How to prepare for the 'Cortisol' test: rules for taking it

Cortisol is the main glucocorticoid hormone that regulates glucose metabolism, the stress response, inflammation and blood pressure. For athletes it is often associated with overtraining and «catabolism». But cortisol is one of the most difficult tests to take correctly: its level changes several-fold over the course of a day, and it reacts even to anxiety before the needle. The editorial team explains how to prepare so that the result is informative.

The daily rhythm: why the time of the draw is decisive

Cortisol secretion follows a circadian rhythm. The highest level is observed in the morning: after waking it rises further over 30–45 minutes — this is the so-called cortisol awakening response. Then the concentration gradually declines and reaches a minimum around midnight.

Therefore the same level can be normal in the morning and pathologically high in the evening. For a standard blood test, cortisol is usually determined between 7 and 9 o'clock in the morning, preferably no later than 1–2 hours after waking.

People who work night shifts or have a shifted sleep schedule should discuss with their doctor exactly when to take the test: the cortisol rhythm is tied to the sleep-wake cycle, not to the clock.

After a flight with a change of time zones, the rhythm readjusts over several days. A routine test is better not taken immediately after travel; instead, wait until the sleep schedule stabilizes.

Stress, sleep and training

Cortisol is a stress hormone, and it responds even to minor stimuli. Fear of the needle, rushing, being late, an argument in the queue can raise the level. Therefore, before the draw it is advisable to sit quietly for 15–20 minutes, and in some cases doctors recommend placing a venous catheter in advance.

Sleep deprivation and poor sleep quality alter the daily profile and can raise evening values. The day before the test it is important to go to bed at your usual time and get enough sleep.

Physical exertion raises cortisol, and the strength of the response depends on the intensity. Studies show that moderate and intense exertion, roughly from 60% of maximal oxygen uptake, noticeably increases its level, while light activity has no significant effect. A workout the day before the test is better made light, and on the day of the test — do not train before the draw.

Prolonged fasting, a strict caloric deficit and low-carbohydrate diets can also change the cortisol level. For athletes who are cutting this is important: values against a background of an aggressive deficit will differ from values in the usual regimen.

Medications and hormones that distort the test

Glucocorticoid drugs have the strongest effect on the test. Prednisolone, hydrocortisone and some others have a similar structure and can cross-react with immunoassays, inflating the result. Dexamethasone, on the contrary, suppresses the body's own cortisol secretion. It is important to remember that even inhaled, nasal, topical and intra-articular forms can affect the results.

Oral estrogens, in particular combined contraceptives and hormone replacement therapy, raise the level of cortisol-binding globulin. Because of this, total cortisol in the blood rises, although the active free fraction may remain normal. The same is observed during pregnancy.

High-dose biotin can interfere with some immunoassays, so its use is usually paused 2–3 days before the test. Licorice in significant amounts affects cortisol metabolism in the tissues and can distort the results of specific tests.

Prescribed drugs must not be discontinued on your own: withdrawal of glucocorticoids after prolonged use is dangerous. It is enough to inform the doctor about all medications, including creams and inhalers.

Blood, saliva or urine: which test to choose

Morning cortisol in the blood is the most common test. It is useful for an initial assessment, especially when the doctor suspects adrenal insufficiency: a very low morning level is grounds for further examination, for example a stimulation test.

For diagnosing an excess of cortisol (Cushing's syndrome), the clinical guidelines of the Endocrine Society recommend other tests: late-evening salivary cortisol, 24-hour urinary free cortisol excretion, or an overnight 1 mg dexamethasone test. A single morning blood cortisol is of little use for this.

• Saliva:collected late in the evening, no earlier than 30 minutes after eating, drinking or brushing teeth; you must not smoke, and bleeding gums distort the result.
• 24-hour urine:it is important to collect all urine over 24 hours, not missing a single portion, and not to drink excessively large amounts of fluid.
• Dexamethasone test:carried out only as prescribed by a doctor, with precise observance of the timing of the dose and the draw.

The so-called «overtraining test» based on a single morning cortisol has no scientific basis. Cortisol in athletes depends on many factors, and to assess the condition repeated measurements under standard conditions and the clinical picture are needed.

Regardless of the type of test, a common rule applies: the conditions of each collection must be identical. If the doctor has ordered several saliva samples on different evenings, collect them at the same time, on ordinary days without competitions and late workouts, so that the variability reflects the body and not the circumstances.

Checklist and interpretation

A short list of rules for morning cortisol in the blood:

1. draw blood between 7 and 9 o'clock, at your usual time after waking;
2. the day before — usual sleep, no alcohol, no intense training;
3. in the morning — fasting or as instructed by the laboratory, no smoking and no coffee;
4. before the draw — sit quietly for 15–20 minutes;
5. inform the doctor about all glucocorticoids, contraceptives, hormones and biotin.

Reference intervals depend on the method and the time of day, so the result should be compared with the interval of the specific laboratory. Modern methods based on mass spectrometry and new immunoassays may give lower values than older methods.

A single result that slightly exceeds the norm is rarely a diagnosis. Hypercortisolism and adrenal insufficiency are confirmed only by a combination of tests, and they should be interpreted by an endocrinologist.

If you are being examined because of fatigue, poor recovery or declining results, it is worth assessing, along with cortisol, a complete blood count, ferritin, thyroid function and energy balance.

Editorial conclusions

Cortisol has a pronounced daily rhythm, so for a blood test the time of the draw is critically important — usually 7–9 o'clock in the morning.

Stress, sleep deprivation, intense training, glucocorticoids in any form and oral estrogens can substantially change the result.

For diagnosing an excess of cortisol, saliva, 24-hour urine or a dexamethasone test are used, not a single blood test.

We also recommend reading our materials on preparing for the fasting glucose test, on ferritin, and on creatine phosphokinase.

References

1. Nieman LK, Biller BMK, Findling JW, et al. The diagnosis of Cushing's syndrome: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2008;93(5):1526–1540.
2. Bornstein SR, Allolio B, Arlt W, et al. Diagnosis and treatment of primary adrenal insufficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2016;101(2):364–389.
3. Hill EE, Zack E, Battaglini C, et al. Exercise and circulating cortisol levels: the intensity threshold effect. J Endocrinol Invest. 2008;31(7):587–591.
4. Fleseriu M, Auchus R, Bancos I, et al. Consensus on diagnosis and management of Cushing's disease: a guideline update. Lancet Diabetes Endocrinol. 2021;9(12):847–875.
5. Melmed S, Auchus RJ, Goldfine AB, et al (eds). Williams Textbook of Endocrinology. 14th ed. Elsevier; 2020.