Immune checkpoint inhibitor (ICI)-related hypophysitis (RH) is a common immune-related adverse event. The early detection of ICI-RH prevents life-threatening adrenal insufficiency. However, good predictors of secondary adrenal insufficiency in ICI-RH have not yet been reported. We hypothesized that fluctuations in plasma adrenocorticotropic hormone (ACTH) and cortisol levels occur similarly to those in thyroid-stimulating hormone and thyroid hormone (thyroxine and triiodothyronine) levels in ICI-related thyroiditis. Here, we sought to test this hypothesis. Patients who used ICI and had a history of measurement of plasma ACTH and serum cortisol concentrations were retrieved from electronic medical records, and those with a history of glucocorticoid use were excluded from the analysis. We evaluated fluctuations in plasma ACTH and serum cortisol concentrations and the development of ICI-RH. For patients with ICI-RH, data at three points (before ICI administration (pre), maximum ACTH concentration (peak), and onset of ICI-RH) were analyzed to evaluate hormone fluctuations. A total of 202 patients were retrieved from the medical record. Forty-three patients were diagnosed with ICI-RH. Twenty-six out of 43 patients had sufficient data to evaluate fluctuations in plasma ACTH and serum cortisol concentrations and no history of glucocorticoid use. ACTH concentrations changed from 37.4 (29.9–48.3) (pre) to 64.4 (46.5–106.2) (peak) pg/mL (1.72–fold increase, p=0.0026) in the patients with ICI-RH before the onset. There were no differences in cortisol concentrations between the pre and peak values in patients with ICI-RH. We also evaluated the fluctuations in plasma ACTH and serum cortisol levels in patients who did not receive ICI-RH (62 cases). However, elevation of plasma ACTH levels was not observed in patients without ICI-RH, suggesting that transient elevation of plasma ACTH levels is a unique phenomenon in patients with ICI-RH. In conclusion, plasma ACTH levels were transiently elevated in some patients with ICI-RH before the onset of secondary adrenal insufficiency. Monitoring the ACTH levels and their fluctuations may help predict the onset of ICI-RH.