Cortisol, also known as hydrocortisone, is a glucocorticoid produced by the adrenal gland. This stable labeled internal standard is suitable for quantitation of cortisol levels in whole blood by LC-MS/MS for newborn screening of congenital adrenal hyperplasia (CAH) and in serum or plasma by LC-MS/MS for determination of several diseases including Cushing’s syndrome, Addison’s disease, and Sheehan’s syndrome.