Serum cortisol concentrations follow a circadian rhythm, with maximal secretion occuring at 9 am. Samples for analysis should be taken as close to 9 am as possible. 

Indications for measurement include:

  • Investigation of adrenal insufficiency
  • Assessment of response to synacthen
  • Assessment of response to dexamethasone suppression

9 am/random serum cortisol measurements are not recommended for investigation of Cushing's syndrome. In these cases, a dexamethasone suppression test or 24-hour urine cortisol is the most appropriate test.

For short synacthen tests, please send 0 and 30 minute samples together, ensuring time of sample draw is indicated on the tubes.

For overnight dexamethasone suppression tests, please indicate this in clinical details to enable correct interpretation.

The cortisol assay may cross-react with steroid medications - if in doubt, please state which medications are in use when requesting.

Other Comments:

For more information, please see the following: Cortisol - Lab Tests Online

See also: ACTH, salivary cortisol, urine free cortisol

Content approved in Clinical Biochemists meeting 13.12.18

Last updated: Thursday 20th December 2018

About This Test

Department: Biochemistry


  • Gel/clotted (yellow/red top)

Add-on requests are accepted up to 48 hours post sampling

Turnaround Time:

Within 24 hours