What is the optimal time of day to check cortisol levels?

Optimal Time for Cortisol Level Testing

The optimal time to check cortisol levels is between 7:00-9:00 AM when cortisol naturally peaks as part of the circadian rhythm, making this the standard collection time for baseline cortisol assessment. 1

Morning Cortisol Testing (7:00-9:00 AM)

• Morning cortisol testing is the standard for evaluating adrenal function as cortisol follows a circadian rhythm with peak levels occurring between 7:00-9:00 AM 1
• Values <50 nmol/L (<1.8 μg/dL) in morning samples strongly suggest adrenal insufficiency and warrant further evaluation 1
• Values >138 nmol/L (>5 μg/dL) in morning samples may indicate hypercortisolism, especially when clinical features are present 1

Special Testing Scenarios

For Hypercortisolism/Cushing's Syndrome Evaluation

• Overnight 1-mg dexamethasone suppression test (DST) is the preferred screening test, requiring administration of dexamethasone at 11 PM with cortisol measurement at 8 AM the following morning 2
• Late Night Salivary Cortisol (LNSC) should be collected at bedtime (typically between 11 PM-midnight) to assess for loss of normal circadian rhythm 2
• 24-hour Urinary Free Cortisol (UFC) requires complete collection over a full day and serves as an ancillary test when other results are equivocal 2

For Adrenal Insufficiency Evaluation

• The 30-minute cortisol response to ACTH stimulation is consistent regardless of basal cortisol level or time of day, making it a reliable measure of adrenal function 3
• Some evidence suggests that a single 60-minute post-Synacthen (ACTH) cortisol measurement may be sufficient and cost-effective for excluding adrenal insufficiency 4

Physiological Considerations

• The cortisol awakening response is distinct from the circadian rise in HPA activity and primarily reflects processes specific to the sleep-wake transition 5
• Individuals may have different morning cortisol patterns based on their chronotype (morning vs. evening types), with "evening types" potentially showing a delay in their early-morning cortisol peak 6

Common Pitfalls to Avoid

• Do not interpret cortisol levels in isolation; always correlate with clinical presentation and other diagnostic tests 1
• Avoid collecting samples in patients with altered circadian rhythms (e.g., shift workers) without adjusting interpretation accordingly 1, 2
• Consider medications that may affect cortisol metabolism when interpreting results 1, 2
• Remember that normal morning cortisol values do not completely exclude cyclic Cushing's syndrome 1
• Do not confuse pseudo-Cushing states (depression, alcoholism, obesity) with true Cushing's syndrome 1

Alternative Testing Approaches

• The midnight-to-morning urinary cortisol increment (comparing midnight and waking samples) may provide a noninvasive method for assessing HPA axis function 7
• For patients with disrupted circadian rhythms such as shift workers, the dexamethasone suppression test is preferred over time-dependent cortisol measurements 2