Cortisolnmol/L
Adrenal stress-hormone output (morning sample)
Also known as: Cortisol (9am)
Typical optimal range
133–537 nmol/L
A morning cortisol broadly in the region of 133–537 nmol/L is considered within the expected range, though exact reference intervals differ between laboratories and analysers, so your result should always be interpreted against the range given on your own report. These figures are indicative — the range printed on your own lab report is the definitive one for your results.
Interpretation of cortisol and investigation of adrenal insufficiency or excess follows NHS and Society for Endocrinology guidance, with NICE having produced relevant surveillance and referral recommendations; your laboratory report will include the reference range and flagging used by your specific analyser.
What is Cortisol?
Cortisol is the body's primary stress hormone, produced by the adrenal glands which sit just above each kidney. It follows a predictable daily rhythm, rising sharply in the early morning to help you wake and mobilise energy, then falling through the day — which is why a blood sample is almost always taken around 9am. Measuring morning cortisol gives clinicians a reliable snapshot of adrenal output and helps identify conditions where this axis is over- or underactive.
What a high Cortisol means
A raised morning cortisol is a signal worth investigating further, particularly if it is persistently elevated or accompanied by symptoms — it is not a diagnosis in itself, but it prompts a closer look at what may be driving excess cortisol output.
- Chronic stress and poor sleep. Sustained psychological stress and inadequate or disrupted sleep are among the most common reasons for a mildly elevated cortisol reading. The hypothalamic-pituitary-adrenal (HPA) axis responds to both perceived threat and sleep deprivation by increasing cortisol secretion, which over time can affect mood, blood sugar, and weight.
- Cushing's syndrome. Persistently high cortisol — especially when accompanied by symptoms such as central weight gain, easy bruising, stretch marks, or a rounded face — may point towards Cushing's syndrome, caused by either a cortisol-secreting adrenal tumour or excess ACTH from the pituitary or elsewhere. This requires specialist investigation.
- Steroid medication. Oral, inhaled, or topical corticosteroids prescribed for conditions such as asthma, inflammatory bowel disease, or arthritis can raise measured cortisol or, paradoxically, suppress natural production — always tell your GP about any steroid use before a cortisol test.
A single elevated result can also reflect the stress of the blood draw itself, a slightly late sample, or a poor night's sleep before the test; repeat testing and clinical context are essential before drawing conclusions.
What a low Cortisol means
A low morning cortisol reading can occasionally be clinically significant, as it may indicate that the adrenal glands are not producing enough cortisol — a situation that warrants prompt discussion with a GP, particularly if symptoms are present.
- Addison's disease. Primary adrenal insufficiency (Addison's disease) occurs when the adrenal glands themselves are damaged and cannot produce sufficient cortisol. Symptoms can include persistent fatigue, dizziness, salt craving, and unexplained weight loss; it is rare but important not to miss.
- Steroid suppression. Long-term use of corticosteroid medications can suppress the body's own cortisol production via HPA axis feedback, sometimes leading to secondary adrenal insufficiency if steroids are stopped too quickly — this is why steroid courses should always be tapered under medical supervision.
A low result should always be considered alongside symptoms and the clinical picture; some individuals may have naturally lower-normal morning values without any underlying problem.
Should I be worried?
A single cortisol reading outside the expected range is rarely cause for alarm on its own — levels fluctuate considerably depending on the time of sampling, recent sleep, physical exertion, acute illness, and anxiety. If your result is mildly outside range and you feel well, your GP will often recommend repeating the test under standardised conditions before taking further action. However, if your cortisol is markedly low, if you have symptoms such as persistent extreme fatigue, dizziness on standing, or unexplained weight loss, or if your result is very high alongside physical symptoms, it is worth contacting your GP promptly. They may arrange a repeat 9am sample, a short Synacthen (ACTH stimulation) test to assess adrenal reserve, or referral to an endocrinologist depending on the picture.
What actually helps
Several evidence-backed lifestyle habits can support a healthier cortisol rhythm, particularly where elevated levels are linked to stress, sleep, or blood-sugar instability.
- Prioritise sleep. Cortisol and the sleep-wake cycle are tightly linked; consistently getting seven to nine hours of good-quality sleep helps regulate the morning cortisol peak and prevents the HPA axis from remaining in a state of heightened alert.
- Mindfulness and breath-work. Regular mindfulness meditation and slow, controlled breathing (such as diaphragmatic or 4-7-8 breathing) have been shown in randomised trials to reduce both perceived stress and salivary or serum cortisol — even short daily practice is beneficial.
- Limit caffeine in the afternoon. Caffeine stimulates cortisol release; consuming it after midday can interfere with the natural afternoon decline and disrupt sleep, creating a cycle that keeps cortisol elevated — switching to decaffeinated drinks from around noon is a practical first step.
- Stabilise blood sugar. Large swings in blood glucose trigger cortisol release as a counter-regulatory response; eating regular balanced meals with adequate protein, fibre, and complex carbohydrates helps keep glucose steady and reduces unnecessary HPA activation.
There is good supporting evidence that certain dietary patterns and specific foods help buffer the physiological stress response and support adrenal function.
Oily fish (salmon, mackerel, sardines)
Rich in long-chain omega-3 fatty acids, which have been shown in controlled studies to attenuate cortisol responses to psychological stress and reduce systemic inflammation that can activate the HPA axis.
Dark leafy greens (spinach, kale)
Excellent sources of magnesium, a mineral whose deficiency is associated with heightened HPA reactivity; adequate magnesium intake is linked to a more measured stress response.
Wholegrains and legumes
Provide slow-release carbohydrates and B vitamins that support steady blood glucose and healthy neurotransmitter production, both of which moderate cortisol secretion throughout the day.
Fermented foods (live yogurt, kefir)
Emerging evidence suggests the gut-brain axis influences HPA activity; probiotic-rich foods may help modulate the stress response, though research in humans is still developing and claims should be considered preliminary.
Berries
High in polyphenols and vitamin C, which have antioxidant properties that may help the adrenal glands cope with oxidative stress associated with sustained cortisol production.
Track your own Cortisol
Upload a lab PDF and Better Days trends Cortisol over time, flags it when it drifts, and suggests foods that help.
More in Hormones
Sources & further reading
These are reputable UK health resources for further reading. This page is written to be consistent with their guidance but is not a substitute for them.
For information only — not medical advice, and not individually reviewed by a clinician. Reference ranges vary by lab and individual. Discuss any results with a qualified clinician before making changes.