiness, and fever. He was found to be clinically dehydrated, and a chest x ray showed a left lower lobe pneumonia. Biochemical investigations revealed hyponatraemia, with a serum sodium concentration of 121 mmol/l. His serum potassium concentration was 5.0 mmol/l. The hyponatraemia was investigated and ascribed to the syndrome of inappropriate antidiuretic hormone secretion (SIADH). A random serum cortisol concentration was 163 nmol/l. After treatment with intravenous fluids and antibiotics, the …