The Importance of Vigilant Prescreening and Monitoring: Missed Adrenal Insufficiency in a Patient on Pembrolizumab.

Immune checkpoint inhibitors (ICIs), such as pembrolizumab, have transformed cancer treatment by enhancing antitumour immunity. However, they are associated with immune-related adverse events (irAEs), including endocrinopathies like adrenal insufficiency. While existing literature extensively covers the risk of pembrolizumab-induced adrenal insufficiency, this case highlights the consequences of failing to act on an abnormal pretreatment cortisol result, leading to delayed diagnosis and clinical deterioration. We report the case of a 76-year-old male with metastatic squamous cell carcinoma (SCC) of the oesophagus undergoing treatment with pembrolizumab. A routine pretreatment blood test in mid-September revealed a cortisol level of 55 nmol/L (low), but this result was not flagged or acted upon. Three weeks later, the patient presented with worsening lethargy, vomiting and hyponatraemia. Follow-up testing confirmed persistently low cortisol levels, leading to a delayed diagnosis of adrenal insufficiency. Hydrocortisone replacement therapy was initiated, with subsequent symptom improvement. This case underscores the need for rigorous pretreatment hormone profiling, systematic review of laboratory test results and safeguarding mechanisms such as automated alerts for abnormal findings. Implementing structured endocrine monitoring in immunotherapy prescribing protocols could prevent similar occurrences and improve patient outcomes.