Cortisol PM measures cortisol levels in the afternoon or evening, typically collected between 4pm and 8pm. In a normal circadian rhythm, cortisol is at its daily peak in the early morning and should be substantially lower by evening. Evening cortisol that remains high is clinically significant because it may indicate Cushing's syndrome or disruption of the normal cortisol circadian pattern.
Loss of the normal diurnal cortisol decline is one of the most sensitive indicators of Cushing's syndrome — in most forms of cortisol excess, the late-night nadir is lost before absolute morning cortisol becomes distinctly elevated. Evening or late-night cortisol testing therefore offers earlier and more sensitive detection of pathological cortisol excess than a single morning measurement alone.
FAQs
What is late-night salivary cortisol and how does it differ from blood cortisol PM?
Late-night salivary cortisol (collected at 11pm) is the most sensitive screening test for Cushing's syndrome. It measures free cortisol in saliva and can be done at home. Blood cortisol PM is a useful alternative when salivary testing is not available, though it requires a blood draw at a specific time of day.
Can poor sleep cause elevated evening cortisol?
Yes. Chronic sleep deprivation, irregular sleep schedules, and conditions like obstructive sleep apnoea are associated with elevated evening cortisol and disruption of the normal cortisol circadian rhythm, contributing to metabolic and cardiovascular risk.
Does stress cause high cortisol PM?
Chronic psychological stress activates the HPA axis and can blunt the normal cortisol decline, producing persistently elevated evening cortisol. This is distinct from pathological Cushing's syndrome but can still have metabolic and health consequences if sustained.
Is elevated evening cortisol always Cushing's syndrome?
No. Many people have elevated evening cortisol from stress, sleep disorders, or lifestyle factors without Cushing's syndrome. Cushing's syndrome requires a consistent pattern of cortisol excess confirmed by multiple tests including urinary free cortisol, late-night salivary cortisol, and dexamethasone suppression.
