What is Ante Meridiem (AM) cortisol?

AM Cortisol: Definition and Clinical Significance

AM cortisol refers to a morning blood test that measures cortisol levels typically drawn between 7-9 AM, when cortisol naturally peaks as part of its diurnal rhythm, serving as a critical screening tool for adrenal insufficiency. 1

Understanding AM Cortisol Testing

Morning cortisol measurement is a fundamental component in evaluating adrenal function for several reasons:

• Optimal Timing: The American College of Endocrinology recommends drawing AM cortisol between 7-9 AM to accurately reflect the normal diurnal peak of cortisol secretion 1
• Diagnostic Value: Morning cortisol levels provide crucial information about the hypothalamic-pituitary-adrenal (HPA) axis function
• Screening Tool: AM cortisol serves as an initial screening test before proceeding to more complex dynamic testing like the ACTH stimulation test

Interpretation of AM Cortisol Results

AM cortisol results should be interpreted according to these evidence-based thresholds:

• <250 nmol/L (9 μg/dL) with elevated ACTH: Strongly suggestive of primary adrenal insufficiency 1
• <275 nmol/L: Identifies subnormal cortisol response with 96.2% sensitivity, indicating need for further testing 2
• ≥300 nmol/L: Generally excludes the possibility of adrenal insufficiency in non-acute cases 3
• <110 nmol/L: Likely indicates subnormal cortisol response to ACTH, suggesting adrenal insufficiency 3

Clinical Applications

AM cortisol testing is particularly valuable in:

1. Initial screening for suspected adrenal insufficiency
2. Monitoring patients on glucocorticoid therapy
3. Evaluating patients with symptoms of cortisol excess or deficiency
4. Differentiating between primary and secondary adrenal insufficiency when measured alongside ACTH levels

Limitations and Considerations

Several factors can affect AM cortisol interpretation:

• Assay variability: Different laboratory methods (immunoassays vs. LC-MS/MS) may yield different results 4
• Timing issues: Samples collected later than 9 AM may show lower values due to diurnal variation 5
• Medication effects: Exogenous steroids can suppress morning cortisol levels 1
• Stress response: Acute illness or stress can elevate cortisol levels, potentially masking underlying insufficiency 1

Follow-up Testing

When AM cortisol results are indeterminate or suggestive of adrenal insufficiency:

• ACTH stimulation test: Gold standard for confirming adrenal insufficiency 1
• Additional pituitary hormone testing: Including TSH, Free T4, LH, FSH, and testosterone/estradiol 1
• Imaging: Adrenal CT for primary insufficiency or pituitary MRI for secondary causes 1

Clinical Pearls

• Never delay treatment for suspected adrenal crisis to obtain diagnostic tests 1
• AM cortisol measurement is cost-effective as an initial screening tool 3
• In patients taking exogenous steroids, morning cortisol may be suppressed, requiring careful interpretation 1
• Always evaluate cortisol results in clinical context, as laboratory values alone may not capture the full clinical picture

AM cortisol testing represents a critical first step in the evaluation of adrenal function, providing valuable diagnostic information that guides further testing and management decisions.