Diurnal variation in cortisol
Plasma cortisol levels are highest on waking and lowest before early sleeping.
Dexamethasone is used to suppress ACTH production to determine if there is something wrong with the adrenal-pituitary axis. An advantage is that dexamethasone does not cross react with other cortisol assays. The caveats are that the cytochrome p450 system metabolizes dexamethasone, so agents that stimulate the p450 system may cause this test not to work (e.g. in alcoholics). Obese patients with a higher volume of distribution may not suppress on the screening dose.
Low dose and high doses of dexamethasone are used to differentiate Cushing's syndrome and Cushing's disease.
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