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BML-ADR-006adrenal stressUSA

Cortisol, Serum (Total)

The Cortisol Serum test quantifies total circulating cortisol — bound and free fractions combined — providing the primary biochemical assessment of HPA axis function and adrenocortical output. Morning collection (07:00–09:00) is the standard for adrenal insufficiency screening — a value above 18–20 µg/dL effectively excludes adrenal insufficiency; below 3 µg/dL strongly confirms it;

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Biomarkers

Blood (serum)

Sample

1–3 business days

Turnaround

No — fasting is not required for serum cortisol. However, collection timing is the critical variable superseding all other preparation requirements. For adrenal insufficiency screening: collect between 07:00 and 09:00 hours — the earlier in this window the better for maximum cortisol peak. Document exact collection time (not just 'morning') on the requisition form. For HPA axis diurnal assessment alongside a morning specimen: schedule a second collection between 15:00 and 17:00. For Cushing's screening: a late-night specimen (22:00–24:00) requires an in-patient, closely supervised hospital collection or a home salivary cortisol alternative.

Fasting

What is the Serum Cortisol test?

The Cortisol Serum test quantifies total circulating cortisol — the combined bound and free fractions — in serum, providing the primary biochemical assessment of adrenocortical output and HPA axis function. Collection timing is the single most critical variable for serum cortisol interpretation — a morning specimen (07:00–09:00) captures the diurnal peak and is the reference standard for adrenal insufficiency screening; an afternoon specimen (15:00–17:00) reflects the mid-cycle decline; and a late-night specimen (22:00–24:00) is used for Cushing's syndrome screening, where failure to suppress cortisol to the expected nocturnal nadir is pathognomonic. Without documented and standardised collection timing, a serum cortisol value is clinically uninterpretable — it is impossible to determine whether a given cortisol concentration represents a normal morning peak, an inappropriate afternoon elevation, or a normal nocturnal value without knowing when in the diurnal cycle the specimen was drawn. In mainstream endocrinology, serum cortisol is ordered most frequently for adrenal insufficiency screening and the ACTH stimulation test protocol — where a morning basal cortisol is followed by intramuscular or intravenous ACTH (Synacthen/cosyntropin) with cortisol remeasurement at 30 and 60 minutes, assessing the cortisol increment as a measure of adrenal reserve. In the functional and integrative medicine context, serum cortisol provides a valuable single-time-point objective measure of adrenal secretory output that contextualises and complements the multi-time-point salivary cortisol diurnal profile — with serum total cortisol useful when CBG-independent free cortisol measurement is not the primary requirement, and when a single morning cortisol provides a rapid objective adrenal status snapshot alongside DHEA-S and the cortisol:DHEA-S ratio.

What does the Serum Cortisol test measure?

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Cortisol(Cortisol, Serum (Total — Bound and Free Fractions))
Range: AM (07:00–09:00): 6.2–19.4 µg/dL (171–535 nmol/L); PM (15:00–17:00): 2.3–11.9 µg/dL (63–327 nmol/L) # REVIEWER FLAG: Confirm Access Medical Laboratories reference ranges for morning and afternoon serum cortisol — values are time-of-collection-dependent and platforms vary. Some laboratories apply a single AM reference range only; others provide diurnal time-banded ranges. Confirm whether Access Medical Laboratories provides a single reference range or multiple time-specific ranges, and whether they require the collection time to be documented on the requisition form to apply the correct reference interval. Confirm assay methodology — most platforms use electrochemiluminescence immunoassay (ECLIA) or chemiluminescent microparticle immunoassay (CMIA); confirm whether Access Medical Laboratories uses one of these or an alternative platform, as this affects reference range calibration.Optimal: Morning (08:00–09:00): 10–18 µg/dL (functional medicine optimal — mid-to-upper reference range reflecting robust but not excessive morning HPA axis activation); afternoon (15:00–17:00): 3–8 µg/dL (reflecting adequate but declining cortisol for afternoon biological function without evening excess). # REVIEWER FLAG: Functional medicine optimal ranges for serum cortisol are not standardised in mainstream endocrine guidelines — reviewer to confirm appropriate framing distinguishing the functional optimal from the adrenal insufficiency diagnostic threshold.µg/dL / nmol/L

Cortisol is a glucocorticoid steroid hormone synthesised in the zona fasciculata of the adrenal cortex from cholesterol through a cascade of CYP450 enzymatic reactions, under the stimulation of adrenocorticotropic hormone (ACTH) secreted by the anterior pituitary in response to hypothalamic corticotropin-releasing hormone (CRH). The hypothalamic-pituitary-adrenal (HPA) axis operates under tight negative feedback: rising cortisol suppresses both CRH and ACTH secretion, limiting further adrenal stimulation. Cortisol exhibits a pronounced diurnal rhythm — peaking in the 30–60 minutes following waking (the cortisol awakening response, CAR) then declining progressively through the day to a nocturnal nadir before beginning to rise again approximately 2–3 hours before habitual waking time. In serum, approximately 90–95% of cortisol is protein-bound — predominantly to cortisol-binding globulin (CBG, also called transcortin, approximately 80%) and albumin (approximately 15%) — with only 5–10% circulating as free, biologically active cortisol. Serum total cortisol measurement captures both bound and free fractions and is therefore sensitive to changes in CBG concentration; free cortisol measurement by equilibrium dialysis is the gold-standard bioavailable cortisol assay but is not routinely available through standard clinical networks.

Serum cortisol is the cornerstone test of adrenal cortical function assessment in mainstream endocrinology and is ordered across several distinct clinical contexts that require different timing, interpretive frameworks, and reference standards. As a morning cortisol screen, it is the first-line test for suspected adrenal insufficiency — a random morning cortisol below 3 µg/dL strongly suggests adrenal insufficiency in a patient who is not acutely stressed, while a morning cortisol above 18–20 µg/dL effectively excludes it; values between these thresholds require dynamic stimulation testing (Short Synacthen/ACTH stimulation test). As a Cushing's syndrome screen, late-night serum cortisol — while not the primary recommended method (late-night salivary cortisol and 24-hour urinary free cortisol are preferred) — can identify failure of normal diurnal suppression. In the context of critical illness, acute stress, surgery, and major trauma, cortisol rises dramatically as part of the stress response; a cortisol below 18–20 µg/dL in a critically ill patient may indicate relative adrenal insufficiency. In functional and integrative medicine, serum cortisol is used to assess HPA axis status alongside DHEA-S and the cortisol:DHEA-S ratio, characterising the adrenal hormonal landscape for stress adaptation assessment and adrenal support protocol guidance.

Why is the Serum Cortisol test ordered?

  • Adrenal insufficiency screening — morning cortisol (07:00–09:00) as the first-line biochemical screen in patients with unexplained fatigue, weight loss, hypotension, hyponatraemia, hypoglycaemia, hyperpigmentation, or a clinical picture suggesting primary adrenal insufficiency (Addison's disease) or secondary adrenal insufficiency from pituitary or hypothalamic pathology
  • ACTH stimulation test baseline — basal morning cortisol collection immediately before intramuscular or intravenous ACTH (Synacthen) administration in patients with morning cortisol in the indeterminate zone (3–18 µg/dL) or with clinical features of adrenal insufficiency; the baseline cortisol is required for peak and increment response calculation
  • Cushing's syndrome screening — while late-night salivary cortisol and 24-hour urinary free cortisol are the primary Endocrine Society-recommended Cushing's screening tools, a late-night serum cortisol (22:00–24:00) above 7.5 µg/dL is used as a supportive confirmatory screen in some specialist protocols; confirm clinical indication with specialist
  • Critical illness adrenal function assessment — random serum cortisol in acutely ill patients (sepsis, surgery, major trauma, acute respiratory failure) where relative adrenal insufficiency is suspected as a contributor to haemodynamic instability; interpretive thresholds differ from ambulatory reference ranges in critical illness
  • HPA axis and adrenal stress pattern assessment in functional medicine — morning cortisol alongside DHEA-S for cortisol:DHEA-S ratio characterisation of adrenal balance; sequential morning and afternoon cortisol to map diurnal decline trajectory; baseline HPA axis assessment before initiating adaptogenic or adrenal support protocols
  • Corticosteroid therapy and HPA axis suppression assessment — serum cortisol to assess the degree of endogenous HPA axis suppression in patients on long-term systemic or high-dose inhaled corticosteroid therapy, and to guide safe taper strategies before discontinuation
  • Pituitary disease and secondary adrenal insufficiency evaluation — morning cortisol alongside ACTH measurement in patients with known or suspected pituitary adenoma, craniopharyngioma, or other sellar pathology where loss of ACTH drive (secondary adrenal insufficiency) is a potential complication of tumour, surgery, or radiotherapy
  • Integrative evaluation of metabolic syndrome, insulin resistance, and visceral adiposity — morning cortisol as part of the adrenal-metabolic assessment, given the established bidirectional relationship between cortisol excess (subclinical hypercortisolaemia), visceral adiposity, insulin resistance, hypertension, and cardiovascular risk

Sample collection and turnaround

Sample type

Blood (serum)

Fasting required

No — fasting is not required for serum cortisol. However, collection timing is the critical variable superseding all other preparation requirements. For adrenal insufficiency screening: collect between 07:00 and 09:00 hours — the earlier in this window the better for maximum cortisol peak. Document exact collection time (not just 'morning') on the requisition form. For HPA axis diurnal assessment alongside a morning specimen: schedule a second collection between 15:00 and 17:00. For Cushing's screening: a late-night specimen (22:00–24:00) requires an in-patient, closely supervised hospital collection or a home salivary cortisol alternative.

Collection method

Venipuncture at patient service centre

Turnaround

1–3 business days

Collection notes

Standard venipuncture into a serum separator tube (SST / gold top). The patient should have been resting quietly for a minimum of 30 minutes before the blood draw — physical activity, anxiety, acute pain, or emotional distress immediately before collection elevates cortisol through acute HPA stress activation and must be avoided or documented. Document on the requisition form: (1) exact collection time — hour and minute, not just AM/PM; (2) waking time that morning (for CAR context); (3) all current exogenous corticosteroids — name, dose, route, frequency, and timing of most recent dose relative to draw; (4) oral contraceptives, HRT, or other oestrogen-containing medications — for CBG interpretation; (5) acute illness, hospitalisation, or significant psychological stress on the collection day; (6) indication for test — adrenal insufficiency screening, Cushing's evaluation, functional HPA assessment, or critical illness assessment; (7) whether ACTH stimulation testing is planned (baseline cortisol must be collected immediately before ACTH injection). # REVIEWER FLAG: Confirm Access Medical Laboratories SST tube type, minimum serum volume, maximum time from collection to centrifugation, and cold-chain shipping requirements for cortisol before publication. Confirm whether Access Medical Laboratories applies time-banded reference ranges from collection time documentation or uses a single reference range with collection time noted as contextual.

Specimen requirements

Minimum 0.5 mL serum. Centrifuge and separate serum within 2 hours of collection — cortisol is relatively stable in serum but prompt separation is best practice. Stable at 2–8°C for up to 7 days; stable frozen at −20°C for extended storage. # REVIEWER FLAG: Confirm minimum volume and stability data with Access Medical Laboratories assay documentation.

What can affect Serum Cortisol results?

variableCollection timing — the dominant pre-analytical variable for serum cortisol

Serum cortisol declines by approximately 50–80% from its morning peak (6–19 µg/dL) to its afternoon nadir (2–12 µg/dL) and to near-zero at the nocturnal nadir (<3 µg/dL) through the diurnal cycle. A single numerical cortisol result without documented collection time cannot be interpreted clinically — the same value of 8 µg/dL represents a subnormal morning cortisol suggesting adrenal insufficiency, a normal afternoon value, or an elevated late-night value suggesting Cushing's syndrome, depending entirely on collection timing. Collection time must be documented precisely (to within 30 minutes) on the Access Medical Laboratories requisition form for the correct reference range to be applied and for the result to have any clinical meaning.

variableCorticosteroid-binding globulin (CBG) alterations — oestrogen, liver disease, nephrotic syndrome

Serum total cortisol reflects the combined bound and free fractions — approximately 80% CBG-bound and 5–10% free. CBG is elevated by oestrogen exposure (oral contraceptives, HRT, pregnancy), elevating total serum cortisol without increasing bioavailable free cortisol — a patient on OCP may have total cortisol of 25–30 µg/dL with normal free cortisol. CBG is reduced by liver failure, nephrotic syndrome, and critical illness (CBG is a negative acute phase reactant), lowering total cortisol without reducing free cortisol. In all these states, serum free cortisol by equilibrium dialysis — or salivary cortisol, which measures the free fraction — provides a more accurate bioavailable cortisol assessment than serum total cortisol.

elevatesAcute physical and psychological stress at collection

Acute physiological and psychological stress activates the HPA axis through the limbic-hypothalamic-pituitary pathway, producing ACTH-driven cortisol surges within 20–30 minutes of the stressor. Pain, anxiety about the blood draw itself, vigorous exercise, acute illness, or any emotionally or physically stressful event immediately before or during phlebotomy will elevate serum cortisol above the baseline diurnal level. Patients should be in a rested, non-stressed state for a minimum of 30 minutes before collection; the clinical context of any acute stressor on the collection day must be documented on the requisition form.

variableExogenous corticosteroids — all routes

All exogenous corticosteroid formulations — oral prednisone and dexamethasone, intravenous hydrocortisone and methylprednisolone, high-dose inhaled fluticasone and budesonide, intranasal, topical, and injectable depot steroids — suppress endogenous HPA axis cortisol secretion through negative feedback at the hypothalamic and pituitary level. The degree of suppression depends on the potency, dose, route, and duration of use. Dexamethasone cross-reacts minimally with most cortisol immunoassays, so post-dexamethasone cortisol suppression tests give a genuine measure of pituitary feedback; prednisolone and hydrocortisone DO cross-react substantially with standard cortisol immunoassays, producing falsely elevated cortisol results in patients whose assay medication exposure is not accounted for. All current and recent corticosteroid use must be documented precisely — agent, dose, route, frequency, and most recent dose timing relative to collection.

elevatesTiming within the cortisol awakening response (CAR) window

Serum cortisol peaks in the 30–45 minutes following waking — the cortisol awakening response — as a discrete neurally-mediated amplification of the morning cortisol peak. A specimen collected within 30–45 minutes of waking will be approximately 50–100% higher than a specimen collected at waking itself. If an 'early morning' cortisol is collected within 30–45 minutes of waking time, the CAR peak may cause the result to appear higher than the standard 08:00 AM reference range suggests. Documentation of exact waking time alongside collection time allows the CAR context to be applied.

variableCompetitive protein binding immunoassay cross-reactivity — other steroids

Standard cortisol immunoassays exhibit variable cross-reactivity with structurally similar steroids — including 11-deoxycortisol, prednisolone, cortisone, and synthetic glucocorticoids. In most routine clinical contexts this cross-reactivity is clinically insignificant; however, in patients with congenital adrenal hyperplasia (where 11-deoxycortisol may be substantially elevated), or those on prednisolone therapy, immunoassay cortisol may be spuriously elevated by cross-reacting steroids. LC-MS/MS-based cortisol assays provide isomer-specific measurement without cross-reactivity and are preferable when high diagnostic precision is required. # REVIEWER FLAG: Confirm Access Medical Laboratories cortisol assay platform and its cross-reactivity profile for prednisolone and other glucocorticoids.

What do Serum Cortisol results mean?

Elevated

Elevated morning cortisol (above 19–20 µg/dL) with clinical features of cortisol excess — central adiposity, purple striae, proximal myopathy, hypertension, glucose intolerance, osteoporosis, easy bruising — warrants Cushing's syndrome evaluation with the Endocrine Society-recommended tests: late-night salivary cortisol on two separate occasions, 24-hour urinary free cortisol, and low-dose dexamethasone suppression test (1 mg overnight DST — failure to suppress to <1.8 µg/dL constitutes a positive screen). Mildly elevated morning cortisol (20–25 µg/dL) in an acutely stressed patient or one on oestrogen therapy may reflect physiological HPA activation or CBG elevation rather than pathological hypercortisolaemia. Elevated afternoon cortisol with failure of the expected diurnal decline is associated with metabolic syndrome, visceral adiposity, insomnia, and HPA dysregulation in functional medicine assessment. In critical illness, elevated cortisol is expected — values below 18–20 µg/dL in a critically ill patient may represent relative adrenal insufficiency requiring hydrocortisone support per intensive care protocols.

Normal

A morning serum cortisol within the reference range (6.2–19.4 µg/dL) in a non-acutely stressed ambulatory patient effectively excludes primary adrenal insufficiency as a primary diagnosis when the value is above 18 µg/dL. A morning cortisol above 18–20 µg/dL provides high-confidence biochemical evidence of intact HPA axis output sufficient to exclude primary and most secondary adrenal insufficiency without requiring ACTH stimulation testing in most guidelines. In the functional medicine context, a normal morning cortisol does not characterise the full adrenal functional status — the diurnal cortisol pattern, afternoon decline rate, nocturnal suppression, CAR amplitude, and DHEA-S level together provide the comprehensive HPA axis picture that a single morning cortisol value cannot deliver alone.

Reduced

Morning cortisol below 3 µg/dL in an ambulatory, non-critically ill patient who is not on exogenous corticosteroids is strongly consistent with adrenal insufficiency — the diagnosis should be confirmed with ACTH measurement (low ACTH confirms pituitary/hypothalamic secondary insufficiency; high ACTH confirms primary adrenal failure) and formally evaluated with ACTH stimulation testing. Morning cortisol 3–18 µg/dL represents the diagnostic grey zone where adrenal insufficiency cannot be excluded or confirmed — ACTH stimulation testing is required. A morning cortisol above 18–20 µg/dL effectively excludes primary and most secondary adrenal insufficiency in a non-critically ill ambulatory patient. In functional medicine, a morning cortisol in the lower tertile of the reference range (6–10 µg/dL) alongside low DHEA-S and clinical features of HPA hyporesponsiveness may indicate adrenal exhaustion or burnout — interpreted as a functional HPA depletion pattern warranting adrenal support, though this interpretation is not validated as an endocrine diagnosis in mainstream guidelines.

Note: The three most clinically impactful caveats for serum cortisol are: (1) collection timing is the paramount interpretive variable — a result without a precise documented collection time cannot be clinically interpreted; (2) serum total cortisol reflects bound plus free cortisol and is confounded by CBG alterations from oestrogen therapy, liver disease, nephrotic syndrome, and critical illness — salivary cortisol or serum free cortisol by equilibrium dialysis provides the bioavailable fraction in these states; and (3) exogenous corticosteroid immunoassay cross-reactivity — prednisolone and hydrocortisone cross-react with standard immunoassays, producing falsely elevated serum cortisol in patients on these medications, while dexamethasone does not. A morning cortisol that appears inappropriately high in a patient on prednisolone reflects assay cross-reactivity rather than true endogenous cortisol. In addition, a normal serum cortisol does not exclude Cushing's syndrome — the diurnal variation of cortisol in autonomous cortisol excess may produce intermittent normal morning values; late-night salivary cortisol, 24-hour urinary free cortisol, and overnight DST are the validated Cushing's screening tools, not a single serum morning cortisol.

Frequently asked questions

Serum cortisol measures total circulating cortisol — the combined bound (approximately 90–95%) and free (approximately 5–10%) fractions — providing the primary biochemical index of adrenocortical output and HPA axis function. Collection timing is the most critical pre-analytical variable because cortisol follows a pronounced diurnal rhythm, falling from a morning peak of 6–19 µg/dL to an afternoon value of 2–12 µg/dL and a near-zero nocturnal nadir. Without a precisely documented collection time, the same numerical cortisol value can represent a normal morning peak, an inappropriately elevated afternoon value, or a pathological nocturnal elevation — making the result clinically uninterpretable.

How to access the Serum Cortisol test

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About this page

Published: May 24, 2026

Last reviewed: May 24, 2026

This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.

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