Cortisol Test in Australia

What a cortisol test is

A cortisol test measures the concentration of cortisol — the principal glucocorticoid hormone produced by the adrenal cortex — in a blood sample taken at a specified time of day.

Cortisol is released by the adrenal glands under instruction from ACTH, produced by the pituitary, which is in turn driven by CRH from the hypothalamus. This hypothalamic-pituitary-adrenal axis has a strong daily rhythm: cortisol peaks about 30 to 45 minutes after waking, declines through the day, and reaches its lowest point around midnight.

The size of that swing is the reason timing dominates interpretation. A cortisol of 200 nmol/L at 8am is unremarkable; the same number at 11pm would be distinctly abnormal. A result reported without a collection time is close to uninterpretable, and Australian laboratories therefore record the time on the report.

Cortisol also circulates mostly bound to cortisol-binding globulin. Anything that raises that binding protein — oral oestrogen, the combined oral contraceptive pill, pregnancy — raises total measured cortisol without any change in the biologically active free fraction. This is one of the commonest causes of an apparently high result in an otherwise well woman.

A single blood cortisol is a screening test, not a diagnostic one. Definitive assessment uses dynamic tests arranged by a doctor: a short Synacthen (ACTH stimulation) test where insufficiency is suspected, and an overnight dexamethasone suppression test, 24-hour urinary free cortisol or late-night salivary cortisol where excess is suspected.

Why cortisol is measured

Cortisol is measured when there is clinical suspicion of adrenal insufficiency or cortisol excess — not as a general 'stress' or 'burnout' marker, which it does not reliably reflect.

Adrenal insufficiency, whether primary (Addison's disease) or secondary to pituitary disease or corticosteroid withdrawal, presents with fatigue, weight loss, low blood pressure, dizziness on standing, nausea, salt craving and, in the primary form, skin darkening. It is uncommon but genuinely dangerous, because an untreated adrenal crisis can be fatal. A morning cortisol is the standard first-line screen.

Cortisol excess (Cushing's syndrome) presents with central weight gain, a rounded face, thin skin with easy bruising and purple stretch marks, proximal muscle weakness, new or worsening high blood pressure and raised glucose. By far the most common cause in Australia is prescribed corticosteroid medicine rather than a tumour.

It is worth being direct about what cortisol testing does not do. The concept of 'adrenal fatigue' — that ordinary chronic stress causes a measurable partial failure of the adrenal glands producing tiredness — is not supported by the evidence, and a systematic review of dozens of studies found no consistent relationship between fatigue and cortisol measurements. Tiredness is far more often explained by iron deficiency, thyroid dysfunction, anaemia, sleep apnoea, depression or simple sleep debt, all of which are more usefully tested first.

• Unexplained weight loss, low blood pressure, dizziness on standing or salt craving.
• Persistent nausea, vomiting or abdominal pain with fatigue.
• Darkening of skin creases, scars or gums.
• Central weight gain with thin skin, easy bruising or purple stretch marks.
• Long-term corticosteroid use, or symptoms after stopping steroids.
• New high blood pressure or raised glucose with the features above.

What a high result can indicate

A high morning cortisol most often reflects physiological stress, acute illness or raised binding proteins, and only occasionally genuine cortisol excess.

Cortisol rises with any acute physical or psychological stress: infection, injury, surgery, pain, intense exercise, a difficult venepuncture, alcohol withdrawal and depression can all elevate it. A single high value in an unwell or anxious person usually means very little.

Oral oestrogen, the combined oral contraceptive pill and pregnancy raise cortisol-binding globulin and therefore total cortisol, sometimes to strikingly high numbers, with normal free cortisol. Australian practice is generally to interpret with that in mind or to use an alternative test.

Where genuine excess is suspected, the diagnosis is not made on a random blood cortisol. Your GP or an endocrinologist will use an overnight dexamethasone suppression test, late-night salivary cortisol or 24-hour urinary free cortisol, often repeated, because Cushing's syndrome is uncommon and false positives are frequent.

What a low result can indicate

A low morning cortisol raises the possibility of adrenal insufficiency and should be discussed with a doctor promptly, though the commonest explanation is exogenous corticosteroid suppression.

In Australian practice a morning cortisol below roughly 100 nmol/L is highly suggestive of adrenal insufficiency and warrants urgent medical assessment. Values between roughly 100 and 300 nmol/L are indeterminate and usually lead to a short Synacthen test, in which cortisol is measured before and after synthetic ACTH to see whether the adrenals can respond. A morning cortisol above roughly 400 to 500 nmol/L makes insufficiency unlikely.

The most frequent cause of a suppressed result is corticosteroid medicine — oral, but also inhaled, topical, nasal and injected preparations — suppressing the axis. Never stop a prescribed corticosteroid because of a blood result; abrupt withdrawal can precipitate an adrenal crisis. That decision belongs entirely with your prescribing doctor.

If you have a low cortisol together with dizziness on standing, vomiting, weight loss or profound weakness, seek medical care promptly rather than waiting for a routine appointment. FORM does not provide emergency care; call your doctor, or 000 if you are seriously unwell.

Australian reference ranges

Australian laboratories report serum cortisol in nmol/L against time-of-day-specific intervals, with the morning interval the one used for screening.

Who should consider a cortisol test

Cortisol testing is appropriate where there are specific features of adrenal insufficiency or cortisol excess; for undifferentiated tiredness, other tests come first.

If your question is 'why am I tired?', a full blood count, iron studies, thyroid function, HbA1c and vitamin D answer that question far more often than cortisol does. That is the sequence most Australian GPs follow, and it is the sensible order to test in.

• Weight loss, low blood pressure, dizziness on standing, nausea or salt craving.
• Skin darkening in creases, scars or gums.
• Central weight gain with thin skin, bruising, purple stretch marks or muscle weakness.
• Long-term corticosteroid use, or symptoms during or after tapering.
• Known pituitary disease under specialist care.

How testing works with FORM in Australia

You choose the test, we issue an Australian pathology request form, you attend an accredited collection centre between 7am and 9am, and your result is returned in nmol/L with the collection time and a written explanation.

• No referral from your own GP is required. Privately requested pathology is arranged under a request from a registered medical practitioner working with our accredited lab partner.
• Collection is a walk-in at Laverty (Healius) pathology centres Australia-wide; attend between 7am and 9am.
• No fasting required, but avoid testing during an acute illness or immediately after intense exercise.
• Tell us about any corticosteroid medicine, the combined pill or pregnancy — each changes interpretation.
• Dynamic tests such as the short Synacthen test are performed under medical supervision and are arranged by your doctor, not by us.
• FORM is a diagnostic testing service. We do not diagnose adrenal disease, prescribe or adjust steroid medicines — take any abnormal result to your GP.
• Australian ordering is currently pre-sale. Join the waitlist for launch notification.