How is a diagnosis of exogenous Cushing’s syndrome made?

A 48-year-old lady has been referred from her primary physician to rule out Cushing’s disease. She has been suffering with bronchial asthma for 20 years, with acute exacerbations at least 3 times/year requiring hospital admissions. She is on inhaled steroids everyday with use of oral steroids for 1–2 weeks at least 4–6 times/year. She is also diagnosed to have diabetes and hypertension for 5 years and is on treatment. She had sustained fracture of the left wrist following a trivial fall a year ago. On examination, she has central obesity, abdominal striae, thin skin and hypertension. Her primary physician ordered for an 8 am cortisol which showed 28 mg/dL. After receiving the patient, it is revealed that the patient was continuing to use prednisolone 10 mg/day without disclosing to her doctor. She is educated about being off all medications and a repeat 8 am cortisol is ordered which is 2 mg/dL this time suggesting exogenous cortisol use.

How is a diagnosis of exogenous Cushing’s syndrome made?