Abstract
Summary: A 52-year-old female patient with breast cancer presented with a history of fatigue and malaise 1 year prior. She was diagnosed with isolated adrenocorticotropic hormone (ACTH) deficiency (IAD) on endocrinological examination. Her pituitary gland showed normal morphology. Paraneoplastic IAD associated with breast cancer was suspected; however, immunofluorescence staining revealed no ectopic ACTH or proopiomelanocortin expression in the tumor tissue. Subsequently, the patient was diagnosed with idiopathic acquired IAD concurrent with breast cancer, ruling out paraneoplastic syndrome. Although malignancy should be considered a potential cause of IAD, not all patients with concurrent IAD and malignancy necessarily develop paraneoplastic syndrome. Learning points: Several adrenal insufficiency symptoms are similar to the nonspecific symptoms associated with malignancies, and therefore, the diagnosis of IAD remains challenging, especially in patients with cancer. When we encounter a case of IAD accompanied by a malignant tumor, it is important to suspect that paraneoplastic IAD, a novel clinical condition as secondary hypophysitis, may be the etiologic agent. Although malignant tumours should be considered a potential cause of IAD, not all patients with concurrent IAD and malignancy necessarily develop paraneoplastic autoimmune hypophysitis.