Normal Morning Serum Cortisol Levels in Adults
A morning (approximately 8 AM) serum cortisol level above 500–550 nmol/L (18–20 μg/dL) is considered normal and effectively excludes adrenal insufficiency in adults. 1, 2
Diagnostic Thresholds for Morning Cortisol
Values that exclude adrenal insufficiency:
- >550 nmol/L (>18–20 μg/dL): Normal response that rules out adrenal insufficiency 1, 2
- >500 nmol/L (>18 μg/dL): Considered the standard threshold for excluding adrenal insufficiency in most guidelines 1
Values that suggest adrenal insufficiency:
- <250 nmol/L (<9 μg/dL) with elevated ACTH in acute illness: Diagnostic of primary adrenal insufficiency 1, 2
- <400 nmol/L (<14 μg/dL) with elevated ACTH in acute illness: Raises strong suspicion of primary adrenal insufficiency 1
Indeterminate values requiring further testing:
- Between 250–500 nmol/L (9–18 μg/dL): Requires ACTH stimulation testing to definitively rule in or rule out adrenal insufficiency 1, 2, 3
Important Clinical Context
Timing considerations:
- Morning cortisol should ideally be measured between 8:00 AM and 9:00 AM when cortisol levels are at their physiological peak 1, 2
- Afternoon measurements (12 PM–6 PM) have different thresholds and are less reliable for diagnostic purposes 3
Population variations:
- These thresholds apply to adults; children and adolescents may have lower baseline values that vary by pubertal stage 4
- Sex and pubertal status significantly affect morning cortisol levels, with Tanner stage 5 individuals having higher values than prepubertal children 4
Critical pitfall to avoid:
- A single normal morning cortisol does not exclude early or partial adrenal insufficiency—approximately 10% of patients with confirmed primary adrenal insufficiency may present with normal basal cortisol concentrations but have elevated ACTH levels 5
- In patients with clinical symptoms highly suggestive of adrenal insufficiency, proceed with ACTH stimulation testing even if morning cortisol appears borderline normal 6