Is a Fasting Cortisol of 12.4 mcg/dL Normal?
Yes, a fasting morning cortisol level of 12.4 mcg/dL (342 nmol/L) is within the normal range and effectively rules out adrenal insufficiency.
Normal Reference Range
Morning cortisol levels in healthy individuals typically range from 5-23 μg/dL (138-635 nmol/L), with most values clustering between 10-20 μg/dL (276-552 nmol/L). 1
Your value of 12.4 mcg/dL falls comfortably within this normal range and is near the middle of the typical distribution. 1
Clinical Significance for Ruling Out Adrenal Insufficiency
Morning cortisol levels >14 μg/dL (>386 nmol/L) effectively rule out adrenal insufficiency. 1 While your value of 12.4 mcg/dL is slightly below this threshold, it still provides strong reassurance:
Morning cortisol >300 nmol/L (10.9 mcg/dL) almost excludes ACTH-cortisol insufficiency. 2 Your level exceeds this cutoff.
A mean morning cortisol ≥234 nmol/L (8.5 mcg/dL) predicts normal adrenal response with 80.6% sensitivity and 91.4% specificity. 3 Your value is well above this threshold.
Morning cortisol >444.7 nmol/L (16.1 mcg/dL) best predicts normal response to stimulation testing with 96.9% sensitivity. 4 While your value doesn't reach this higher threshold, it remains solidly normal.
When Further Testing Would Be Needed
Further evaluation with stimulation testing (synacthen test) would only be indicated if: 5, 1
- Morning cortisol was <100 nmol/L (<3.6 mcg/dL), which strongly suggests adrenal dysfunction 2
- Morning cortisol was between 100-300 nmol/L (3.6-10.9 mcg/dL) in the presence of clinical symptoms 3
- Clinical suspicion remains high despite normal cortisol (unexplained hypotension, hyperpigmentation, hyponatremia, hyperkalaemia) 5
Important Caveats
This interpretation assumes you were not taking exogenous steroids (oral prednisolone, dexamethasone, or inhaled fluticasone), which can confound cortisol measurements. 1
The sample should have been drawn in the morning (ideally around 8 AM) when cortisol is at its physiologic peak. 1, 6
If you have acute severe illness or sepsis, different diagnostic criteria apply, as cortisol levels that appear "normal" may be inadequate for the stress state. 5, 6