What does the Cortisol at 08:00 test measure?

Why is Cortisol at 08:00 measured?

The 08:00 cortisol is the anchor of the six-specimen series. Values below approximately 100 nmol/L (3.6 µg/dL) raise concern for adrenal insufficiency and typically prompt dynamic testing, while values above 500 nmol/L (18 µg/dL) make the diagnosis unlikely. Isolated morning elevation may be physiological stress or early Cushing syndrome; serial sampling across the day reveals whether the normal diurnal decline is preserved.

Reference ranges

Standard reference

Approximately 140–690 nmol/L (5–25 µg/dL) at 08:00, assay-specific.

Functional medicine optimal

A robust morning cortisol is usually above 400 nmol/L (14 µg/dL), with preservation of the diurnal decline by evening.

Units

nmol/L, µg/dL

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