What does the Cortisol (morning, ~30 minutes post-waking) test measure?

Why is Cortisol (morning, ~30 minutes post-waking) measured?

Morning cortisol is the first-line screening test for adrenal insufficiency; a level below 80–100 nmol/L in an unstressed state strongly suggests impaired reserve and warrants a dynamic stimulation test, while values above 500 nmol/L make insufficiency unlikely. It is also the foundation of Cushing syndrome screening — an elevated or unsuppressed morning level after overnight dexamethasone is typical of autonomous cortisol production.

Reference ranges

Standard reference

Morning (08:00): approximately 140–690 nmol/L (5–25 µg/dL); diurnal reference intervals are assay- and time-specific.

Functional medicine optimal

No universally defined optimal level; interpretation is pattern-based. Morning values in the upper half of the reference range in an appropriate circadian context reflect intact adrenal reserve, while persistently low or absent diurnal variation suggests pathology.

Units

nmol/L, µg/dL, ng/mg creatinine

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