Pembrolizumab-Induced Addison's Disease Leading to Severe Hyponatremia in a Breast Cancer Survivor: A Case Report With Implications for Emergency Department Practice.

Pembrolizumab, a PD-1 inhibitor, has become a cornerstone in the treatment of various cancers, including breast cancer. However, its use is associated with immune-related adverse effects (irAEs), particularly those involving the endocrine system. This case report presents a rare instance of pembrolizumab-induced Addison's disease leading to severe hyponatremia. The case is supported by detailed laboratory findings and evaluated using the Naranjo adverse drug reaction probability scale. A 53-year-old female with a history of breast cancer presented with dizziness and fatigue while on a cruise. Initial laboratory tests revealed severe hyponatremia (serum sodium 117 mEq/L). Further evaluation revealed low cortisol (1.7 µg/dL) and elevated adrenocorticotropic hormone (ACTH) (452 pg/mL), indicative of adrenal insufficiency. Although thyroid function was normal, low IGF-1 levels suggested secondary adrenal insufficiency. The administration of hydrocortisone resulted in rapid symptom improvement, and the patient was discharged with a prescription for ongoing corticosteroid therapy. The Naranjo scale score of 4 indicated a possible relationship between pembrolizumab and the development of Addison's disease. This case underscores the critical need for awareness of irAEs in patients undergoing treatment with immune checkpoint inhibitors. The application of the Naranjo scale provided a quantitative assessment of the likelihood that pembrolizumab induced adrenal insufficiency. Emergency department protocols should incorporate endocrine evaluations for patients presenting with non-specific symptoms while undergoing immunotherapy. Pembrolizumab can lead to severe endocrine disorders, such as Addison's disease, which can result in life-threatening conditions like severe hyponatremia. Emergency clinicians must remain vigilant in recognizing and treating these adverse effects to optimize patient outcomes.