Cortisol & dexamethasone suppression
What it means
Cortisol interpretation hinges on suppressibility and ACTH. Cushing's screen: overnight/low-dose dexamethasone should suppress cortisol to <50 nmol/L — failure to suppress = Cushing's syndrome. Then localise: ACTH suppressed = adrenal (tumour) or exogenous steroid; ACTH high = pituitary (Cushing's disease — suppresses on high-dose dex) or ectopic (no suppression). Short Synacthen test: cortisol fails to rise after ACTH = adrenal insufficiency (Addison's). Random cortisol is unreliable (diurnal); interpret against the stimulus/suppression.
Worked example
A woman with irregular periods and hirsutism has cortisol post-1 mg dexamethasone of 40 nmol/L (<50) → cortisol SUPPRESSED normally → NOT Cushing's (look elsewhere, e.g. PCOS).
In the exam
A hirsute woman's cortisol after a 1 mg overnight dexamethasone dose is 40 nmol/L, i.e. it has suppressed below the 50 nmol/L threshold.
What settles it
Failure to suppress on dexamethasone = Cushing's; then ACTH high (pituitary/ectopic) vs suppressed (adrenal/exogenous). Failure to rise on Synacthen = Addison's.
Source: NICE CKS — Cushing's; Endocrine Society