Clinical Research Directory

Inclusion Criteria: * Male or female participants must be aged 16 years or older at the time of signing the informed consent/assent. * In participants aged \<18 years, height velocity must be less than 2 cm/year in the last year and puberty must be completed (Tanner stage V). * Participants with known classic CAH due to 21 hydroxylase deficiency diagnosed in childhood with documented (at any time) elevated 17-OHP and with or without elevated A4 and currently treated with hydrocortisone, prednisone, prednisolone or dexamethasone (or a combination of the aforementioned glucocorticoids) and on stable glucocorticoid therapy for a minimum of 3 months. * Participants who are receiving fludrocortisone must be on a documented stable dose for a minimum of 3 months prior to enrollment and must have stable renin levels at screening. * Female participants of childbearing potential and all male participants must agree to the use of an accepted method of contraception during the study. * A female participant is eligible to participate if she is not pregnant, not breastfeeding, and she is either not a woman of childbearing potential (WOCBP) or has a negative pregnancy test at entry into the study. Note: females presenting with oligomenorrhea or amenorrhea who are aged ≤55 years should be considered potentially fertile and therefore should undergo pregnancy testing like all other female participants. * Capable of giving signed informed consent/assent which includes compliance with requirements and restrictions listed in the informed consent form (ICF) and in this protocol. Exclusion Criteria: * Clinical or biochemical evidence of hepatic or renal disease e.g. creatinine \>2 times the upper limit of normal (ULN) or elevated liver function tests (alanine aminotransferase \[ALT\] or aspartate aminotransferase \[AST\] \>2 times the ULN). * History of bilateral adrenalectomy. * History of malignancy (other than basal cell carcinoma successfully treated \>26 weeks prior to entry into the study). * Participants who have type 1 diabetes or receive regular insulin, have uncontrolled diabetes, or have a screening HbA1c greater than 8%. * Persistent signs of adrenal insufficiency or the participant does not tolerate treatment at the end of the 4-week run-in period. * Participants with any other significant medical or psychiatric conditions that in the opinion of the Investigator would preclude participation in the study. * Participants on regular daily inhaled, topical, nasal or oral steroids for any indication other than CAH. * Co-morbid condition requiring daily administration of a medication or consumption of any material that interferes with the metabolism of glucocorticoids. * Participants who are receiving \<10 mg hydrocortisone dose at screening or the hydrocortisone dose equivalent. * Participants anticipating regular prophylactic use of additional steroids e.g. for strenuous exercise. * Participation in another clinical study of an investigational or licensed drug or device within the 12 weeks prior to screening. * Inclusion in any natural history or translational research study that would require evaluation of androgen levels during the study period outside of this protocol's assessments. * Participants who have previously been exposed to Chronocort in any Diurnal study. * Participants who routinely work night shifts and so do not sleep during the usual night-time hours. * Participants, who in the opinion of the Investigator, will be unable to comply with the requirements of the protocol. * Participants with a known hypersensitivity to any of the components of the Chronocort capsules, the Cortef tablets, or the placebo capsules. * Participants with congenital galactosemia, malabsorption of glucose and galactose, or who are lactase deficient. * Participants with a body weight of 45 kg or less.