Cortisol
What it measures, and what changes the number.
Cortisol is a steroid hormone made by the adrenal glands. It helps regulate energy use, blood pressure and the body's response to stress. This blood test measures total cortisol, which includes both the portion bound to carrier proteins and the small free portion.
It is often read alongside ACTH, the pituitary hormone that drives cortisol production, and sometimes with urinary free cortisol or DHEA-S to give a fuller picture of adrenal activity.
Cortisol follows a strong daily rhythm, peaking in the early morning and falling through the day, so the time of collection matters a great deal and reports often state it, as in 'CORTISOL, A.M.'. Stress, illness, sleep patterns and anything that changes carrier protein levels, such as oral estrogen, can shift the result. Immunoassay and mass spectrometry methods can give different numbers.
Reference ranges labs publish
Each lab states an interval for its own method and the population it measured. They differ, they are not interchangeable, and none of them is a verdict on a single result.
- Everyone, 18 years and over, morning (a.m.) draw: 7 to 25 mcg/dL (193 to 690 nmol/L) · Immunoenzymatic assay · Source: Mayo Clinic Laboratories, CORT (Cortisol, Serum) (read 2026-09-24)
- Everyone, 18 years and over, afternoon (p.m.) draw: 2 to 14 mcg/dL (55.2 to 386 nmol/L) · Immunoenzymatic assay · Source: Mayo Clinic Laboratories, CORT (Cortisol, Serum) (read 2026-09-24)
- Everyone, morning (a.m.) draw: 6.2 to 19.4 μg/dL (171 to 535 nmol/L) · ECLIA (immunoassay) · Source: Labcorp, 004051 (Cortisol) (read 2026-09-24)
- Everyone, afternoon (p.m.) draw: 2.3 to 11.9 μg/dL (63.5 to 328 nmol/L) · ECLIA (immunoassay) · Source: Labcorp, 004051 (Cortisol) (read 2026-09-24)
Usually reported in nmol/L or ug/dL.
Often read alongside: ACTH, Urinary free cortisol (24h), DHEA-S, Urinary free cortisol