Cortisol, Total (Serum) Test
The total serum cortisol test measures the principal glucocorticoid to assess adrenal function, interpreted against its strong diurnal rhythm. A low early-morning value can suggest adrenal insufficiency and a high late-night value can support hypercortisolism, but confirmation usually requires dynamic testing.
1
Biomarkers
Blood (serum)
Sample
1–3 business days
Turnaround
No — however, the collection time is critical and must be documented; an early-morning (approximately 8 am) sample is standard for adrenal insufficiency assessment
Fasting
What is the Total Cortisol test?
The Cortisol, Total Test measures serum cortisol, the principal glucocorticoid, to assess adrenal function. Because cortisol follows a strong diurnal rhythm and rises with stress, the result is interpreted against the time of collection, typically as an early-morning measurement or as part of a dynamic test. Within adrenal assessment, total serum cortisol is a component of a structured pathway rather than a standalone diagnostic. A low early-morning cortisol raises the possibility of adrenal insufficiency, and a raised late-night cortisol can support hypercortisolism, but confirmation usually requires dynamic testing — the short Synacthen (ACTH stimulation) test for insufficiency, and the overnight dexamethasone suppression test, late-night salivary cortisol, or 24-hour urinary free cortisol for Cushing's syndrome. Total cortisol measures bound and free hormone together, so binding-protein changes from oestrogens, pregnancy, or critical illness can mislead unless considered. It is interpreted with the collection time, clinical picture, and, where relevant, ACTH and dynamic testing.
What does the Total Cortisol test measure?
Cortisol is the principal glucocorticoid, secreted by the adrenal cortex under hypothalamic-pituitary (CRH-ACTH) control with a pronounced diurnal rhythm — highest shortly after waking and lowest around midnight. Most circulating cortisol is bound to cortisol-binding globulin and albumin, with only a small free fraction biologically active; total serum cortisol measures bound and free together. It has wide-ranging effects on metabolism, immune function, and the stress response, and its secretion rises acutely with physical and psychological stress.
Total serum cortisol is used, with awareness of its diurnal rhythm, in the assessment of adrenal function — a low early-morning value can suggest adrenal insufficiency and a high late-night value can support hypercortisolism, though both usually require dynamic testing to confirm. Because it measures total hormone, changes in binding proteins (from oestrogens, pregnancy, or critical illness) shift the result without altering free cortisol, an important interpretive caveat. It is interpreted against the collection time and clinical context and often as part of dynamic tests rather than as a single value.
Why is the Total Cortisol test ordered?
- Assessment of suspected adrenal insufficiency, where a low early-morning cortisol prompts confirmatory dynamic (ACTH stimulation) testing
- Evaluation of suspected hypercortisolism (Cushing's syndrome) as part of a structured pathway with dynamic and rhythm-based testing
- Investigation of unexplained fatigue, hypotension, hyponatraemia, or hyperpigmentation where adrenal insufficiency is considered
- Assessment of patients on or withdrawing from glucocorticoid therapy where adrenal suppression is a concern
- Evaluation of the cortisol rhythm using paired morning and late-night sampling where loss of diurnal variation is being assessed
- Baseline and stimulated cortisol measurement within an ACTH stimulation test protocol
- Assessment of adrenal contribution to acute illness or shock where cortisol adequacy is clinically relevant
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No — however, the collection time is critical and must be documented; an early-morning (approximately 8 am) sample is standard for adrenal insufficiency assessment
Collection method
Venipuncture at patient service centre
Turnaround
1–3 business days
Collection notes
Record the exact collection time, as cortisol interpretation is time-dependent. Draw early-morning samples for adrenal insufficiency assessment and late-night samples where hypercortisolism is being evaluated. Document glucocorticoid therapy, oestrogen use, pregnancy, and acute illness, and note that dynamic testing is usually required for diagnosis.
Specimen requirements
Minimum 0.5 mL serum. Separate serum from cells per Quest protocol. Specimen stable refrigerated; follow Quest transport guidelines.
What can affect Total Cortisol results?
Cortisol is highest after waking and lowest near midnight, so the collection time is essential for interpretation; a value normal in the morning may be abnormal at night and vice versa.
Oestrogen therapy and pregnancy raise cortisol-binding globulin, increasing total cortisol without changing the active free fraction. This can falsely suggest hypercortisolism unless recognised.
Physical and psychological stress, including the stress of venepuncture and acute illness, raises cortisol, so results must be interpreted against the clinical state and may not reflect baseline function.
Synthetic glucocorticoids suppress endogenous cortisol via ACTH feedback, lowering measured cortisol, while some corticosteroids may or may not cross-react with the assay depending on the compound. Therapy must be documented.
Low cortisol-binding globulin and albumin in critical illness lower total cortisol while free cortisol may be preserved or elevated, so total cortisol can underestimate the active hormone in this setting.
High-dose biotin can interfere with some immunoassays. Patients should withhold high-dose biotin for 48–72 hours before collection where relevant.
What do Total Cortisol results mean?
Elevated
A raised cortisol, particularly a high late-night value or loss of the normal diurnal fall, can support hypercortisolism but requires confirmation with dynamic testing (dexamethasone suppression, late-night salivary cortisol, or 24-hour urinary free cortisol). Raised total cortisol from oestrogen or pregnancy-related binding-protein increases should be excluded.
Normal
A clearly normal early-morning cortisol makes significant adrenal insufficiency less likely, and a normal late-night value argues against hypercortisolism, but single values rarely confirm or exclude adrenal disease. Results are interpreted with the collection time and usually alongside dynamic testing.
Reduced
A low early-morning cortisol raises the possibility of adrenal insufficiency and prompts an ACTH stimulation test to confirm, along with ACTH measurement to localise the cause. Very low values with recent glucocorticoid use suggest suppression of the axis.
Note: Single random cortisol values are of limited diagnostic value; interpretation depends on the collection time and usually on dynamic testing. Total cortisol measures bound and free hormone, so binding-protein changes from oestrogens, pregnancy, or critical illness can mislead. Stress and acute illness raise cortisol, and exogenous glucocorticoids suppress it. It is interpreted within a structured pathway with ACTH and confirmatory tests, not as an isolated value.
Frequently asked questions
It measures serum cortisol, the principal glucocorticoid, to assess adrenal function. Because cortisol follows a strong diurnal rhythm, the result is interpreted against the time of collection.
How to access the Total Cortisol test
The Total Cortisol reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
Create Free AccountAbout this page
Published: June 2, 2026
Last reviewed: June 2, 2026
- Bornstein SR, et al. Diagnosis and Treatment of Primary Adrenal Insufficiency: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. (2016)
- Nieman LK, et al. The diagnosis of Cushing's syndrome: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. (2008)
- Raff H, et al. Cushing syndrome: diagnosis and surveillance using salivary and serum cortisol. Compr Physiol. (2014)
- Charmandari E, et al. Adrenal insufficiency. Lancet. (2014)
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.
Related tests
adrenal stress
DHEA-S
The DHEA-S (Dehydroepiandrosterone Sulfate) Serum test measures the sulfated storage form of the primary adrenal androgen prohormone — the most analytically stable adrenal androgen marker with minimal diurnal variation, suitable for collection at any time of day.
adrenal stress
Saliva Hormones 5 + 4-Point Cortisol
The Saliva LC-MS Hormones 5 with 4-Point Cortisol panel measures five salivary steroid hormones — oestradiol, progesterone, testosterone, and DHEA-S — plus a four-point diurnal cortisol curve, analysed by liquid chromatography-mass spectrometry from at-home saliva collections.