What does the Total Cortisol (Serum) test measure?

Why is Total Cortisol (Serum) measured?

Total serum cortisol is used, with awareness of its diurnal rhythm, in the assessment of adrenal function — a low early-morning value can suggest adrenal insufficiency and a high late-night value can support hypercortisolism, though both usually require dynamic testing to confirm. Because it measures total hormone, changes in binding proteins (from oestrogens, pregnancy, or critical illness) shift the result without altering free cortisol, an important interpretive caveat. It is interpreted against the collection time and clinical context and often as part of dynamic tests rather than as a single value.

Reference ranges

Standard reference

Morning (approx. 8 am): 6–23 µg/dL (165–635 nmol/L); lower later in the day # REVIEWER FLAG: Reference intervals are time-of-day-specific and assay-dependent — reviewer to confirm against Quest's assay-specific ranges and the collection time assumed.

Functional medicine optimal

No functional 'optimal' single value; interpreted against the collection time and, where relevant, dynamic testing

Units

µg/dL, nmol/L

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