What the cortisol test shows
Cortisol is the body’s main glucocorticoid, made by the adrenal glands above the kidneys. It runs on a daily rhythm — highest in the early morning, lowest around midnight — and rises with stress, which is why it is called the stress hormone. A single test captures only that moment, so when it is drawn matters as much as the number, per MedlinePlus.
It is run by the HPA axis: the pituitary’s ACTH tells the adrenals how much to make, and cortisol feeds back to switch the signal off. So it is rarely read alone — adding ACTH shows where a problem sits, adrenal or pituitary, and separates it from the adrenal androgen DHEAS, as StatPearls describes. The standard assay measures total cortisol, so estrogen, pregnancy and the pill can raise the reading without raising the active free hormone.
Cortisol normal range
Cortisol swings across the day, so there is no single normal value — it is read against the draw time. Serum cortisol is reported in µg/dL (US) or nmol/L elsewhere, and the two differ (1 µg/dL ≈ 27.6 nmol/L). Orientation ranges for a healthy adult:
| Time of draw | µg/dL (conventional) | nmol/L (SI) |
|---|---|---|
| Morning (~8 AM), the daily peak | ~5–25 | ~140–690 |
| Afternoon (~4 PM) | ~3–16 | ~80–440 |
| Late night / midnight (asleep) | below ~5 | below ~140 |
Two cut-offs matter more than the band. A morning cortisol below 5 µg/dL (140 nmol/L) raises concern for adrenal insufficiency; above about 15–18 µg/dL makes it unlikely. After an overnight 1 mg dexamethasone dose, a normal axis suppresses cortisol below 1.8 µg/dL (50 nmol/L); a higher value screens positive for Cushing’s. Cut-offs are assay-dependent, so read your result against your own lab’s range and draw time.
Why cortisol is high
High cortisol (hypercortisolism) has many causes, and most raised values are not Cushing’s. Roughly by frequency:
- Stress and acute illness — pain, infection, surgery, injury or even the anxiety of the draw briefly push it up; normal physiology, not disease.
- Physiological (“pseudo-Cushing’s”) states — poorly controlled diabetes, obesity, heavy alcohol use, severe depression.
- Estrogen and pregnancy — raise the binding protein, lifting total cortisol while the free hormone stays normal.
- True Cushing’s syndrome — a pituitary tumour over-making ACTH (Cushing’s disease, the commonest endogenous cause), an adrenal tumour, or an “ectopic” ACTH tumour such as some lung cancers, per the Endocrine Society.
One trap: steroid medicines (prednisone, or potent inhaled and topical steroids) cause the look of Cushing’s but suppress the measured cortisol — so a person can appear Cushingoid while the blood level reads low.
When it is urgent: severe, fast-progressing hypercortisolism (often ectopic ACTH) can drive very high blood sugar, low potassium and high blood pressure, and needs prompt specialist care.
Why cortisol is low
A low cortisol points to adrenal insufficiency — the adrenals cannot make enough. Less common than a high reading, but potentially life-threatening. By type:
- Secondary (pituitary), the commonest — usually from medication. Long glucocorticoid courses suppress the axis, so stopping steroids abruptly leaves the body unable to make its own (ACTH is low); pituitary tumours, surgery or radiation are rarer causes.
- Primary — Addison’s disease. The adrenals themselves fail, most often from autoimmune damage (tuberculosis leads globally). ACTH runs high and aldosterone is usually lost too — causing salt craving, low sodium and skin darkening, as StatPearls explains.
Symptoms are vague and easily missed — fatigue, weight loss, low blood pressure, dizziness, nausea — so diagnosis is often delayed. A morning cortisol screens; an ACTH (cosyntropin) stimulation test confirms it, a peak below about 18 µg/dL (500 nmol/L) indicating insufficiency, per the Endocrine Society.
When it is urgent: an adrenal crisis — collapse, vomiting, confusion or shock, often triggered by infection or a missed steroid dose — is an emergency needing immediate IV hydrocortisone and fluids. Call emergency services; do not wait for a result.
What to test alongside
Cortisol is read with the hormones around it and the markers it moves:
- DHEAS — the main adrenal androgen; low in adrenal failure, high with some adrenal tumours.
- 17-OH-progesterone — screens for congenital adrenal hyperplasia.
- Prolactin, FSH and LH — other pituitary hormones, checked when a pituitary cause is suspected.
- TSH and free T4 — the thyroid axis; a failing pituitary lowers these too, and thyroid disease mimics the fatigue.
- Glucose and HbA1c — cortisol raises blood sugar (high in Cushing’s); low cortisol can cause hypoglycaemia.
- Total cholesterol — often raised in the metabolic picture of Cushing’s.
(A doctor usually adds ACTH, sodium, potassium and a confirmatory salivary or urinary cortisol.)
What to do about an abnormal result
- Never start or stop steroids on your own. Abruptly stopping a glucocorticoid can trigger an adrenal crisis; long-term users should know their “sick-day” rules.
- Mind the timing and confounders. A value means little without the draw time — tell your doctor about steroid medicines, the pill or HRT, pregnancy, recent illness and stress.
- Confirm before you conclude. High results are checked with dexamethasone suppression, late-night salivary or 24-hour urinary cortisol; low results with an ACTH stimulation test.
- See your primary-care doctor or GP first. They order the confirmatory test and refer to an endocrinologist for suspected Cushing’s or adrenal insufficiency.
- Treat crisis symptoms as an emergency. Collapse, severe vomiting or confusion in someone with known or suspected adrenal insufficiency needs same-day care.
Mini-FAQ
What time of day should cortisol be tested?
First thing in the morning, around 8 AM, at its natural peak — a low morning value best screens for adrenal insufficiency. For Cushing’s syndrome, doctors use a late-night sample, when cortisol should be lowest.
Does a high cortisol level mean I have Cushing’s syndrome?
Usually not on one blood test. Stress, illness, obesity, alcohol, depression, pregnancy and the pill all raise cortisol. Cushing’s is confirmed only with dedicated tests — dexamethasone suppression, late-night salivary or 24-hour urine cortisol.
What does a low cortisol result mean?
It can signal adrenal insufficiency — Addison’s disease (the glands fail) or a pituitary problem — but the commonest cause is steroid medication suppressing the glands. A low morning value is confirmed with an ACTH stimulation test.
Can stress or the blood test itself raise cortisol?
Yes. Cortisol is the stress hormone, so physical or emotional stress — even the anxiety of the needle — briefly raises it. That is normal physiology, not a cortisol disorder, which is why timing and repeat testing matter.
Can birth control pills or pregnancy change a cortisol result?
Yes. Estrogen from the pill, HRT or pregnancy raises the protein that carries cortisol, lifting the total measured but not the active free hormone. Tell your doctor, who may use a saliva or urine test instead.
