8 AM Fasting Cortisol Test Purpose
The 8 AM fasting cortisol test is primarily used to diagnose adrenal insufficiency, serving as an initial screening tool to evaluate the hypothalamic-pituitary-adrenal (HPA) axis function. 1, 2
Diagnostic Applications
Primary Use: Adrenal Insufficiency Evaluation
- Morning cortisol levels below 250 nmol/L (9 μg/dL) with elevated ACTH levels during acute illness are diagnostic of primary adrenal insufficiency 1
- Cortisol levels between 250-400 nmol/L with elevated ACTH raise strong suspicion of adrenal insufficiency 1
- The test reflects the normal diurnal rhythm of cortisol, which should be highest in the early morning
Secondary Uses
- Serves as a baseline measurement before dynamic testing (such as ACTH stimulation tests)
- Can be used before metyrapone testing to assess adrenal function 3
- May provide initial data in the evaluation of Cushing's syndrome, though not definitive for diagnosis
Interpretation Considerations
Normal Results
- Normal morning cortisol levels generally indicate intact HPA axis function
- However, a normal result does not definitively rule out adrenal insufficiency, especially in early or mild cases
Abnormal Results
- Low morning cortisol (<250 nmol/L) with elevated ACTH suggests primary adrenal insufficiency 1
- Low morning cortisol with low or normal ACTH may indicate secondary adrenal insufficiency (pituitary or hypothalamic origin)
- High morning cortisol may suggest Cushing's syndrome, though further testing is required for confirmation 2
Limitations and Caveats
- Single measurements have limited diagnostic value and often require confirmation with dynamic tests
- The test should be performed at 8 AM to capture the peak of the circadian rhythm
- Results can be affected by:
- Stress (which can elevate cortisol)
- Medications (especially exogenous steroids)
- Sleep disturbances or shift work
- Acute illness
Follow-up Testing
If the 8 AM cortisol result is abnormal or equivocal:
- For suspected adrenal insufficiency: ACTH stimulation test (Synacthen/tetracosactide test) is recommended 1
- For suspected Cushing's syndrome: Additional tests such as overnight dexamethasone suppression test, late-night salivary cortisol, or 24-hour urinary free cortisol are indicated 2
Clinical Context
The 8 AM fasting cortisol test should be interpreted within the clinical context, considering:
- Presenting symptoms (fatigue, weight loss, hyperpigmentation for adrenal insufficiency)
- Electrolyte abnormalities (hyponatremia, hyperkalemia)
- Presence of hypotension or orthostatic changes
- History of steroid use or abrupt discontinuation
Remember that treatment of suspected acute adrenal insufficiency should never be delayed for diagnostic testing 1, as this condition can be life-threatening.