Low Cortisol on Your Blood Test
What a low cortisol result means, why timing and steroid use matter so much, and the tests your GP should run next — in plain English.
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The Quick Answer
Cortisol is the body's main stress hormone, made by the adrenal glands sitting on top of each kidney. It follows a strong daily rhythm — highest first thing in the morning and lowest late at night — which is why timing matters enormously. The standard diagnostic sample is a morning cortisol taken between 8 and 9am.
A low morning cortisol raises the question of adrenal insufficiency. By far the most common explanation is current or recent steroid medication(tablets, inhalers, injections or creams), which suppresses your own adrenal glands. Less commonly it points to Addison disease (the adrenal glands themselves are underperforming) or a pituitary gland problem (the signal telling the adrenal glands to work is too weak). All three are manageable once identified.
What Cortisol Does, and Why Timing Is Everything
Cortisol helps regulate blood sugar, blood pressure, inflammation and the body's response to physical and emotional stress. Production is controlled by a chain of command called the hypothalamic-pituitary-adrenal (HPA) axis: the hypothalamus signals the pituitary gland, the pituitary releases ACTH (adrenocorticotropic hormone), and ACTH travels through the blood to tell the adrenal glands how much cortisol to make.
Cortisol is not released at a steady rate. It follows a distinct circadian rhythm: levels peak shortly after waking, fall steadily through the day, and reach their lowest point around midnight. A blood sample taken at 4pm can look "low" simply because that is when cortisol is supposed to be lower — which is why an isolated afternoon or evening result is rarely diagnostic on its own.
This is also why steroid medication is such a common cause of a low cortisol result. When you take a steroid medicine like prednisolone, your brain senses plenty of steroid already circulating and switches off its own ACTH signal. Your adrenal glands, no longer being told to work, quieten down — sometimes for months after the medication stops. This is a normal, expected feedback loop, not a disease, but it does mean the cortisol result needs to be interpreted alongside your medication history.
Causes of Low Cortisol
Causes are grouped as steroid-related (by far the most common), primary adrenal (the adrenal glands themselves are affected, with a high ACTH), or secondary/pituitary (the pituitary signal is too weak, with a low or inappropriately normal ACTH).
Oral corticosteroid medication (prednisolone, dexamethasone)
More than about three weeks of prednisolone 5mg or more daily can suppress your own adrenal glands. This is the single most common cause of a low cortisol result seen in Australian general practice, and the medication itself is often the reason nobody thought to ask why testing was done in the first place.
Inhaled corticosteroids (high-dose, for asthma or COPD)
High-dose fluticasone or budesonide inhalers are absorbed into the bloodstream enough, especially in children and over months to years, to suppress the hypothalamic-pituitary-adrenal (HPA) axis. Easy to overlook because a puffer does not feel like a steroid.
Injected or topical steroids (joint injections, potent skin creams)
A single joint injection rarely causes lasting suppression, but repeated injections, epidurals, or long-term potent topical steroid use over large areas of skin can add up over months.
Recent steroid withdrawal (stopped too quickly)
The adrenal glands can take weeks to months to wake back up after steroids are stopped. Stopping suddenly after more than three weeks of use is the classic setup for a low cortisol result and, occasionally, an adrenal crisis.
Autoimmune adrenalitis (Addison disease)
The immune system attacks the adrenal cortex directly. Usually develops slowly over months, often alongside other autoimmune conditions such as thyroid disease or type 1 diabetes. Low sodium and high potassium are classic companions.
Tuberculosis or other adrenal infection
A historically important cause worldwide and still relevant in people with a relevant travel or exposure history, though rare in Australian practice today.
Adrenal haemorrhage or infarction
Can occur with severe sepsis (Waterhouse-Friderichsen syndrome), anticoagulant medication, or antiphospholipid syndrome. Usually presents suddenly and severely, sometimes as an adrenal crisis.
Pituitary tumour, surgery or radiotherapy
Damage to the pituitary gland reduces ACTH output, which in turn reduces cortisol. Sodium and potassium are often normal because aldosterone production is largely independent of ACTH. Other pituitary hormones (thyroid, sex hormones, growth hormone) may also be affected.
Critical illness or sepsis (relative adrenal insufficiency)
Severe illness can temporarily blunt the cortisol response even when the adrenal glands are structurally normal. This is usually identified and managed by the treating hospital team and resolves as the underlying illness improves.
Symptoms That Can Accompany Low Cortisol
Symptoms of adrenal insufficiency are often vague and develop gradually over weeks to months, which is why the condition is sometimes missed at first. Some symptoms, like skin darkening, only occur when ACTH is very high (primary adrenal insufficiency).
Profound fatigue
Often described as an exhaustion that sleep does not fix, worse as the day goes on and worse still during illness or stress, when cortisol demand rises but supply cannot.
Unintentional weight loss
Loss of appetite combined with the metabolic effects of low cortisol commonly leads to gradual, unexplained weight loss over weeks to months.
Dizziness or fainting on standing
Postural (orthostatic) hypotension is common because cortisol helps maintain blood pressure and blood vessel tone. Standing up quickly can bring on light-headedness or fainting.
Nausea, vomiting or abdominal pain
Gastrointestinal symptoms are common and can be mistaken for a stomach bug or gastritis, delaying the correct diagnosis.
Salt craving
A distinctive symptom of primary adrenal insufficiency (Addison disease), where low aldosterone causes the body to lose salt. Some people report craving salty foods or adding extra salt to meals.
Darkening of the skin, gums and scars (hyperpigmentation)
Occurs only in primary adrenal insufficiency, where a very high ACTH also stimulates skin pigment cells. Look for darkening in skin creases, scars, pressure points and the inside of the mouth. Does not occur in secondary (pituitary) causes.
Low blood sugar (hypoglycaemia)
Cortisol helps keep blood sugar stable, so low levels can cause shakiness, sweating and confusion between meals, particularly in children.
Low mood, irritability or difficulty concentrating
Cortisol deficiency affects energy and mental clarity and can be mistaken for depression or burnout, especially when the physical symptoms are mild.
Red Flags — Adrenal Crisis Is a Medical Emergency
Most people with a low or borderline cortisol result can wait for a routine GP follow-up. But anyone with known or suspected adrenal insufficiency who develops the following should seek emergency care immediately — call an ambulance rather than driving yourself:
Severe weakness or collapse
A sudden, dramatic drop in energy and strength, sometimes with an inability to stand, can signal an adrenal crisis and needs emergency care.
Persistent vomiting or diarrhoea
If someone on steroid replacement cannot keep their tablets down, cortisol levels can fall rapidly. This is an emergency in anyone with known adrenal insufficiency.
Very low blood pressure or shock
Cortisol helps maintain blood vessel tone. A dramatic drop in blood pressure, especially with a fast heart rate, is a hallmark of adrenal crisis and needs immediate hospital treatment.
High fever with vomiting after a missed steroid dose
Illness increases the body’s cortisol requirement. Missing a dose during a fever or infection is one of the most common triggers of adrenal crisis.
Severe abdominal pain
Adrenal crisis can mimic a surgical abdomen with severe pain, cramping and vomiting. Always mention known or suspected adrenal insufficiency to any treating doctor.
Confusion or reduced consciousness
A late and serious sign of adrenal crisis, often accompanied by very low blood sugar and very low blood pressure. Call an ambulance immediately.
What Your GP Will Do Next — The Workup
Australian GPs follow a fairly standard pathway when investigating a low cortisol result. Knowing the sequence helps you understand why each test is being ordered and what the next step might be.
Repeat as a proper 9am sample
Cortisol follows a strong daily rhythm, highest around waking and lowest late at night, so a random afternoon or evening sample that looks low may simply reflect normal timing. The standard diagnostic sample is taken between 8 and 9am.
Check ACTH
A same-sample or same-morning ACTH level tells your GP whether the pituitary gland is trying hard to stimulate a failing adrenal gland (high ACTH, suggesting Addison disease) or is itself under-functioning (low or inappropriately normal ACTH, suggesting a pituitary cause).
Check electrolytes and glucose
Sodium, potassium and blood glucose are checked alongside cortisol. Low sodium with high potassium points toward primary adrenal insufficiency; normal electrolytes are more typical of a pituitary (secondary) cause.
Review every form of steroid exposure
Your GP will ask about tablets, inhalers, nasal sprays, joint injections, epidurals and topical creams, including any recently stopped. This single conversation resolves the majority of low cortisol results.
Short Synacthen (ACTH stimulation) test
The definitive test. Synthetic ACTH is injected and cortisol is measured before and 30 to 60 minutes afterwards. A healthy adrenal gland responds with a sharp rise; a blunted or absent rise confirms adrenal insufficiency.
Adrenal antibodies and imaging
If a primary adrenal cause is suspected, blood tests for adrenal (21-hydroxylase) antibodies and a CT scan of the adrenal glands can help identify autoimmune disease, infection or haemorrhage.
Pituitary MRI and other pituitary hormones
If a secondary (pituitary) cause is suspected, an MRI of the pituitary gland and blood tests for thyroid, sex hormones, growth hormone and prolactin help build the full picture.
Referral to endocrinology
Confirmed or strongly suspected adrenal insufficiency is managed by an endocrinologist, who confirms the diagnosis, identifies the cause, starts hormone replacement, and teaches sick day rules and emergency planning.
Treatment — What Happens Once You Know the Cause
Steroid-induced suppression
If the cause is your own steroid medication, treatment usually means a slow, carefully supervised taper rather than stopping abruptly, giving your adrenal glands time to wake back up. Your GP or endocrinologist may recommend temporary hydrocortisone cover during the taper and for any period of illness or surgery.
Addison disease (primary adrenal insufficiency)
Lifelong daily hydrocortisone (usually split into two or three doses through the day to mimic the natural rhythm) replaces the missing cortisol, and fludrocortisone replaces the missing aldosterone that controls salt and water balance. Doses are increased temporarily during illness, injury or surgery under "sick day rules".
Secondary (pituitary) adrenal insufficiency
Treated with hydrocortisone alone in most cases, since aldosterone production usually remains intact. Any underlying pituitary problem — a tumour, prior surgery, or radiotherapy effects — is managed alongside cortisol replacement, and other pituitary hormones are checked and replaced if needed.
Living well with adrenal insufficiency
Everyone on long-term steroid replacement is taught sick day rules (doubling or tripling the dose during fever, infection or before surgery), and is advised to carry a steroid card or medical alert bracelet plus an emergency hydrocortisone injection kit for use if vomiting prevents tablets being taken. With this plan in place, most people live a full, active, completely normal life expectancy.
Low Cortisol — Frequently Asked Questions
What does it mean if my cortisol is low?
Cortisol is the body’s main stress hormone, made by the adrenal glands under instruction from the pituitary gland. A low cortisol level, especially on a morning (8-9am) sample, raises the possibility of adrenal insufficiency — a state where your adrenal glands are not producing enough cortisol. By far the most common cause is current or recent steroid medication, which suppresses your own adrenal glands. Less commonly, it points to a problem with the adrenal glands themselves (Addison disease) or with the pituitary gland that controls them.
What is the normal range for cortisol in Australia?
Australian pathology labs typically report morning (8-9am) cortisol reference ranges of roughly 140 to 500 nmol/L, though exact cut-offs vary between labs and the specific assay used. As a general rule, a morning cortisol above about 450-500 nmol/L makes adrenal insufficiency unlikely, a result below about 100 nmol/L strongly suggests it, and anything in between usually needs a Short Synacthen test to clarify.
Can steroid medication cause low cortisol?
Yes, by far the most common cause. Oral steroids like prednisolone or dexamethasone taken for more than about three weeks can suppress your own adrenal glands, sometimes for months after stopping. High-dose inhaled steroids for asthma, and repeated joint injections or potent topical steroid creams, can do the same over time. Always tell your GP about every form of steroid you use, including inhalers, creams and injections, not just tablets.
What is a Short Synacthen test?
It is the definitive test for adrenal insufficiency. Synacthen is a synthetic form of ACTH, the pituitary hormone that normally tells your adrenal glands to make cortisol. You are given an injection, and blood cortisol is measured before and 30 to 60 minutes afterwards. A healthy adrenal gland responds with a sharp rise in cortisol; a poor or absent rise confirms adrenal insufficiency.
Is low cortisol dangerous?
A single low or borderline result is rarely an emergency and is often explained by the timing of the sample or steroid use. However, established adrenal insufficiency needs proper diagnosis and treatment because an untreated or under-treated adrenal gland cannot cope with physical stress such as infection, injury or surgery. This can trigger an adrenal crisis — severe weakness, vomiting, very low blood pressure and collapse — which is a medical emergency requiring immediate hospital treatment.
What is the difference between Addison disease and secondary adrenal insufficiency?
Addison disease is primary adrenal insufficiency: the adrenal glands themselves are damaged, usually by an autoimmune process, so cortisol is low but ACTH (the pituitary signal) is high as the body tries to compensate. This often comes with low sodium and high potassium. Secondary adrenal insufficiency happens when the pituitary gland fails to produce enough ACTH, so both ACTH and cortisol are low; electrolytes are usually normal because aldosterone is largely controlled separately.
Can low cortisol be treated?
Yes. Adrenal insufficiency is very manageable with daily hydrocortisone tablets to replace the missing cortisol, and fludrocortisone as well if the adrenal glands cannot make aldosterone (as in Addison disease). People on replacement therapy are taught sick day rules to increase their dose during illness, and carry a steroid card or medical alert bracelet plus an emergency injection kit for adrenal crisis. With proper treatment, most people live a completely normal life expectancy and activity level.
Related Reading
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This page provides general educational information about low cortisol and adrenal insufficiency. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your GP about abnormal blood test results — they have access to your full medical history and can interpret your results in context. SmarterBlood does not provide medical care.