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 released from the hypothalamus. This hypothalamic-pituitary-adrenal (HPA) axis has a strong and predictable daily rhythm: cortisol peaks about 30 to 45 minutes after waking, declines steadily through the day, and reaches its lowest point around midnight before beginning to climb again before dawn.

The size of that daily swing is the reason timing dominates interpretation completely. A cortisol of 200 nmol/L at 8am is unremarkable; the same number at 11pm would be distinctly abnormal, sitting well above the expected evening trough. A result reported without a documented collection time is close to uninterpretable, which is exactly why Australian laboratories record the time on the report alongside the number.

Cortisol also circulates mostly bound to a carrier protein called cortisol-binding globulin, with only a small unbound (free) fraction biologically active. Anything that raises that binding protein — oral oestrogen, the combined oral contraceptive pill, pregnancy — raises total measured cortisol substantially without any real change in the biologically active free fraction. This single mechanism is one of the commonest causes of an apparently high cortisol result in an otherwise entirely well woman, and it is a routine part of how Australian doctors interpret a raised number.

A single blood cortisol is fundamentally a screening test, not a diagnostic one. It answers 'is this worth investigating further?' rather than 'do you have adrenal disease?'. Definitive assessment, when needed, 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. None of these dynamic tests are something FORM arranges directly — they sit downstream of a screening result and a specific clinical suspicion.

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, caused by damage to the adrenal glands themselves) 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 specifically, darkening of the skin. It is uncommon in the general population but genuinely dangerous if missed, because an untreated adrenal crisis — precipitated by illness, injury or surgery in someone with unrecognised adrenal insufficiency — can be fatal within hours. A morning cortisol is the standard, low-cost first-line screen for this possibility.

Cortisol excess (Cushing's syndrome) presents quite differently: central weight gain, a rounded ('moon') face, thin skin with easy bruising and purple stretch marks, proximal muscle weakness (typically difficulty rising from a chair or climbing stairs), new or worsening high blood pressure, and raised blood glucose. By far the most common cause of this pattern in Australia is prescribed corticosteroid medicine — tablets, but also high-dose inhalers, potent topical steroids used extensively, or repeated joint injections — rather than a hormone-producing tumour, which is comparatively rare.

It is worth being direct about what cortisol testing does not do well. The popular concept of 'adrenal fatigue' — the idea that ordinary chronic stress causes a measurable partial failure of the adrenal glands, producing tiredness — is not supported by the evidence. A systematic review pooling dozens of studies looking specifically for a relationship between fatigue and cortisol measurements found no consistent pattern. Tiredness in the general population is far more often explained by iron deficiency, thyroid dysfunction, anaemia, sleep apnoea, depression or straightforward sleep debt, all of which are more useful — and cheaper — to test for 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, significant pain, intense exercise, a difficult or anxiety-provoking venepuncture, alcohol withdrawal and depression can all elevate a single measurement. A single high value drawn from an unwell, anxious or recently exerted person usually means very little on its own and is not, by itself, evidence of an underlying disorder.

Oral oestrogen, the combined oral contraceptive pill and pregnancy raise cortisol-binding globulin and therefore total cortisol, sometimes to strikingly high numbers, while the free (active) cortisol fraction remains normal. Australian laboratory and GP practice is generally to interpret a raised result in this context, or to choose an alternative test that is not affected by binding-protein changes when the question genuinely needs answering.

Where genuine cortisol excess is suspected on clinical grounds, the diagnosis is never made on a single random blood cortisol. Your GP or an endocrinologist will use a dedicated dynamic test — an overnight dexamethasone suppression test, late-night salivary cortisol, or 24-hour urinary free cortisol — often repeated on more than one occasion, because Cushing's syndrome is genuinely uncommon and false positives on any single screening test are relatively 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 rather than a routine follow-up appointment. Values between roughly 100 and 300 nmol/L are considered indeterminate and usually lead to a short Synacthen test, in which cortisol is measured before and after an injection of synthetic ACTH to see whether the adrenal glands are able to respond appropriately. A morning cortisol above roughly 400 to 500 nmol/L generally makes clinically significant adrenal insufficiency unlikely.

The single most frequent cause of a suppressed morning cortisol is corticosteroid medicine — not just oral tablets, but also inhaled, topical, nasal and injected corticosteroid preparations used regularly — suppressing the HPA axis over time. Never stop a prescribed corticosteroid because of a blood result you have seen yourself; abrupt withdrawal in someone whose axis has been suppressed can precipitate a genuine adrenal crisis. That decision belongs entirely with your prescribing doctor, who will typically taper the dose gradually rather than stop it abruptly.

If you have a low cortisol result together with dizziness on standing, vomiting, significant weight loss or profound weakness, seek medical care promptly rather than waiting for a routine appointment slot to come up. 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.

Tests that pair well with cortisol

Cortisol answers a narrow question; these are the tests more likely to explain fatigue and general symptoms, or to complete an endocrine picture.

• Full blood count — checks for anaemia, a far more common cause of fatigue than adrenal disease.
• Thyroid function test — another HPA-axis-adjacent hormone worth checking when cortisol is being investigated.
• Vitamin D test — low vitamin D is a common, easily tested contributor to fatigue.
• HbA1c test — raised glucose is one of the clinical features that prompts cortisol testing in the first place.
• Build your own Australian panel — combine cortisol with a full blood count, thyroid function and iron studies in one collection.
• FORM Australian panels — see which tiers include cortisol and the wider fatigue work-up.

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 before reaching for cortisol as a general wellness marker.

• 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 (NATA / ISO 15189 accredited).
• Results are returned in Australian SI units with a written, plain-English explanation of what each number means.
• FORM is a diagnostic testing service. We do not diagnose, prescribe or supply any treatment — take your results to your GP or a registered doctor.
• Australian ordering is currently pre-sale. Join the waitlist and we will email you when ordering opens.
• Collection is a walk-in at Laverty (Healius) pathology centres Australia-wide; attend between 7am and 9am specifically.
• 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.