From the Guidelines
The optimal time to measure morning cortisol levels is at 0800 h, as this timing captures cortisol at its peak and allows for the most reliable baseline measurement. This recommendation is based on the natural circadian rhythm of cortisol, with levels typically highest in the early morning and lowest around midnight 1. For accurate results, the blood sample should be collected after fasting overnight and before taking any medications. It's essential to be awake for at least one hour before the test, as cortisol levels surge upon waking. Some key considerations to keep in mind include:
- Avoiding strenuous exercise, smoking, and consuming alcohol the day before testing, as these can affect results
- Informing the healthcare provider about any corticosteroid medications like prednisone, hydrocortisone, or dexamethasone, as these can interfere with test results
- Adjusting the timing for patients working night shifts or with irregular sleep patterns to correspond with their sleep-wake cycle The reference range for plasma cortisol is 140–700 nmol.l1 at 0900, but only 80–350 nmol.l1 at midnight, highlighting the importance of timing in cortisol measurement 1. By measuring cortisol levels at 0800 h, healthcare providers can properly assess adrenal function and diagnose conditions like Cushing's syndrome or Addison's disease.
From the Research
Optimal Time to Measure Morning Cortisol Levels
The optimal time to measure morning cortisol levels is not explicitly stated in the provided studies. However, the studies suggest that morning cortisol levels can be used as a screening test for adrenal insufficiency and that the timing of the measurement may not be critical.
- The study by 2 measured cortisol levels every 2 hours over 38 hours and found that the acrophases (peak times) of the cortisol rhythm occurred earlier in the morning in the morningness group.
- The studies by 3, 4, and 5 measured morning serum cortisol levels and found that they could be used to predict adrenal sufficiency and diagnose adrenal insufficiency.
- The study by 6 measured urine cortisol/creatinine ratios in the early morning and found that they could be used to evaluate pituitary-adrenal function.
Key Findings
- Morning cortisol levels can be used as a screening test for adrenal insufficiency 3, 4, 5.
- The timing of the measurement may not be critical, but early morning measurements may be more useful 6.
- The optimal cutoff value for morning cortisol levels to predict adrenal sufficiency is not established, but studies suggest that values above 350-400 nmol/L may be indicative of normal adrenal function 3, 4, 5.
Measurement Timing
- The study by 2 suggests that cortisol levels peak in the morning, but the exact timing is not specified.
- The study by 6 measured urine cortisol/creatinine ratios in the early morning (AM) and found that they could be used to evaluate pituitary-adrenal function.
- The other studies do not provide specific guidance on the optimal timing of morning cortisol measurements.