Adrenocorticotropic hormone (ACTH) is a hormone produced by the pituitary gland that stimulates the production of cortisol. Cortisol is a steroid hormone produced by the adrenal glands that is important for regulating glucose, protein, and lipid metabolism; suppressing the immune system’s response; and helping to maintain blood pressure. This process of stimulating ACTH production in the pituitary is done via the hypothalamic-pituitary-adrenal axis (HPA). Laboratory evaluation of ACTH can be assessed by ACTH. For most accurate results, testing should be done between 6 and 10 a.m. ACTH laboratory tests require a special refrigerated centrifuge to immediately separate plasma from cells, then the specimen needs to be immediately frozen. ACTH concentrations vary considerably depending on physiologic conditions and therefore should always be evaluated simultaneously with cortisol. Different assay methods can vary and cannot be used interchangeably. ACTH can also be measured as part of the ACTH stimulation test.