Low cortisol: what a low morning result means and what to check next

One low cortisol result usually has a dull explanation: the blood was drawn in the afternoon, you used a steroid in the past few months, or the lab's test method (assay) reads lower than older ones. Cortisol is the hormone your adrenal glands make every morning to keep blood pressure, blood sugar and salt balance steady. A clearly low 8 a.m. value in someone who also feels unwell is different, and it needs a stimulation test, because true adrenal insufficiency (the adrenal glands not making enough cortisol) is treatable and can turn dangerous during an infection. Your next step: ask your doctor for a repeat cortisol drawn between 7 and 9 a.m. with ACTH measured at the same time, and tell them about every steroid you have used in the past few months, including inhalers, creams and injections. If you came here looking for ways to bring a high cortisol down, the page you want is high morning cortisol.

What counts as low cortisol

Cortisol is reported in µg/dL in the US and nmol/L in most other countries; 1 µg/dL equals 27.6 nmol/L. The cutoffs below apply to blood drawn between about 7 and 9 a.m., when cortisol should be near its daily peak. The Endocrine Society guideline treats a morning cortisol below 140 nmol/L (5 µg/dL) together with a high ACTH (the pituitary hormone that tells the adrenals to make cortisol) as suggestive of primary adrenal insufficiency ¹. The 2024 European and Endocrine Society guideline for people coming off steroids uses about 150 nmol/L as the line below which steroid replacement is still needed, and about 300 nmol/L (roughly 11 µg/dL) as the level that suggests the system has recovered ⁴.

A healthy person's cortisol at 4 p.m. can be half the 8 a.m. value, so an afternoon result cannot be judged against morning cutoffs. The afternoon cortisol page covers what an afternoon or evening value is meant to look like.

Is your cortisol really low? Five things to check first

1. 1The time of the drawFind the collection time on the report. A cortisol drawn after about 10 a.m. reads lower by design, and a night-shift worker's peak may fall at a different clock time.
2. 2Any steroid in the past monthsTablets such as prednisone, but also asthma inhalers, nasal sprays, strong skin creams and joint or spine injections. A meta-analysis found adrenal insufficiency in about 52% of people after joint injections, 49% on oral steroids, 8% on inhaled, 5% on topical and 4% on nasal steroids ².
3. 3Opioid painkillersLong-term opioid use can quieten the signal from the brain to the adrenals. A meta-analysis estimated adrenal insufficiency in roughly 15% of long-term opioid users ⁷, so a low cortisol in someone on daily opioids needs that context.
4. 4Low binding proteinMost cortisol in blood rides on a carrier protein. Severe liver disease, a kidney condition that leaks protein into the urine, and serious illness in hospital lower that protein, so total cortisol can read low while the free, active cortisol is normal ⁸.
5. 5The test method and supplementsNewer lab test methods read lower than the older tests that set the classic cutoffs, so a value can look low on one platform and normal on another ³. High-dose biotin supplements can upset some cortisol tests, so ask whether to pause them before a repeat, and retest at the same lab so the numbers compare.

What causes low cortisol

A low cortisol that is confirmed on repeat testing has three possible causes, and a paired ACTH result usually tells them apart. When the adrenals themselves fail, ACTH climbs as the pituitary shouts louder. When the pituitary is the problem, or steroid medicine has quietened it, ACTH stays low or normal.

Primary adrenal insufficiency also wipes out aldosterone, the salt-keeping hormone, which is why low sodium and high potassium travel with it, while pituitary causes usually leave potassium normal ¹. The high ACTH in primary disease is what darkens the skin. Addison's disease is rare, and the full condition is covered on the Addison's disease page.

Low cortisol symptoms

Low cortisol symptoms are vague and build slowly, so the diagnosis is often late.

Low cortisol tends to cause weight loss, not weight gain. Most people with long-standing tiredness turn out to have normal cortisol, and the fatigue page and dizziness page cover the more common causes.

How doctors confirm low cortisol

1. Repeat an early-morning cortisol with an ACTH drawn at the same time.
2. An ACTH stimulation test (also called a Synacthen test): cortisol is measured before and 30 to 60 minutes after an injection of synthetic ACTH. The Endocrine Society treats a peak below 18 µg/dL (500 nmol/L) as diagnostic of adrenal insufficiency ¹. On newer specific assays a peak of about 14 to 15 µg/dL (roughly 400 nmol/L) is a proposed cutoff, to avoid labelling healthy people ³.
3. If ACTH is high: tests for renin and aldosterone (the salt-balance hormones) and for adrenal antibodies, to confirm Addison's disease and find its cause ¹.
4. If ACTH is low and there is no steroid explanation: other pituitary hormones and a pituitary MRI.
5. After steroids: a doctor may check morning cortisol during a slow taper once the dose is near the body's own output; testing while still on higher doses is not advised ⁴.

If you feel well, a repeat test within the next week or two is usual. If you feel faint, are vomiting or are getting worse, do not wait for an appointment.

Thyroid often fails alongside autoimmune adrenal disease, so a TSH is usually checked as well; see underactive thyroid. If both are low, doctors replace cortisol before thyroid hormone, because starting thyroxine first can bring on a crisis ¹. The electrolytes explainer covers how sodium and potassium are read together.

Is adrenal fatigue the same as low cortisol?

Adrenal fatigue is a different idea from low cortisol: a label sold with saliva panels and supplements, with no agreed test or definition. A systematic review of 58 studies concluded:

There is no substantiation that "adrenal fatigue" is an actual medical condition.Cadegiani and Kater, BMC Endocrine Disorders, 2016 ⁶

True adrenal insufficiency shows up on a blood test and a stimulation test. Someone who is exhausted with a normal cortisol is better served by a look at thyroid, iron, sleep and mood, and by the chronic fatigue syndrome page when tiredness has lasted six months or more.

Can you raise cortisol naturally?

If adrenal insufficiency is confirmed, no food, sleep habit or supplement brings cortisol back; treatment is replacement hormone (hydrocortisone, with fludrocortisone, a replacement for aldosterone, added in primary disease) prescribed and adjusted by an endocrinologist ¹. People on replacement are taught sick-day rules for raising their dose during a fever and carry a steroid card, because illness is when crises happen ⁵.

If cortisol is borderline after a course of steroids, the adrenal axis usually recovers over weeks to months as the dose comes down slowly, and sometimes a year or more after long courses ⁴. A low reading while taking or tapering prednisone, using an inhaler or after a joint injection is expected. Stopping the steroid suddenly is the step that causes harm, so talk to the prescriber before changing any dose.

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