Is an morning (AM) cortisol level of 18 micrograms per deciliter (mcg/dL) considered low or high?

Morning Cortisol Level of 18 mcg/dL is Normal to High

A morning cortisol level of 18 mcg/dL (approximately 500 nmol/L) is considered normal to high and indicates adequate adrenal function. This value is well above the threshold that would raise concern for adrenal insufficiency.

Understanding Cortisol Reference Ranges

Morning cortisol levels are interpreted based on established reference ranges:

• Low cortisol: <3-5 mcg/dL (<83-138 nmol/L)

  • Strongly suggestive of adrenal insufficiency 1
  • Values <4 mcg/dL (<110 nmol/L) typically warrant further investigation 2

• Indeterminate range: 5-15 mcg/dL (138-414 nmol/L)

  • May require additional testing such as ACTH stimulation test 3
  • Values between 10-15 mcg/dL (276-414 nmol/L) often indicate adequate adrenal function but may need confirmation in certain clinical contexts

• Normal to high range: >15 mcg/dL (>414 nmol/L)

  • Values >15-18 mcg/dL (>414-500 nmol/L) effectively exclude adrenal insufficiency 3
  • Your value of 18 mcg/dL falls in this category

Clinical Significance

A morning cortisol of 18 mcg/dL has important clinical implications:

• Rules out primary adrenal insufficiency: This level is significantly higher than the cutoff of <250 nmol/L (<9 mcg/dL) that would suggest primary adrenal insufficiency in the setting of acute illness 1

• No need for ACTH stimulation testing: According to multiple studies, morning cortisol values >14-16 mcg/dL (>375-444 nmol/L) have high specificity (95%) for predicting normal adrenal response to ACTH stimulation, making further dynamic testing unnecessary 4, 3

• Potential for hypercortisolism: While this value is within the upper normal range, persistent levels at this range might warrant consideration of mild hypercortisolism in appropriate clinical contexts 5

Important Considerations

• Timing matters: Confirm this sample was truly collected in the morning (7-9 AM) when cortisol levels naturally peak due to circadian rhythm 5

• Clinical context: Interpret this value in conjunction with clinical presentation:

  • Acute illness or stress can elevate cortisol levels 6
  • Medications (especially exogenous steroids) can affect results 1
  • Shift work or disrupted sleep patterns may alter normal cortisol rhythms 5

• Follow-up testing: If there are persistent symptoms suggesting adrenal dysfunction despite this normal value, consider:

  • Late-night salivary cortisol if Cushing's syndrome is suspected 5
  • 24-hour urinary free cortisol collection 5
  • ACTH level measurement to evaluate the hypothalamic-pituitary-adrenal axis 1

Conclusion

A morning cortisol level of 18 mcg/dL is normal to high and effectively rules out adrenal insufficiency. This value provides reassurance of adequate adrenal function and does not require further testing for adrenal insufficiency in most clinical scenarios.