When should cortisol levels be measured in patients with symptoms of adrenal insufficiency or excess—such as unexplained fatigue, weakness, weight loss, nausea, vomiting, abdominal pain, orthostatic hypotension, hyponatremia, hyperpigmentation, recurrent hypoglycemia, Cushingoid features (central obesity, facial rounding, dorsocervical fat pad, purple striae, hypertension, glucose intolerance, osteoporosis, muscle weakness), adrenal incidentaloma larger than 1 cm, early virilization, rapid growth, precocious puberty, or a history of pituitary or hypothalamic disease, bilateral adrenalectomy, chronic glucocorticoid withdrawal, or abrupt cessation of long‑term steroids?

When to Check Cortisol Levels

Check cortisol levels whenever a patient presents with unexplained fatigue, weakness, weight loss, nausea, vomiting, hypotension, hyponatremia, hyperpigmentation, Cushingoid features, or has risk factors including chronic glucocorticoid use, pituitary disease, or adrenal incidentaloma >1 cm. 1, 2

Clinical Scenarios Requiring Cortisol Testing

Suspected Adrenal Insufficiency

Primary indicators:

• Unexplained hypotension or orthostatic hypotension, particularly if vasopressor-resistant 1
• Hyponatremia with hypo-osmolality—present in 90% of newly diagnosed adrenal insufficiency cases 1, 2
• Unexplained collapse with gastrointestinal symptoms (vomiting or diarrhea) 1
• Hyperpigmentation of skin creases, scars, or mucous membranes (suggests primary adrenal insufficiency due to elevated ACTH) 1
• Unexplained fatigue, weakness, weight loss, nausea, or poor appetite 1, 2
• Recurrent hypoglycemia, especially in children 1

Critical pitfall: Hyperkalemia occurs in only ~50% of primary adrenal insufficiency cases—its absence does NOT rule out the diagnosis. 1, 2

High-risk populations requiring testing:

• Any patient taking ≥20 mg/day prednisone (or equivalent) for ≥3 weeks who develops unexplained hypotension 1
• History of pituitary or hypothalamic disease, pituitary surgery, or pituitary microadenoma 1
• Bilateral adrenalectomy 1
• Abrupt cessation of long-term glucocorticoid therapy 1
• Critically ill patients with refractory shock despite high-dose vasopressors 1

Suspected Cushing's Syndrome

Check cortisol when patients present with:

• Cushingoid features: central obesity, facial rounding, dorsocervical fat pad ("buffalo hump"), purple striae (>1 cm wide), easy bruising 3
• Hypertension with glucose intolerance or new-onset diabetes 3
• Osteoporosis or pathologic fractures, particularly in young patients 3
• Proximal muscle weakness 3
• Adrenal incidentaloma >1 cm on imaging 3, 4

Pediatric-specific indicators:

• Early virilization 1
• Rapid growth velocity or growth deceleration 1
• Precocious puberty 1

Diagnostic Testing Protocol

Initial Morning Cortisol and ACTH

Obtain paired 8 AM serum cortisol and plasma ACTH as first-line testing: 1, 2

Interpretation thresholds:

• Morning cortisol <250 nmol/L (<9 μg/dL) with elevated ACTH in acute illness is diagnostic of primary adrenal insufficiency 1, 2
• Morning cortisol <275 nmol/L (<10 μg/dL) warrants ACTH stimulation testing 3, 5
• Morning cortisol <400 nmol/L (<14.5 μg/dL) with elevated ACTH raises strong suspicion and requires confirmatory testing 2
• Morning cortisol >550 nmol/L (>20 μg/dL) effectively rules out adrenal insufficiency in most clinical contexts 3

Pattern recognition:

• Primary adrenal insufficiency: Low cortisol + high ACTH, often with hyponatremia and hyperkalemia 1, 2
• Secondary adrenal insufficiency: Low cortisol + low or inappropriately normal ACTH, hyponatremia without hyperkalemia 1

Cosyntropin (ACTH) Stimulation Test

Perform when morning cortisol is indeterminate (between 275-550 nmol/L or 10-20 μg/dL): 1, 2, 6

Protocol:

• Administer 0.25 mg (250 mcg) cosyntropin IV or IM 1, 2
• Measure serum cortisol at baseline, 30 minutes, and 60 minutes 1, 2
• Peak cortisol <500 nmol/L (<18 μg/dL) is diagnostic of adrenal insufficiency 1, 2, 6
• Peak cortisol >550 nmol/L (>20 μg/dL) is normal and excludes adrenal insufficiency 1, 3

Important considerations:

• The high-dose (250 mcg) test is preferred over low-dose (1 mcg) due to easier administration, comparable accuracy, and FDA approval 1
• Test can be performed at any time of day—fasting is not required 1
• Hold hydrocortisone for 24 hours before testing; other steroids require longer washout 1

Screening for Cushing's Syndrome

When clinical suspicion exists, obtain at least TWO of the following: 3

• Late-night salivary cortisol (2-3 measurements)—values above upper limit of normal indicate loss of circadian rhythm 3
• 24-hour urinary free cortisol (2-3 collections)—values above upper limit of normal suggest Cushing's 3
• Overnight 1 mg dexamethasone suppression test—post-dexamethasone cortisol >50 nmol/L (>1.8 μg/dL) is concerning 3
• Midnight serum cortisol ≥50 nmol/L (≥1.8 μg/dL) has 100% sensitivity in pediatric populations 3

For adrenal incidentalomas >1 cm:

• Perform overnight dexamethasone suppression test as screening 4
• Post-dexamethasone cortisol >138 nmol/L (>5 μg/dL) is highly suggestive of autonomous cortisol production 3

Critical Pitfalls to Avoid

Never Delay Treatment for Testing

If suspected acute adrenal crisis (hypotension, shock, altered mental status, severe vomiting): 1, 2

• Give IV hydrocortisone 100 mg immediately PLUS 0.9% saline at 1 L/hour 1, 2
• Draw blood for cortisol and ACTH before steroids if possible, but do NOT wait for results 1, 2
• If diagnosis uncertain and you still need to perform future testing, use dexamethasone 4 mg IV instead—it doesn't interfere with cortisol assays 1

Factors That Confound Results

Do NOT attempt cortisol testing in these situations:

• Patients actively taking corticosteroids (prednisone, prednisolone, inhaled fluticasone)—causes iatrogenic secondary adrenal insufficiency with falsely low cortisol 1
• Wait until corticosteroids discontinued with adequate washout before testing 1
• For patients on long-term steroids, wait 3 months after weaning to maintenance hydrocortisone before testing HPA axis recovery 1

Conditions affecting cortisol-binding globulin (falsely alter total cortisol): 3

• Increased CBG (falsely elevated): oral estrogens, pregnancy, chronic hepatitis 3
• Decreased CBG (falsely lowered): nephrotic syndrome, malnutrition 3

Drug interactions: 1

• CYP3A4 inducers (phenytoin, carbamazepine, rifampin) increase cortisol clearance—may need higher replacement doses 1
• CYP3A4 inhibitors (grapefruit juice, licorice) decrease cortisol clearance 1

Special Populations

Adrenal insufficiency must be excluded BEFORE diagnosing SIADH: 1

• Both present with euvolemic hypo-osmolar hyponatremia, low serum sodium, inappropriately high urine osmolality, and elevated urinary sodium 1
• Perform cosyntropin stimulation test to rule out adrenal insufficiency in all patients with hyponatremia and hypo-osmolality 1

When treating concurrent hypothyroidism and adrenal insufficiency:

• Start corticosteroids several days BEFORE initiating thyroid hormone replacement to prevent precipitating adrenal crisis 1

Night-shift workers:

• Do not use late-night cortisol testing due to disrupted circadian rhythm 3