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Abstract Background Several immune checkpoint inhibitors have been implemented for cancer treatment which have shown some degree of antitumor effcacy, while immune-related adverse events (irAEs) that affect multiple organ functions ensue which obviously should not be neglected. Though less common than other kinds of irAEs, Immune checkpoint inhibitors (ICIs) related Isolated ACTH deficiency (IAD) may cause long-term damage to pituitary-adrenal axis. Several case reports are available about IAD during anti-PD-1 therapy. We report the first case of immune checkpoint inhibitor-induced IAD following 3 month of sintilimab therapy. Case presentation A 66-year-old Chinese man was diagnosed with stage IIIB lung adenocarcinoma with involving ipsilateral intrapulmonary and hilar lymph node metastasis. After 3 months of combination therapy of nedaplatin, pemetrexed and sintilimab, the patient presented with general fatigue, nausea and vomiting. Laboratory investigation at admission revealed hyponatremia and hypokalemia. Further investigation revealed adrenocorticotropic hormone and cortisol levels were far below than normal limits. His other pituitary hormone levels were normal, except for mild elevation of follicle stimulating hormone and estradiol. Cranic magnetic resonance imaging showed a normal pituitary gland. Isolated adrenocorticotropic hormone deficiency was diagnosed, and corticosteroid replacement therapy was administered, leading to a significant improvement of his symptoms while ACTH level maintaining low level. Conclusions Our patient developed isolated ACTH deficiency during combination cancer treatment with chemotherapy and sintilimab. Although isolated ACTH deficiency due to anti-PD-1 including sintilimab therapy is rare occurrence, it can often cause severe clinical symptoms. Its diagnosis basically relies on clinical symptoms and endocrinological examination. Unlike traditional hypophysitis diagnosed by cranial MRI, pituitary MRI of IAD due to anti-PD-1 often indicates normal pituitary gland implying that over-reliance on imaging findings is not recommended. Even if clinical symptoms have relieved after corticosteroid replacement therapy was commenced, low levels of ACTH or cortisol could maintain for a long period which highlights the need for long term corticosteroid therapy. The purpose of the current report was to provide increased awareness of early detection and therapy of IAD.