What does the Cortisol (afternoon specimen) test measure?

Why is Cortisol (afternoon specimen) measured?

Comparing afternoon and morning cortisol reveals whether the normal diurnal rhythm is preserved. An afternoon level that remains as high as or higher than the morning value suggests loss of hypothalamic-pituitary control and is a red flag for Cushing syndrome. In suspected adrenal insufficiency the afternoon specimen is less discriminatory, but paired sampling clarifies whether low morning cortisol reflects true deficiency or simply late sampling on the descending slope.

Reference ranges

Standard reference

Afternoon (16:00): approximately 50–250 nmol/L (2–9 µg/dL); continuing decline toward evening nadir.

Functional medicine optimal

Afternoon cortisol is expected to fall to roughly half the morning peak; functional frameworks propose a mid-range afternoon value (around 200–350 nmol/L) as optimal, though evidence is limited.

Units

nmol/L, µg/dL

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