SummaryShort synacthen tests (SST) are frequently used for assessing adrenocorticotropin hormone (ACTH) deficiency. In this study, we present the case of a 53-year-old man receiving immunotherapy for metastatic melanoma, who subsequently developed immune checkpoint inhibitor (ICI)-induced hypothyroidism and was investigated for the presence of ICI-induced hypocortisolaemia on different occasions. Despite two reassuring SSTs, he subsequently developed clinical and biochemical evidence of ACTH deficiency. The ACTH on local measurement was not conclusive in keeping with ICI-related ACTH deficiency but when repeated using an alternative assay confirmed the diagnosis. The case illustrates the evolution of ACTH deficiency and exposes the potential pitfalls of screening strategies. Two important lessons may be gleaned from this case: (i) SSTs can be normal in early cases of secondary adrenal insufficiency, for example, hypophysitis due to adrenal reserve and (ii) when there is mismatch between the clinical and biochemical presentation, the ACTH should be repeated using a different assay.Learning pointsShort synacthen tests, useful for ruling out adrenalitis and primary adrenal failure, may be normal in early adrenocorticotrophic hormone deficiency and secondary adrenal failure due to residual adrenal reserve.If clinical suspicion of adrenal insufficiency persists despite an initial satisfactory SST, it is important to recognise the need for re-assessment of cortisol levels.Insulin tolerance test remains a useful tool in the investigation of secondary adrenal insufficiency.Levothyroxine replacement may accelerate the metabolism of cortisol in cases of concurrent hypothyroidism and hypoadrenalism, and therefore, glucocorticoid replacement must precede levothyroxine treatment, to avoid adrenal crisis.The prevalence of immunotherapy-related endocrinopathies is likely to increase with increasing use of ICI and it is crucial that clinicians are alert to their subtle symptoms.