Low Dose
Overnight: 8 AM plasma cortisol: <5 ng/dL
Standard: Urinary free cortisol on day 3: <10 ng/day
High Dose
Overnight: >50% reduction in plasma cortisol.
Standard: >90% reduction in urinary free cortisol.
. Red-top tube.
■ Administration of dexamethasone suppresses ACTH and
should suppress cortisol levels in healthy people.
■ Helpful in determining cause of increased cortisol levels
(adrenal tumor, pituitary tumor, or ectopic ACTH-producing
tumor).
