What does the Cortisol test measure?

Why is Cortisol measured?

Cortisol measurement is central to the diagnosis of Cushing syndrome (elevated) and adrenal insufficiency (low). Single time-point values are less useful than paired serum cortisol with ACTH, late-night salivary cortisol, or 24-hour urinary free cortisol, but a dried blood spot offers a convenient screening option when logistics or patient preference preclude venepuncture.

Reference ranges

Standard reference

Morning cortisol typically 5–25 ng/mL, with marked diurnal variation; single dried blood spot values must be interpreted with collection time noted; method-specific.

Functional medicine optimal

Optimal morning cortisol typically sits in the mid-to-upper part of the reference range (10–20 ng/mL); very low values (<5 ng/mL) raise concern for insufficiency, while sustained elevation suggests hypercortisolism. Interpretation requires correlation with time of collection and clinical context.

Units

ng/mL, nmol/L

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