Is a Morning Serum Cortisol of 10 µg/dL (276 nmol/L) Normal?
A morning serum cortisol of 10 µg/dL (276 nmol/L) falls in the equivocal "gray zone" and requires further evaluation with an ACTH stimulation test to definitively rule out adrenal insufficiency. 1
Interpretation of This Specific Value
Morning cortisol >14 µg/dL (>386 nmol/L) effectively excludes adrenal insufficiency, while values <10 µg/dL (<275 nmol/L) in the setting of suggestive symptoms strongly indicate the diagnosis. 1
Your value of 10 µg/dL (276 nmol/L) sits precisely at the threshold where neither diagnosis nor exclusion of adrenal insufficiency can be made with confidence. 1, 2
The European consensus statement identifies that morning cortisol <275 nmol/L should prompt confirmatory ACTH stimulation testing to assess adrenal reserve. 3, 1
Why This Value Is Indeterminate
In healthy adults, optimal morning cortisol ranges from 5-23 µg/dL (138-635 nmol/L), with most values concentrated between 10-20 µg/dL. 1 Your value of 10 µg/dL is technically within the broad reference range but at the lower boundary.
Research demonstrates that a morning cortisol of <275 nmol/L identifies subnormal ACTH-stimulated cortisol with 96.2% sensitivity, meaning this cutoff is designed to capture nearly all cases of adrenal insufficiency. 2
Conversely, a morning cortisol >375 nmol/L (>13.6 µg/dL) predicts adrenal sufficiency with 95% specificity, reliably excluding the diagnosis. 4
Values between 275-375 nmol/L (10-13.6 µg/dL) represent the gray zone where dynamic testing is mandatory. 2, 4
Recommended Next Step: ACTH Stimulation Test
You should proceed directly to a standard-dose ACTH (cosyntropin) stimulation test using 0.25 mg administered IV or IM. 3, 1
Measure serum cortisol at baseline, 30 minutes, and 60 minutes after injection. 1
A peak cortisol response <500 nmol/L (<18 µg/dL) confirms primary adrenal insufficiency. 3, 1
A peak cortisol >550 nmol/L (>20 µg/dL) confirms adrenal sufficiency. 4
This dynamic test directly assesses adrenal reserve and is the gold-standard diagnostic method. 1
Distinguishing Primary from Secondary Adrenal Insufficiency
Simultaneously measure plasma ACTH with the morning cortisol to differentiate the etiology. 3, 1
In primary adrenal insufficiency (e.g., autoimmune adrenalitis, Addison's disease), ACTH levels are markedly elevated (often >100 pg/mL) because the pituitary attempts to stimulate a failing adrenal gland. 1
In secondary adrenal insufficiency (pituitary or hypothalamic disease), ACTH is low or inappropriately normal despite low cortisol. 3, 1
Clinical Context Matters
If you have symptoms suggestive of adrenal insufficiency—unexplained fatigue, weight loss, hypotension, hyperpigmentation, hyponatremia, or hyperkalemia—a cortisol of 10 µg/dL is concerning and warrants urgent dynamic testing. 3, 1
In acutely ill patients, a random cortisol <10 µg/dL (<276 nmol/L) suggests relative adrenal insufficiency, as stress should substantially elevate cortisol. 1
In the absence of symptoms and in a stable outpatient, this value may simply reflect normal diurnal variation, but dynamic testing is still recommended to definitively exclude subclinical insufficiency. 2, 4
Common Pitfalls to Avoid
Do not rely on a single morning cortisol measurement alone to make or exclude the diagnosis of adrenal insufficiency when the value falls in the 10-14 µg/dL range. 2, 4
Exogenous steroid use (oral prednisolone, dexamethasone, inhaled fluticasone) can suppress endogenous cortisol and confound interpretation; ensure the patient has not been exposed to glucocorticoids. 3, 1
Timing is critical: cortisol must be drawn between 08:00-09:00 hours to capture the physiologic peak; afternoon samples have lower reference ranges. 1, 2
Age and pubertal status affect cortisol levels: prepubertal children have significantly lower morning cortisol than adults, and sex differences emerge after puberty. 5 In adults, older age and comorbidity are associated with higher cortisol levels. 6