From the Guidelines
Normal cortisol levels are typically considered to be less than 1.8 μg/dL (50 nmol/L) at 0800 h in the morning after 1 mg dexamethasone given between 2300 h and midnight, as indicated by the most recent guideline update 1.
Context of Cortisol Levels
Cortisol levels vary throughout the day due to the hormone's natural circadian rhythm.
- In the morning, cortisol levels are typically higher to help the body wake up and face daily activities.
- As the day progresses, cortisol levels gradually decrease to allow for rest and recovery at night.
Interpreting Cortisol Test Results
When interpreting cortisol test results, it's essential to consider the time of collection and use the appropriate reference range for that specific time.
- Abnormal cortisol levels may indicate conditions like Cushing's syndrome (too high) or Addison's disease (too low).
Factors Affecting Cortisol Levels
Several factors can affect cortisol levels, including:
- Time of day
- Dexamethasone absorption and metabolism
- Concomitant medications, such as CYP3A4 inducers or inhibitors
- Corticosteroid binding globulin (CBG) levels, which can be increased by oral estrogens, pregnancy, or chronic active hepatitis
Clinical Implications
Accurate interpretation of cortisol test results is crucial for diagnosing and managing conditions related to cortisol imbalance.
- A serum cortisol level of less than 1.8 μg/dL (50 nmol/L) after 1 mg dexamethasone suppression test is considered a normal response, as stated in the most recent guideline update 1.
From the Research
Normal Cortisol Levels
The normal cortisol level can be measured in various ways, including 24-hour urinary free cortisol (24-h UFC) and serum cortisol.
- The study 2 measured 24-h UFC in a general population cohort and found that the stability of 24-h UFC was moderate, with a day-to-day stability of 0.69 and 0.72, and a long-term stability of 0.60.
- The study 3 found that a cutoff of 170 nmol per 24 hours for 24-h UFC had a sensitivity of 97% and a specificity of 91% for diagnosing Cushing's syndrome.
- The study 4 compared the sensitivity and specificity of 24-h UFC and serum cortisol in the diagnosis of Cushing's syndrome and found that the sensitivity of 24-h UFC was 92.54% with a cutoff point of <32 nmol.
- The study 5 found that suppression of urine free cortisol greater than 90% was associated with 100% specificity for pituitary disease.
- The study 6 discussed the accuracy and pitfalls of using overnight 1 mg dexamethasone suppression test and 24-h urine free cortisol for screening for Cushing's syndrome, but did not provide specific normal cortisol levels.
Factors Affecting Cortisol Levels
Several factors can affect cortisol levels, including:
- Urine volume: The study 2 found that urine volume was a strong predictor of 24-h UFC.
- Glomerular filtration rate: The study 2 found that glomerular filtration rate was also a predictor of 24-h UFC.
- Sex: The study 2 found that sex explained a small part of the variation in 24-h UFC.
- Time of day: Cortisol levels can vary throughout the day, with typically higher levels in the morning and lower levels at night.
- Other factors such as stress, exercise, and certain medications can also affect cortisol levels 3, 6.