What is cortisol: what the stress hormone does, what too high looks like and what a scan can show

In this article you’ll find the answer to the question of what cortisol is, how the hormone moves through your day, what happens when you have too much or too little of it, and what a scan can and cannot show about it. That matters, because online, cortisol gets the blame for stress, a round belly and tiredness, and what the studies really found is calmer and more useful. BioProfile’s hormone scan gives you an indication of where you stand today in this area. It is insight, to see where you stand and to help you make better choices in your diet and lifestyle. The scan does not measure cortisol. It is not a diagnosis; only a doctor can make one.

What is cortisol?

To the question “cortisol, what is it?”, the short answer is: a hormone made by the adrenal cortex. Addison’s disease is what happens when the adrenal glands make too little of it, and Cushing’s syndrome is what happens when the adrenal glands make too much of it.⁠¹ Cortisol is called the stress hormone for a reason: in a review of 66 studies on hair cortisol, groups living with stress had on average about a fifth more cortisol in their scalp hair than groups without that stress.⁠⁴

Cortisol also follows a daily rhythm. It rises in the period after you wake up. A review of 147 studies found that this rise was larger in people with work stress or general stress in their lives, and smaller in people with fatigue, burnout or exhaustion.⁠² Put simply: your morning cortisol responds to how your life is going, and it can go up or down.

Measuring cortisol is harder than it sounds. Cortisol in blood, saliva and urine varies a lot from day to day, which is why studies using those samples contradict each other, while scalp hair reflects cortisol over a longer period.⁠³ Scalp hair has its quirks too: in 760 Dutch adults without a psychiatric condition, age, diabetes, how often people washed their hair and the season all shifted the hair cortisol value.⁠⁵ Hair dye and the contraceptive pill shift that value as well.⁠⁴ So a single cortisol value says little on its own. That value only means something once you know how and when the sample was taken.

What if your cortisol is too high?

Genuinely too much cortisol has a name: Cushing’s syndrome, in which the adrenal cortex makes too much. The syndrome is rare. The Netherlands has about 150 to 200 patients, a GP will probably see at most 1 patient with it in an entire career, and the syndrome is three times more common in women than in men.⁠¹ Nine in ten patients are often intensely tired and almost two in three report severe muscle and joint complaints. Typical signs are a full, round face, thin, fragile skin, purple-red striae and bruising easily, and early on the complaints are often wrongly seen as psychological.⁠¹ What this means for you: tiredness is common and Cushing’s syndrome is rare. The signs belong together, and a doctor assesses the whole picture.

Higher cortisol is also seen around depression. In a Dutch study of 1,588 people, both people with current depression and people whose depression had passed showed a larger rise in cortisol in the first hour after waking than the comparison group.⁠⁶ Mood and morning cortisol rise together. A study like this cannot show whether mood drives cortisol or the other way round.

Over the longer term, the Lifelines study in the north of the Netherlands measured cortisol and cortisone, a substance closely related to cortisol, in the scalp hair of 6,341 adults and followed those adults for 5 to 7 years. People with more cortisone in their scalp hair clearly developed heart and vascular disease more often, most clearly under the age of 60; for cortisol itself, the link remained a weak trend.⁠⁷ In plain words: in a Dutch population, high stress hormones over a long period went together with diseases of the heart and blood vessels, especially in people under 60.

What is a cortisol belly?

“Cortisol belly” is an internet term that the studies below do not use. What those studies do describe: in Cushing’s syndrome, fat is stored mainly on the upper half of the body, with thin arms and legs.⁠¹ And in a Dutch comparison of 175 adults, people with obesity had more than three times as much cortisol in their scalp hair as people who were overweight or of normal weight, while overweight and normal weight showed no clear difference.⁠³ So the link between cortisol and body fat only appears with obesity. This comparison cannot tell whether the cortisol came first or the body fat.

What lowers cortisol?

None of the studies mentioned here tested a product, a supplement or a habit that lowers cortisol. What those studies do show is that hair cortisol follows stress over time: while the stress was still going on, the level was more than 40 per cent higher than in comparison groups, and once the stress was over, the level was about a tenth lower.⁠⁴ What this means for you: the strongest signal in the data is the stress itself, and your scalp hair follows along when a stressful period ends.

What the hormone scan can show

The hormone scan asks questions about how you feel and gives you a score as a baseline. The scan does not measure cortisol in scalp hair, saliva or blood. The two would not tell the same story either: in the review of hair studies, how stressed people felt according to their questionnaire answers showed no consistent link with their hair cortisol.⁠⁴ A questionnaire and a hormone value measure two different things. The scan is not a diagnosis. Repeat the scan after two weeks and you have two numbers instead of a feeling. If your questions are about hormonal changes around midlife, the article on menopause symptoms explains which symptoms belong to the menopause and how long those symptoms last.

When to see a doctor

If you recognise several signs of Cushing’s syndrome at once (a full, round face, fat mainly on the upper body with thin arms and legs, purplish-red stretch marks, bruising easily), see your GP.⁠¹ Too little cortisol can also be urgent. In Addison’s disease, the adrenal glands make too little cortisol, and diarrhoea and vomiting with low blood pressure and a tendency to faint are warning signs of an Addisonian crisis, an acutely life-threatening situation: go straight to A&E at the hospital.⁠¹

In summary

Cortisol is a hormone from the adrenal glands that rises after you wake up and follows the stress in your life. Genuinely too much, as in Cushing’s syndrome, is rare and shows up as a set of symptoms that occur together. Higher cortisol goes hand in hand with depression, obesity and, over many years, diseases of the heart and blood vessels. A single value says little on its own. The hormone scan gives you a baseline for how you feel, and a doctor looks at the symptoms.