Before I found out about my ferritin, before I tested anything at all, I was convinced my problem was stress.
And honestly, it was partly stress. The months before I started this testing journey were rough. I was overworked, sleeping badly, anxious about things I couldn't control, and running on caffeine and willpower. I had that specific flavour of exhaustion where you're simultaneously tired and wired. Too drained to function, too on-edge to rest.
I did what a lot of people do in that situation: I Googled my symptoms. And Google gave me a diagnosis within about three clicks. Adrenal fatigue.
The internet was very confident about this. My adrenals were burned out. They couldn't produce enough cortisol anymore. I needed adaptogens, ashwagandha, maybe some DHEA. There were entire supplement stacks designed for exactly this condition. Podcasts. YouTube channels. Instagram accounts with millions of followers. A whole ecosystem built around a term that sounded medical, felt validating, and pointed directly at a solution I could buy.
The problem? "Adrenal fatigue" isn't a recognised medical diagnosis. It's not in any medical textbook. The Endocrine Society has explicitly stated that no scientific evidence supports it as a true medical condition. Mainstream endocrinology doesn't use the term.
That doesn't mean the symptoms aren't real. They are. I felt them. Millions of people feel them. But the explanation (that your adrenal glands are "fatigued" and can't produce cortisol) doesn't match the physiology. What does match is something more nuanced: HPA axis dysfunction. Understanding the difference matters, because it changes what you test, how you interpret results, and what interventions actually help.
This article covers cortisol: what it does, how to test it, what the results mean, and how to navigate the confusing space between wellness culture and clinical evidence.
A note before we get into it
General information only. I'm not an endocrinologist. Cortisol is a complex hormone with a circadian rhythm that makes single blood test interpretation challenging. If you suspect a genuine cortisol disorder (Cushing's syndrome, Addison's disease), work with an endocrinologist.
This article is primarily aimed at people experiencing stress-related symptoms and wondering whether cortisol testing might provide useful information, not at people with suspected endocrine disease.
What cortisol actually does
Think of cortisol as your body's natural alarm clock and dimmer switch. It rises in the morning to help you wake, peaking roughly 30 minutes after you open your eyes, then gradually dims through the day to prepare for sleep. This pattern is called the diurnal cortisol rhythm. When it's working properly, you barely notice it.
Cortisol is produced by the adrenal glands, but it's controlled by the hypothalamic-pituitary-adrenal (HPA) axis. That's a feedback loop starting in the brain. The hypothalamus detects stress (or simply the need to wake up) and sends a signal to the pituitary gland, which releases ACTH (adrenocorticotropic hormone), which tells the adrenals to produce cortisol. When cortisol levels are adequate, the hypothalamus backs off. The loop regulates itself.
Beyond the wake/sleep cycle, cortisol helps regulate blood sugar (it raises glucose when you need energy), modulates the immune system (dialling down inflammation in the short term), maintains blood pressure, and supports metabolism and cognitive function. It's genuinely essential. You cannot survive without cortisol.
The problem isn't cortisol itself. The problem is when the pattern breaks.
In chronic stress, the HPA axis can become dysregulated. This might look like cortisol that stays elevated when it should be low (wired at night, can't sleep), cortisol that's flattened across the day (no morning spike, persistent low energy), or a cortisol response that's blunted when faced with acute stress. The rhythm is disrupted, not the glands themselves.
That's the critical distinction. The adrenal glands aren't exhausted. The signalling system that controls them is miscalibrated.
Adrenal fatigue vs HPA axis dysfunction
I want to spend some time here because this is where most of the confusion lives.
The "adrenal fatigue" narrative
Chronic stress depletes your adrenals. They can't produce enough cortisol. You feel tired, foggy, anxious, and wired. The solution is supplements (adaptogens, DHEA, vitamin C mega-doses) and lifestyle changes.
What the endocrine evidence says
The adrenal glands are remarkably resilient. True adrenal insufficiency (Addison's disease), where the glands genuinely can't produce cortisol, is rare (roughly 100 to 200 per million people), serious, and requires medical management with cortisol replacement. It's not what happens from being stressed at work.
What the HPA axis research says
Chronic stress does cause measurable changes in cortisol patterns. Studies have shown flattened diurnal curves, blunted cortisol awakening responses, and altered cortisol levels throughout the day in people with chronic stress, burnout, chronic fatigue syndrome, PTSD, and depression. These are real, documented physiological changes. But they're changes in the regulatory system, not in the adrenal glands' capacity to produce cortisol.
Why the distinction matters clinically
If you believe your adrenals are burned out, you might take cortisol-boosting supplements. That could actually make things worse if your cortisol is already elevated at the wrong times. If you understand that your HPA axis rhythm is disrupted, the intervention looks different: restoring the rhythm through sleep hygiene, light exposure, stress management, exercise timing, and sometimes clinical support.
Why the distinction matters emotionally
I'm genuinely sympathetic to why people gravitate toward "adrenal fatigue." When you feel terrible and your GP says your blood tests are normal, it's incredibly frustrating. "Adrenal fatigue" provides a name, an explanation, and a solution, all in one package. HPA axis dysfunction is less tidy. It doesn't have a supplement aisle. It requires nuanced assessment and lifestyle changes that take months. But it's closer to what's actually happening.
What a cortisol blood test measures (and what it misses)
A standard morning cortisol blood test measures your serum cortisol level at a single point in time. It's typically drawn between 7am and 9am, when cortisol should be near its daily peak.
What it's good for
Screening for significantly elevated cortisol (Cushing's syndrome). Screening for significantly low cortisol (Addison's disease). Providing a single data point in a broader investigation. Ruling out major cortisol production disorders.
What it's not good for
Assessing the diurnal cortisol rhythm (it's one snapshot, not the full day). Detecting subtle HPA axis dysregulation (the kind most stressed people have). Diagnosing burnout or chronic stress (these typically don't produce cortisol levels outside the reference range).
This is the honest limitation of a morning cortisol blood test. If you're experiencing the "tired but wired" pattern, a single morning cortisol may come back normal, because your cortisol at 8am is fine. The problem might be that it's too high at 11pm, or that your morning spike is blunted, or that the rhythm is flat across the day. A single morning blood draw doesn't capture any of that.
For a fuller picture, some clinicians use salivary cortisol testing (multiple samples across the day to map the diurnal curve) or 24-hour urinary cortisol (captures total cortisol output over a full day). Bloody Good's cortisol blood test provides the morning serum level. It's a useful screening tool, but not the same as a full diurnal assessment.
When cortisol testing is genuinely useful
Despite its limitations, a morning cortisol blood test has real clinical value in the right context.
Ruling out serious pathology. If you have symptoms suggestive of Cushing's syndrome (weight gain particularly in the face and trunk, easy bruising, purple stretch marks, high blood pressure, muscle weakness) or Addison's disease (severe fatigue, weight loss, low blood pressure, darkened skin, salt craving), a morning cortisol is an important first step.
As part of a broader fatigue investigation. If you're working through the tests covered in this series (iron, thyroid, vitamin D, HbA1c) and everything's come back normal, adding cortisol provides another data point.
Before or during testosterone investigation (men). Chronic cortisol elevation can suppress testosterone production. If a man has low testosterone and is chronically stressed, cortisol context is relevant. See the testosterone article.
As a baseline. Some people, particularly those in high-stress occupations, shift workers, or people with a history of burnout, find value in establishing a morning cortisol baseline they can compare against over time.
Other markers that add context
DHEAS (Dehydroepiandrosterone Sulphate). Produced by the adrenal glands, sometimes measured alongside cortisol. The cortisol-to-DHEA ratio can provide insight into catabolic/anabolic balance.
Thyroid function (TSH, Free T4, Free T3). Chronic stress can suppress thyroid function. If you're investigating fatigue in a stress context, thyroid should be on the panel.
HbA1c and fasting glucose. Cortisol raises blood sugar. Chronic cortisol elevation can contribute to insulin resistance. Blood sugar markers provide metabolic context.
Iron studies. Fatigue from iron deficiency feels very similar to fatigue from stress or cortisol dysregulation. Checking both prevents treating one while missing the other.
Testosterone (men). Cortisol and testosterone have an inverse relationship. Testing both together gives a more complete hormonal picture.
How to get accurate results
Test between 7am and 9am. This is when cortisol should be at its daily peak.
Don't exercise before the test. Physical activity acutely raises cortisol.
Get adequate sleep the night before. Sleep deprivation alters the cortisol rhythm.
Mention all medications. Corticosteroids (prednisone, hydrocortisone, inhaled steroids, topical steroids) can dramatically affect results. Opioids can suppress cortisol. Oral contraceptives can elevate cortisol-binding globulin.
Minimise stress on the morning of the test. Easier said than done, but the blood draw itself can spike cortisol in anxious individuals.
Understanding your results
Morning cortisol reference range
Approximate range
185–624 nmol/L. Ranges vary between labs.
Below 185 nmol/L (low)
May suggest adrenal insufficiency. This warrants further investigation, typically a Synacthen stimulation test.
185–624 nmol/L (normal range)
Morning cortisol production is adequate. If you're symptomatic, the problem may lie in the cortisol pattern rather than the amount.
Above 624 nmol/L (high)
May suggest excessive cortisol production. Warrants investigation for Cushing's syndrome.
The most common scenario: your morning cortisol comes back normal. This rules out the extremes. But if your symptoms are stress-related, a normal morning cortisol doesn't mean "your cortisol is fine." It means your cortisol at 8am was within range. The pattern across the day may tell a different story.
What happens if cortisol is abnormal
Significantly low. Your GP will refer for confirmatory testing, likely a Synacthen stimulation test and ACTH measurement. Addison's disease requires lifelong cortisol replacement.
Significantly high. Confirmatory tests follow: 24-hour urinary free cortisol, late-night salivary cortisol, dexamethasone suppression test. If Cushing's is confirmed, investigation for the source follows.
Normal with persistent symptoms. Focus shifts to lifestyle-based HPA axis support and investigation of other factors: thyroid, iron, blood sugar, sleep quality, mental health.
The lifestyle interventions that actually affect cortisol
Sleep consistency. Number one. Consistent bed and wake times anchor the cortisol rhythm. Aim for 7 to 9 hours at roughly the same times each day.
Morning light exposure. Bright light in the first 30 to 60 minutes after waking helps set the cortisol awakening response. Step outside. Look at the sky, not the phone.
Exercise (but timing matters). Regular activity reduces cortisol reactivity. Intense exercise late in the evening can elevate cortisol when it should be declining. Morning or early afternoon training aligns better with the rhythm.
Stress reduction practices. Meditation, breathwork, yoga, as regular habits rather than one-off sessions. The evidence is strongest for mindfulness-based stress reduction.
Caffeine management. Caffeine stimulates cortisol. If your rhythm is disrupted, cut off caffeine by noon or earlier.
Alcohol reduction. Alcohol disrupts sleep architecture and the cortisol rhythm. The short-term relaxation comes at the cost of long-term disruption.
Addressing root causes. If the source of your stress is your job, your relationship, your finances, no supplement or sleep protocol will fix the HPA axis while the stressor remains. Sometimes the most important intervention isn't medical.
Tests to consider through Bloody Good
The cortisol test:
Cortisol Blood Test — morning serum cortisol.
Related markers:
DHEAS Blood Test — adrenal androgen.
Thyroid Function Test (TFT) — stress and thyroid overlap.
Iron Studies (Including Ferritin) — fatigue differential.
HbA1c — metabolic context.
Testosterone Free/Total + SHBG — cortisol suppresses testosterone.
Full Blood Count (FBC) — baseline blood health.
Vitamin D (25-OH) — mood and energy.
Comprehensive coverage:
The Bloody Good Test covers 100 biomarkers including cortisol alongside thyroid, iron, cholesterol, blood sugar, liver, kidney, and more.
When to retest
After a clearly abnormal result. Your clinician will advise on confirmatory testing, usually a different type of test rather than repeating the morning blood draw.
After sustained lifestyle changes. If you've implemented HPA-supportive interventions for 3 to 6 months, retesting can show whether the pattern has shifted.
If symptoms change. New symptoms or worsening fatigue warrant retesting.
As ongoing monitoring. Annual morning cortisol provides a comparative data point alongside other markers.
Explore more biomarkers
Browse the Bloody Good Biomarker Directory
This article provides general health information only and is not intended as medical advice, diagnosis, or treatment. Blood test results should be interpreted by a qualified healthcare professional in the context of your individual health circumstances, including symptoms, medical history, and medications. If you are experiencing persistent or concerning symptoms, consult your GP or seek medical attention promptly.