Objective Markers and New Indicators in AI Disease (OMNI-AID Study)

Inclusion Criteria: * Aged 18 - 85 years * Male or female * Participants who are otherwise healthy enough to participate, as determined by pre-study medical history and physical examination Patient groups only: * Diagnosed with AI for over 6 months according to standard diagnostic criteria or with a medical condition requiring acute high dose steroid therapy for anti-inflammatory purposes * If diagnosed with AI, established on stable HC replacement or prednisolone replacement, dose not altered for at least 3 months * Established on a stable dose of Fludrocortisone, if taking, dose not altered for at least 3 months * Participants taking other hormone replacements (e.g. levothyroxine, testosterone or growth hormone in secondary adrenal insufficiency) are accepted providing that their replacement doses have not altered for at least 3 months * Participants who are able and willing to give written informed consent to participate in the study. Exclusion Criteria: * Participants with a diagnosis of Type 1 or Type 2 diabetes mellitus. * Unable to give informed consent. * Excessive caffeine intake above 500 mg per day. * Taking supplements or herbal medications that the participant is unwilling or unable to stop prior to and during the study period e.g. St John's Wort (may decrease prednisolone levels), Cat's claw, Echinacea (immunomodulatory properties). * Currently taking medications that alter CYP3A4 metabolism of glucocorticoids that the participant is unwilling or unable to stop prior to and during the study period e.g. phenytoin, phenobarbital, rifampicin, rifabutin, carbamazepine, primidone, aminogluethimide, itraconazole, ketoconazole, ciclosporin or ritonavir. * Pregnancy, taking the oral contraceptive pill, or oral oestrogen replacement therapy due to the effects on cortisol binding globulin levels and determination of prednisolone levels. Transdermal oestrogen replacement is permitted. Females of child-bearing age will be asked to provide a urine sample for a pregnancy test at each visit. * Diagnosis of growth hormone deficiency, untreated * History of any medical, psychological or other condition, or use of any medications, including over-the-counter products, which, in the opinion of the investigators, would either interfere with the study.