What is the normal reference range for cortisol levels in adult females?

Normal Cortisol Reference Range for Adult Females

The reference range for plasma cortisol in adult females is 140–700 nmol/L (5.1–25.4 μg/dL) at 0900 hours and 80–350 nmol/L (2.9–12.7 μg/dL) at midnight. 1

Time-Dependent Reference Ranges

Cortisol secretion follows a circadian rhythm with marked diurnal variation, making timing of measurement critical for accurate interpretation:

• Morning (0900 hours): 140–700 nmol/L 1
• Midnight: 80–350 nmol/L 1
• Afternoon (1200-1800 hours): Intermediate values between morning and midnight ranges 2

The physiologic peak occurs in early morning hours (0800-0900), with progressive decline throughout the day to reach nadir at midnight 1, 3.

Sex-Specific Considerations

Cortisol reference ranges do not differ between males and females when measured under normal physiologic conditions 4. However, several factors unique to females can affect total cortisol measurements:

Hormonal Contraceptive Use

• Oral contraceptives significantly elevate corticosteroid-binding globulin (CBG), which increases total cortisol levels while free (bioactive) cortisol remains normal 4
• Women on oral estrogen have CBG levels of approximately 89.1 ± 11.2 μg/mL compared to 39.5 ± 3.7 μg/mL in untreated women 4
• Total cortisol in estrogen-treated women averages 395.6 ± 103.3 nmol/L at 0800 hours versus 193.7 ± 35.0 nmol/L in controls, but unbound cortisol remains similar (17.2 ± 7.7 ng/mL vs 16.5 ± 5.6 ng/mL) 4
• For women on hormonal contraceptives, the 1 mg dexamethasone suppression test may be unreliable, and measuring dexamethasone levels should be considered if false-positive results are suspected 5, 3

Pregnancy

• Pregnancy increases CBG levels similar to oral estrogen, elevating total cortisol while free cortisol may remain within normal limits 3
• This must be documented when interpreting cortisol results 3

Clinical Application for Screening

Suspected Adrenal Insufficiency

For outpatient evaluation of suspected adrenal insufficiency:

• Morning cortisol <275 nmol/L (8 AM-12 PM) identifies subnormal adrenal function with 96.2% sensitivity and requires dynamic testing 6
• Morning cortisol >375 nmol/L predicts adequate adrenal function with 95% specificity, potentially obviating need for synacthen testing 7
• Afternoon cortisol <250 nmol/L (12 PM-6 PM) identifies subnormal function with 96.1% sensitivity in outpatients 6

Suspected Cushing's Syndrome

For screening hypercortisolism in females:

• Loss of normal midnight nadir (midnight cortisol ≥50 nmol/L or ≥1.8 μg/dL) has 100% sensitivity and 60% specificity for Cushing's syndrome 1
• Morning ACTH >1.1 pmol/L (>5 ng/L) with elevated cortisol suggests ACTH-dependent hypercortisolism with 68% sensitivity and 100% specificity 1, 3

Critical Pitfalls to Avoid

• Never interpret a single cortisol value without knowing the time of collection – the same value may be normal at 0900 hours but pathologically elevated at midnight 1, 2
• Document all medications affecting CBG including oral contraceptives, as these elevate total cortisol without true hypercortisolism 3, 4
• Avoid cortisol testing in shift workers or those with disrupted circadian rhythms – their cortisol peaks occur at abnormal times, rendering standard reference ranges invalid 5, 3
• Ensure patient avoids strenuous exercise for 24-48 hours before testing, as physical stress significantly elevates cortisol 3
• When measuring ACTH simultaneously, both samples must be drawn at the same time (0800-0900 hours) from the same blood draw, with ACTH processed immediately on ice 3