How Do You Test for Cushing's Syndrome: Cortisol Testing, Normal Ranges, and What Low Cortisol Means

What Cortisol Actually Does in the Body

Cortisol is a glucocorticoid hormone produced by the zona fasciculata of the adrenal cortex. It is not simply a "stress hormone." Cortisol regulates blood glucose, suppresses inflammation, modulates immune responses, and controls blood pressure through mineralocorticoid receptor cross-reactivity. Without adequate cortisol, the body cannot mount a normal response to infection, surgery, or fasting.

The HPA Axis Controls Cortisol Release

The hypothalamic-pituitary-adrenal (HPA) axis governs cortisol in a tight feedback loop. The hypothalamus releases corticotropin-releasing hormone (CRH), which triggers the pituitary to secrete adrenocorticotropic hormone (ACTH). ACTH then signals the adrenal cortex to produce cortisol. Rising cortisol feeds back to suppress both CRH and ACTH, keeping the system in balance. This is why a disrupted HPA axis, not a "fatigued adrenal gland," is the mechanism behind most cortisol dysregulation outside of frank adrenal disease.

The Diurnal Rhythm

Cortisol follows a predictable 24-hour rhythm. Levels peak within 30 to 45 minutes of waking, often reaching 15 to 25 mcg/dL, then fall steadily throughout the day to a nadir near midnight of roughly 1 to 2 mcg/dL. This rhythm is the biological reason that late-night salivary cortisol is such a sensitive screening tool for Cushing's syndrome. When the rhythm is blunted or reversed, it points to autonomous cortisol secretion regardless of feedback signals. The Endocrine Society guideline on Cushing's syndrome specifically notes that "loss of the normal diurnal variation in cortisol secretion is one of the earliest biochemical abnormalities" in the condition (Nieman et al., J Clin Endocrinol Metab, 2008).

How Do You Test for Cushing's Syndrome: The Three First-Line Methods

Two abnormal results from validated tests are required before a diagnosis of Cushing's syndrome can be established. The Endocrine Society 2008 guideline, updated in subsequent reviews, recommends three first-line screening tests. Each has distinct advantages depending on patient circumstances (Nieman et al., 2008).

1. The 1 mg Overnight Dexamethasone Suppression Test (DST)

The overnight DST is the most widely used outpatient screening test. The patient takes 1 mg of dexamethasone orally at 11 PM, and serum cortisol is drawn the following morning between 8 and 9 AM. A post-dexamethasone cortisol <1.8 mcg/dL effectively rules out Cushing's syndrome with a sensitivity exceeding 95%. Patients with cortisol at or above 1.8 mcg/dL require further evaluation. Medications that accelerate dexamethasone metabolism, such as rifampin, phenytoin, and carbamazepine, can cause false-positive results, as can depression, alcoholism, and obesity. The test should not be ordered in patients taking these medications without first accounting for the interaction. Oral contraceptives also raise cortisol-binding globulin and can falsely raise total cortisol in serum-based measurements (PubMed: Meier & Zapf, 1985).

2. Twenty-Four-Hour Urinary Free Cortisol (UFC)

The 24-hour UFC measures integrated cortisol output over a full day, bypassing the binding-globulin problem that affects serum measurements. Normal values are generally <50 mcg/24h, though upper limits vary by assay and lab. Values above 3 to 4 times the upper limit of normal are highly specific for Cushing's syndrome. A single collection is not sufficient; guidelines recommend at least two separate collections because cortisol secretion in early Cushing's can be episodic. Patients with a glomerular filtration rate below 30 mL/min/1.73 m² may have falsely low UFC due to reduced renal excretion, which is a meaningful limitation in patients with chronic kidney disease (Raff et al., J Clin Endocrinol Metab, 2015).

3. Late-Night Salivary Cortisol (LNSC)

Late-night salivary cortisol is collected at or around 11 PM on two separate nights using a Salivette collection device. Salivary cortisol reflects free (unbound) serum cortisol and tracks the diurnal nadir with high accuracy. Most laboratories report a normal upper limit of approximately 0.13 mcg/dL, though assay-specific reference ranges apply. The 2008 Endocrine Society guideline characterizes LNSC as having sensitivity above 90% and specificity above 93% for Cushing's syndrome. It is the preferred test in patients who cannot complete accurate 24-hour urine collections (shift workers, travelers) and in those on oral contraceptives. Shift workers, however, have a physiologically blunted nocturnal nadir and require careful interpretation. Chewing tobacco and bleeding gums can contaminate samples (Kidambi et al., J Clin Endocrinol Metab, 2007).

What Is a Normal AM Cortisol and How Is It Interpreted

A morning serum cortisol drawn between 7 and 9 AM is the standard outpatient starting point for evaluating both excess and deficiency. The generally accepted normal range is 6 to 23 mcg/dL, though laboratories differ slightly. The single number means little without timing and clinical context.

When an AM Cortisol Points Toward Adrenal Insufficiency

A morning cortisol below 3 mcg/dL, drawn during an unstressed state, carries a high likelihood of primary or secondary adrenal insufficiency. Values between 3 and 10 mcg/dL are considered indeterminate and typically require dynamic testing, specifically the 250 mcg cosyntropin (ACTH) stimulation test. A peak cortisol of 18 mcg/dL or greater at 30 or 60 minutes post-injection is generally considered a normal response, ruling out primary adrenal insufficiency. The Endocrine Society guideline on adrenal insufficiency specifies that "the short ACTH stimulation test is the investigation of choice for the diagnosis of primary adrenal insufficiency" (Bornstein et al., J Clin Endocrinol Metab, 2016).

When an AM Cortisol Points Toward Cushing's

A clearly elevated AM cortisol, above 23 mcg/dL, combined with symptoms of Cushing's syndrome does not confirm the diagnosis alone. The next step is a confirmatory screening test from the three first-line options above. Cortisol rises with acute illness, severe depression, and morbid obesity, all of which can produce pseudo-Cushing states with lab values that overlap with true Cushing's syndrome.

The AM Cortisol After Dexamethasone

The post-DST morning cortisol is the most clinically actionable single cortisol value. Results <1.8 mcg/dL after the 1 mg overnight DST make Cushing's syndrome very unlikely. Results at or above this threshold in a symptomatic patient warrant referral to endocrinology for further workup, including potential CRH stimulation and inferior petrosal sinus sampling to determine the source of ACTH excess.

Is Adrenal Fatigue Real? What the Evidence Says

"Adrenal fatigue" is not a recognized medical diagnosis. The term describes a state of exhaustion, brain fog, salt craving, and poor stress tolerance allegedly caused by adrenal glands that are too worn out to produce cortisol. In 2016, the Endocrine Society released a position statement concluding that "no scientific proof exists that 'adrenal fatigue' is a real medical condition" (Endocrine Society, 2016). A systematic review published in BMC Medicine evaluated 58 studies and found no consistent evidence supporting the construct (Cadegiani & Kater, BMC Med, 2016).

What Is Actually Happening in These Patients

The symptoms attributed to adrenal fatigue are real. Exhaustion, low resilience to stress, disturbed sleep, and persistent fatigue are genuine and often disabling. The problem is the proposed mechanism. Randomized studies show that most patients labeled with "adrenal fatigue" have cortisol levels within the normal range. The physiology driving their symptoms is more likely HPA axis dysregulation, disrupted cortisol pulsatility, altered cortisol receptor sensitivity, or co-existing conditions such as hypothyroidism, sleep apnea, depression, or iron-deficiency anemia.

Why the Label Matters Clinically

Labeling a patient with adrenal fatigue and treating with compounded hydrocortisone or adrenal cortex extracts carries real risk. Exogenous glucocorticoids suppress the HPA axis and can cause iatrogenic adrenal insufficiency on withdrawal. The appropriate workup starts with a proper morning serum cortisol and ACTH, followed by the cosyntropin stimulation test if morning cortisol is indeterminate, not with empirical steroid therapy.

The HealthRX Adrenal Evaluation Framework sequences testing as follows: (1) Symptom screen plus AM cortisol and ACTH. (2) If cortisol is <3 mcg/dL, proceed directly to cosyntropin stimulation. (3) If cortisol is 3 to 10 mcg/dL, cosyntropin stimulation is required before any treatment decision. (4) If cortisol is >10 mcg/dL with symptoms of excess, begin Cushing's screening with at least two first-line tests. (5) Treat confirmed adrenal insufficiency or confirmed Cushing's. Do not treat indeterminate lab results empirically.

What Causes Low Cortisol

Low cortisol, or hypocortisolism, is divided by anatomical location of the defect into primary, secondary, and tertiary adrenal insufficiency.

Primary Adrenal Insufficiency (Addison's Disease)

Primary adrenal insufficiency (Addison's disease) results from destruction or dysfunction of the adrenal cortex itself. In high-income countries, autoimmune adrenalitis accounts for approximately 80 to 90% of cases. Other causes include tuberculosis (the leading cause globally), adrenal hemorrhage, metastatic malignancy, and fungal infections such as histoplasmosis. Biochemically, primary adrenal insufficiency shows low cortisol combined with elevated ACTH, because the pituitary is working correctly and trying to stimulate a gland that cannot respond. Hyponatremia and hyperkalemia are common because aldosterone (also produced by the adrenal cortex) is concurrently deficient. Estimated prevalence in Western countries is 93 to 144 cases per million (Bornstein et al., J Clin Endocrinol Metab, 2016).

Secondary and Tertiary Adrenal Insufficiency

Secondary adrenal insufficiency arises from ACTH deficiency due to pituitary pathology: tumors, surgery, radiation, or autoimmune hypophysitis. Tertiary adrenal insufficiency is the most common form clinically and results from HPA axis suppression by exogenous glucocorticoids. Any patient who has taken prednisone 5 mg/day (or equivalent) for longer than three weeks should be evaluated for HPA axis suppression before abrupt discontinuation. Unlike primary insufficiency, secondary and tertiary forms preserve aldosterone production, so electrolyte abnormalities are usually absent. ACTH levels are low or inappropriately normal in both secondary and tertiary forms (Charmandari et al., Lancet, 2014).

Drug-Induced and Iatrogenic Causes

Inhaled corticosteroids at high doses (fluticasone 500 mcg/day or greater), intranasal steroids used chronically, and intra-articular corticosteroid injections can all suppress the HPA axis to a degree that produces measurable hypocortisolism. Megestrol acetate, a progestational agent with glucocorticoid activity, is another under-recognized cause. Ketoconazole, etomidate, and metyrapone directly inhibit adrenal steroidogenesis and are used deliberately in Cushing's treatment but can overshoot to cortisol deficiency.

Putting It Together: Diagnosis Pathway for Suspected Cushing's

A patient with central obesity, easy bruising, purple striae wider than 1 cm, proximal muscle weakness, and hypertension needs formal Cushing's evaluation. The Endocrine Society recommends testing in four populations: patients with unusual features for age (osteoporosis before 65, hypertension before 40), incidentally discovered adrenal mass, children with weight gain combined with decreased linear growth, and patients with multiple features of Cushing's syndrome (Nieman et al., J Clin Endocrinol Metab, 2008).

Step-by-Step Outpatient Approach

Start with the overnight 1 mg DST as the most accessible initial test. A post-dexamethasone cortisol <1.8 mcg/dL stops the workup. Cortisol at or above 1.8 mcg/dL requires a second confirmatory test: 24-hour UFC on two separate collections or late-night salivary cortisol on two separate nights. Two abnormal results from any combination of these tests should prompt endocrinology referral. At that point, ACTH measurement distinguishes ACTH-dependent causes (pituitary adenoma, ectopic ACTH) from ACTH-independent causes (adrenal adenoma or carcinoma). ACTH above 20 pg/mL suggests ACTH dependence; ACTH below 5 pg/mL points to autonomous adrenal secretion.

Imaging Comes After Biochemical Confirmation

MRI of the pituitary and CT of the adrenal glands are not first-line tests. Adrenal incidentalomas appear on abdominal CT in approximately 4 to 7% of adults and are usually non-functioning. Ordering an adrenal CT before biochemical confirmation of cortisol excess produces false attribution of symptoms to a radiological finding that may be entirely benign. In confirmed ACTH-dependent disease with a pituitary MRI that does not clearly show a tumor, inferior petrosal sinus sampling (IPSS) is the gold-standard test to confirm a pituitary source, with a central-to-peripheral ACTH ratio above 2 at baseline or above 3 after CRH stimulation indicating a pituitary adenoma. IPSS sensitivity is approximately 94% and specificity approximately 100% for pituitary Cushing's (Findling & Raff, Endocrinol Metab Clin North Am, 2005).

Treatment Overview: What Happens After Diagnosis

Treatment depends on the underlying cause. Pituitary Cushing's disease (the most common form, accounting for roughly 70% of endogenous cases) is treated with transsphenoidal pituitary adenomectomy as first-line therapy, with remission rates of 65 to 90% in experienced centers. Adrenal adenomas causing unilateral cortisol excess are treated with laparoscopic adrenalectomy. Medical therapy with metyrapone, ketoconazole, osilodrostat, or mifepristone is used when surgery is not feasible or as a bridge to surgery. Osilodrostat received FDA approval in March 2020 (NDA 210595) for adults with Cushing's disease for whom pituitary surgery is not an option or has not been curative (FDA approval: osilodrostat, 2020).

After adrenalectomy or successful pituitary surgery, patients face a period of secondary adrenal insufficiency while the suppressed HPA axis recovers. This recovery can take 6 to 18 months. Glucocorticoid replacement with hydrocortisone (15 to 25 mg/day in divided doses, weight-based) is standard during recovery, with gradual tapering guided by morning cortisol and cosyntropin stimulation testing.