Pembrolizumab, a PD-1 inhibitor, has been increasingly used in cancer therapy over the past decade. With its increasing use, more immune-related adverse events (irAE) such as pneumonitis, hepatitis, and endocrinopathies have been reported. We present a case of a 67-year-old man who developed isolated adrenocorticotropic hormone (ACTH) deficiency as an irAE following six cycles of pembrolizumab.

The clinical presentation, laboratory findings, and imaging results of a patient diagnosed with isolated ACTH deficiency secondary to pembrolizumab-induced immune-related adverse events after six treatment cycles.

A 67-year-old man was diagnosed with buccal squamous cell carcinoma (pStage I) and left upper lung squamous cell carcinoma (cT4N3M0, cStage IIIC). He received combination therapy with carboplatin, paclitaxel, and pembrolizumab. After 6 cycles of immunotherapy, he developed primary hypothyroidism with autoimmune thyroiditis and isolated ACTH deficiency leading to secondary adrenal insufficiency. A previous case report described a 44-year-old woman who developed isolated ACTH deficiency after four cycles of pembrolizumab without thyroid involvement, and her symptoms improved following steroid replacement. Given the rarity of endocrine irAEs, particularly secondary adrenal insufficiency, we recommend evaluating adrenal function when suggestive signs or symptoms appear.

Pembrolizumab is increasingly used in cancer treatment, and immune-related endocrine adverse events are being observed more frequently. We suggested to maintain a high level of suspicion and assess endocrine function promptly in patients presenting with symptoms suggestive of endocrinopathy during immunotherapy.