Basal blood cortisol
- Requires a medical indication
Basal blood cortisol measured between 8:00 and 9:00 in the morning. It is the first test when there is a medical suspicion of low cortisol or adrenal insufficiency. The time of day, corticosteroid treatments and oestrogens have an important effect on the result.
This is the first test when the adrenal glands are suspected of producing too little cortisol. It has three possible answers and only two of them are conclusions: if the value falls in the intermediate zone, a stimulation test is needed, which is done in a hospital setting and has to be indicated by a doctor. Talk to your doctor before ordering it.
What it is for
It is used as a first step in investigating possible adrenal insufficiency. If cortisol is clearly low first thing in the morning, suspicion increases; if it is sufficiently high, it can make it unlikely. A large proportion of results fall in a zone that this test does not resolve.
It is not a suitable test for investigating cortisol excess or for measuring your "stress level". Cushing's syndrome is screened for with late-night salivary cortisol, urine free cortisol or dexamethasone suppression.
When it is recommended
- Medical suspicion of adrenal insufficiency because of a combination of weight loss, low blood pressure, dizziness, sodium or potassium abnormalities and other compatible signs
- Darkening of skin creases or scars together with symptoms suggestive of primary adrenal insufficiency
- Assessment of the adrenal axis after prolonged corticosteroid treatment
- It is not recommended for tiredness alone without other clinical findings
Clinical comment
Basal cortisol does not give a yes-or-no answer. Very low values and sufficiently high values can point in opposite directions, but the cut-offs depend on the laboratory method and the context. Between them lies a common intermediate zone.
That zone is investigated with a synthetic ACTH stimulation test and, in specific situations, with other dynamic tests. They require giving a medicine, taking several samples and supervising the procedure, so they are not part of this catalogue.
Bear in mind that corticosteroids can suppress the adrenal axis and, depending on the compound, also interfere with the measurement. Oral oestrogens increase the protein that carries cortisol and can raise total cortisol without increasing the free fraction. That is why this test requires an indication and a full review of medication.
Dr. Antonio Mas
What is tested
• Cortisol: Measures total blood cortisol during the morning peak. A very low or sufficiently high value can point in one direction, while an intermediate zone usually requires a stimulation test.
- Cortisol
How to prepare
Have the sample taken between 8:00 and 9:00 in the morning, after 8 hours of fasting. Rest for a few minutes before the sample.
Report any current or recent corticosteroid, including inhalers, creams and injections, and any contraceptives or other oestrogen treatments. Do not stop any treatment on your own before the test.
You can add
Without another needle. It is offered when you order, so you do not have to decide now.
- General blood testA general blood test with full blood count, glucose, cholesterol and kidney and liver function: the basic parameters included in any general check-up.+€59.00
- General + nutritional blood testEverything in the general blood test (anaemia, kidneys, liver, cholesterol and glucose) plus iron stores, vitamin D, vitamin B12 and folic acid. It is the broadest add-on: it covers the general and the nutritional in one go.+€129.00
- ACTH, glucose and electrolytesCompletes basal cortisol with ACTH, the hormone that regulates it and helps point to where a low cortisol comes from, and with glucose and electrolytes, the basic data when adrenal insufficiency is possible. The sample has to be taken first thing in the morning.+€29.00
Related blood tests
We have other profiles in Other tests with more or fewer tests:
- Dexamethasone suppression test€39.00
- 24-hour urine free cortisol€49.00
- Late-night salivary cortisol€59.00
- High calcium, PTH and mineral metabolism€89.00
- Cortisol assessment, in saliva and urine€99.00
- High prolactin assessment€149.00
- Metabolic assessment of kidney stones (blood and 24-hour urine)€179.00
- Pituitary hormone profile€199.00
The most comprehensive is not always the most useful. If you are not sure, the one we recommend to start with is High prolactin assessment.
What it is based on
Endocrine Society · J Clin Endocrinol Metab 101(2):364-389
This is the source of the lower threshold on this page: a morning cortisol below 140 nmol/L (5 µg/dL), read together with ACTH, points to adrenal insufficiency. The upper limit is less firm and it is worth saying so, because the guideline reviews studies that place it between 285 and 480 nmol/L; that is why the deciding figure is the reference range printed on the report. It is also the guideline that leaves confirmation to the ACTH stimulation test, which is done in a controlled setting and is not sold here. It also confirms the warning about oral oestrogens, which raise the carrier protein and with it total cortisol.
Protocol designed by Dr. Antonio Mas, specialist in Endocrinology and Nutrition, Spanish medical licence no. 080846289, and reviewed in September 2026.
This blood test is a standardised protocol and does not replace a medical consultation. If you have symptoms, a known condition or a complex clinical situation, speak to your doctor.