What does the Cortisol test measure?

Why is Cortisol measured?

Serum cortisol assesses HPA-axis function and is central to the evaluation of both adrenal insufficiency and cortisol excess (Cushing's syndrome). Because of the circadian rhythm, the time of collection is integral to interpretation, and morning measurement is standard for screening adrenal insufficiency. It is typically used within dynamic testing protocols — such as the ACTH stimulation test or dexamethasone suppression test — rather than as a single static value, and in integrative practice it anchors assessment of the stress axis.

Reference ranges

Standard reference

AM (07:00–09:00): 6.2–19.4 µg/dL (171–536 nmol/L); PM (~16:00): 2.3–11.9 µg/dL (64–327 nmol/L) # REVIEWER FLAG: Time-of-day reference intervals are assay- and laboratory-specific — reviewer to confirm against LabCorp cortisol assay ranges before publication.

Functional medicine optimal

Functional medicine practitioners assess cortisol against the expected circadian curve rather than a single optimal value, often favouring diurnal salivary profiling # REVIEWER FLAG: Functional diurnal-curve framing is not standardised — reviewer to confirm whether the platform wishes to present one.

Units

µg/dL, nmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.