How do I check my cortisol levels?

How to Check Cortisol Levels

The optimal approach to checking cortisol levels depends on your clinical question: for suspected adrenal insufficiency, obtain an 8:00-9:00 AM serum cortisol; for suspected Cushing's syndrome, start with an overnight 1-mg dexamethasone suppression test. 1, 2

For Suspected Adrenal Insufficiency

Morning Serum Cortisol (First-Line)

• Draw blood at 8:00-9:00 AM to capture the physiologic peak of cortisol secretion 2
• Morning cortisol >14 μg/dL (>386 nmol/L) effectively rules out adrenal insufficiency 1
• Morning cortisol <3 μg/dL strongly suggests adrenal insufficiency and warrants immediate treatment 3
• Values between 3-15 μg/dL are indeterminate and require further testing with ACTH stimulation 3

ACTH Stimulation Test (Confirmatory)

• Use when morning cortisol is indeterminate (3-15 μg/dL) 3, 4
• Administer 0.25 mg cosyntropin (synthetic ACTH) intramuscularly or intravenously 4
• Obtain baseline cortisol immediately before injection, then at exactly 30 and 60 minutes post-injection 4
• Peak cortisol <18 μg/dL (500 nmol/L) at 30 or 60 minutes is diagnostic of adrenal insufficiency 1, 4

Critical Timing Considerations

• Never perform cortisol testing outside the 8:00-9:00 AM window for adrenal insufficiency evaluation, as cortisol varies dramatically throughout the day 2
• Shift workers and patients with disrupted circadian rhythms should not undergo standard AM cortisol testing, as their cortisol peaks occur at different times 2

Simultaneous ACTH Measurement

• Draw ACTH and cortisol simultaneously at 8:00-9:00 AM from the same blood draw to distinguish primary from secondary adrenal insufficiency 3, 2
• ACTH is extremely labile and requires immediate processing on ice—coordinate with your laboratory before drawing 2
• Low cortisol with elevated ACTH (>1.1 pmol/L) indicates primary adrenal insufficiency 3, 2
• Low cortisol with low ACTH indicates secondary (central) adrenal insufficiency 3, 2

For Suspected Cushing's Syndrome

Overnight 1-mg Dexamethasone Suppression Test (Preferred First-Line)

• Give 1 mg dexamethasone orally at 11:00 PM-midnight, then measure serum cortisol at 8:00 AM the next morning 1, 5
• Normal response: cortisol <1.8 μg/dL (<50 nmol/L) effectively rules out Cushing's syndrome 1, 5
• Cortisol 1.8-5 μg/dL (50-138 nmol/L) suggests possible autonomous cortisol secretion 1
• Cortisol >5 μg/dL (>138 nmol/L) indicates overt Cushing's syndrome 1, 5

Late-Night Salivary Cortisol (Alternative First-Line)

• Collect saliva samples at usual bedtime (around 11:00 PM-midnight) on at least 2-3 separate nights 1, 6
• Normal individuals have cortisol nadir at midnight; Cushing's patients lose this circadian rhythm 1, 6
• Abnormal threshold: >3.6 nmol/L with sensitivity >90% and highest specificity among screening tests 1, 6
• Do not use in shift workers or those with disrupted sleep-wake cycles 1

24-Hour Urinary Free Cortisol (Complementary Test)

• Collect at least 2-3 complete 24-hour urine collections due to 50% random variability between collections 1
• Measure total volume and creatinine to verify completeness of collection 1
• Normal: <70 μg/24h (<193 nmol/24h) 1
• Values >100 μg/24h are typically diagnostic in symptomatic patients 1
• Has lower sensitivity than DST and salivary cortisol, best used when other tests are equivocal 1

Critical Pitfalls to Avoid

Medications That Falsify Results

• Stop glucocorticoids and spironolactone on the day of testing—they falsely elevate cortisol levels 4
• Stop oral estrogens/contraceptives 4-6 weeks before testing, as they increase cortisol-binding globulin and falsely elevate total cortisol 1, 4
• CYP3A4 inducers (phenytoin, rifampin, carbamazepine) accelerate dexamethasone metabolism, causing false-positive DST results 1, 5
• CYP3A4 inhibitors can cause false-negative DST results 5

Conditions Affecting Cortisol-Binding Globulin

• Pregnancy, chronic active hepatitis, and oral estrogens increase CBG, falsely elevating total cortisol 1, 4
• Cirrhosis and nephrotic syndrome decrease CBG, falsely lowering total cortisol 4
• Consider measuring CBG levels concomitantly if these conditions are present 4

Pseudo-Cushing's States

• Depression, alcoholism, severe obesity, and polycystic ovary syndrome can cause mild hypercortisolism that mimics Cushing's syndrome 1, 5
• These conditions may require additional testing such as the Dex-CRH test to distinguish from true Cushing's 5

Sample Collection Issues

• Avoid strenuous exercise for 24-48 hours before testing, as physical stress elevates cortisol 1
• Avoid cognitive assessments or acute psychological stress immediately before blood draw 1
• For salivary cortisol, avoid dental work, teeth brushing, or oral trauma within 1-2 hours of collection 1
• Topical hydrocortisone can contaminate salivary samples and cause falsely elevated results 1

Special Clinical Scenarios

• Never delay treatment of suspected acute adrenal crisis for diagnostic testing—draw baseline cortisol and ACTH, then immediately start stress-dose hydrocortisone 3, 1
• In acute illness, cortisol <250 nmol/L with elevated ACTH is diagnostic of primary adrenal insufficiency 1
• Cyclic Cushing's syndrome can produce normal results for weeks to months—multiple sequential tests may be needed to capture episodes of excess 1, 5