What this test measures
Cortisol is the principal glucocorticoid produced by the adrenal cortex under the drive of pituitary ACTH. It regulates blood sugar, blood pressure, inflammation, immune function, mood, the wake cycle and the stress response. Cortisol rises sharply in the early morning (peak 6–8 am), falls through the day, and reaches its nadir around midnight.
Serum cortisol measures the total hormone (both protein-bound and free). For a clean baseline an 8 am sample is standard. For suspected Cushing's syndrome, a midnight cortisol (or salivary cortisol) is informative because the loss of the normal diurnal fall is one of the earliest features.
Why it matters
Cortisol abnormalities sit at both ends of a wide clinical spectrum — Cushing's syndrome (cortisol excess: weight gain, easy bruising, hypertension, hyperglycaemia, mood changes) and adrenal insufficiency (cortisol deficiency: fatigue, weight loss, low BP, hyponatraemia, hyperpigmentation). Long-term oral or even high-dose inhaled steroid use is the most common cause of HPA-axis suppression seen in Indian clinics; sudden withdrawal can precipitate an adrenal crisis.
A single 8 am cortisol is the first-line test in both directions. If suspecting Cushing's, it is paired with a low-dose dexamethasone suppression test or midnight salivary cortisol. If suspecting adrenal insufficiency, it is paired with ACTH and (if borderline) a synacthen stimulation test.
How to prepare
Morning sample (between 8 and 9 am) is standard. Avoid heavy exercise, stress and alcohol the night before. Stop biotin for 48–72 hours. Tell your doctor about all steroid use — oral, inhaled, topical, joint injections — as these affect interpretation. If a midnight cortisol, dexamethasone suppression test or stimulation test is planned, follow the specific timing your doctor gives. Do not stop chronic steroids without medical advice.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Cortisol (µg/dL)[1][2] | 8 am: 6.2 – 19.4 · 4 pm: 2.3 – 11.9 · Midnight: < 5 (with sleep) | 8 am cortisol < 3 µg/dL essentially confirms adrenal insufficiency in the right clinical context (fatigue, hypotension, hyperpigmentation, weight loss, hyponatraemia). 3–10 µg/dL is indeterminate and needs a synacthen (cosyntropin) stimulation test. Sudden drop after chronic steroid use is a medical emergency. | Persistently raised cortisol (loss of the normal diurnal fall, raised midnight cortisol, or failure to suppress on overnight 1 mg dexamethasone) suggests Cushing's syndrome. Causes include exogenous steroids (commonest), pituitary adenoma (Cushing's disease), adrenal tumour, or ectopic ACTH-producing tumour. Stress, illness, pregnancy and oestrogen pills raise total cortisol without true disease. |
Cortisol — decision thresholds
| 8 am Cortisol (µg/dL) | Interpretation | Next step |
|---|---|---|
| < 3 | Adrenal insufficiency very likely | Start hydrocortisone if symptomatic; ACTH + synacthen test |
| 3 – 10 | Indeterminate | Synacthen stimulation test for definitive answer |
| 10 – 19 | Normal | Rules out adrenal insufficiency in most patients |
| > 19 (with intact diurnal fall) | High-normal / stress | Repeat in calm conditions if Cushing's suspected |
| Midnight > 5 (asleep) | Loss of diurnal rhythm | Suggestive of Cushing's — full workup |
| Post 1 mg dex > 1.8 | Failed suppression | Cushing's syndrome until proven otherwise |
Frequently asked questions
Why does cortisol have to be drawn early morning?
Cortisol follows a strong diurnal rhythm — peak around 6–8 am, nadir around midnight. An 8 am sample is at the natural peak, which is what reference ranges are based on.
I am very stressed — will it show up as high cortisol?
Severe acute stress (major illness, surgery, hospitalisation) raises cortisol substantially. Day-to-day work stress causes much smaller, transient rises that usually do not change interpretation.
I am on steroids for asthma / arthritis — will it affect the result?
Yes — significantly. Oral and high-dose inhaled steroids suppress your own cortisol production. A low cortisol on chronic steroid therapy is expected and is not a separate disease. Never stop steroids abruptly to "get a clean test" — your doctor will plan the timing.
What is a dexamethasone suppression test?
A test for Cushing's syndrome. You take 1 mg of dexamethasone at 11 pm, and serum cortisol is measured at 8 am the next morning. A normal HPA axis suppresses cortisol to < 1.8 µg/dL. Failure to suppress raises strong suspicion of Cushing's.
My doctor is talking about Addison's disease — what is that?
Addison's disease is primary adrenal insufficiency — the adrenal glands fail to produce enough cortisol (and often aldosterone). It causes profound fatigue, weight loss, low BP, salt craving, and a characteristic skin darkening. In India both autoimmune and tuberculous adrenalitis are relevant causes.
My cortisol is normal but I feel exhausted all the time — what now?
A normal 8 am cortisol largely rules out adrenal insufficiency. Chronic fatigue has many other causes — thyroid disease, B-12 / iron / vitamin D deficiency, sleep apnoea, depression, diabetes — which are far more common than adrenal problems.
Is "adrenal fatigue" a real diagnosis?
"Adrenal fatigue" is not a recognised medical diagnosis. The Endocrine Society explicitly cautions against the term. Genuine adrenal insufficiency is rare and well-defined; if your cortisol is normal, the problem lies elsewhere.
Related Hormones / Endocrine tests
Tests commonly ordered alongside CORTISOL, or that help interpret an unexpected result.
Sources & references
- Endocrine Society — Cushing's Syndrome Clinical Practice Guideline · accessed 2026-05-30T00:00:00.000Z
- Endocrine Society — Adrenal Insufficiency Guideline · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Cortisol Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Cortisol · accessed 2026-05-30T00:00:00.000Z
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