Cortisol: What the Test Shows and Why It Matters for Athletes
Cortisol has a bad reputation in fitness circles, where it is often described simply as the "catabolic stress hormone" that eats muscle. In reality, cortisol is essential for life and for exercise itself. Our editorial team explains what a cortisol test can tell you, how the different tests work, and why a single number rarely answers questions about training or recovery.
What cortisol does in the body
Cortisol is a glucocorticoid hormone produced by the adrenal cortex under the control of the hypothalamic-pituitary-adrenal (HPA) axis. The hypothalamus releases corticotropin-releasing hormone, the pituitary responds with ACTH, and ACTH stimulates the adrenals to secrete cortisol. Rising cortisol then feeds back to dampen the signal.
Its roles are wide-ranging. Cortisol mobilizes glucose and fatty acids for energy, maintains blood pressure, modulates inflammation and immune responses, and influences mood and alertness. Without it, a person could not cope with even moderate physical stress, which is why adrenal insufficiency can become life-threatening.
During exercise, cortisol rises in proportion to intensity and duration. Research by Hill and colleagues showed that moderate to high intensity, roughly above 60% of maximal oxygen uptake, reliably increases circulating cortisol, while light activity may even lower it. This acute rise helps supply fuel and is a normal part of training.
Problems arise when cortisol is chronically elevated or chronically low. Persistent excess, as in Cushing's syndrome, causes muscle wasting, central fat gain, high blood pressure, and bone loss. Insufficient cortisol leads to fatigue, weight loss, low blood pressure, and poor tolerance of illness or stress.
Types of cortisol tests
Several different tests measure cortisol, and each answers a different question. Choosing the right one depends on what the physician suspects.
| Test | What it measures | Typical use |
|---|---|---|
| Morning serum cortisol | Total cortisol in blood, usually 7–9 a.m. | Screening for adrenal insufficiency |
| Late-night salivary cortisol | Free cortisol at its expected low point | Screening for Cushing's syndrome |
| 24-hour urinary free cortisol | Total free cortisol excreted in a day | Screening for cortisol excess |
| Dexamethasone suppression test | Whether cortisol falls after a synthetic steroid | Confirming or excluding Cushing's |
| ACTH stimulation test | Adrenal response to ACTH | Confirming adrenal insufficiency |
Blood tests measure total cortisol, most of which is bound to proteins such as cortisol-binding globulin. Conditions or medications that change these proteins, including estrogen-containing contraceptives, can raise total cortisol without changing the active free fraction.
Saliva tests reflect free cortisol and are convenient for collecting samples at specific times. They are widely used in research and in some clinical protocols, but direct-to-consumer saliva panels marketed for "adrenal fatigue" are not validated for that purpose.
Timing, rhythm, and interpretation
Cortisol follows a strong daily rhythm. It peaks shortly after waking, a phenomenon known as the cortisol awakening response, then gradually declines through the day to its lowest point around midnight. A level that is normal at 8 a.m. could be very abnormal at 11 p.m., and vice versa.
Many factors disturb this pattern: night shifts, jet lag, poor sleep, acute illness, pain, recent intense exercise, and even the stress of a blood draw. For this reason, labs and physicians standardize sampling times and conditions, and abnormal results usually need repeat testing.
Reference ranges differ by lab and assay, so results should always be compared with the range printed on the report. Values are commonly expressed in micrograms per deciliter (mcg/dL) in the United States and in nanomoles per liter elsewhere.
Cortisol and training load
Athletes sometimes order cortisol tests to check for overtraining. The scientific picture is more complicated than the popular idea that "high cortisol equals overtraining." Studies on overreaching have shown resting cortisol to be elevated in some cases, unchanged in others, and even blunted in severe, long-standing fatigue.
The joint consensus statement of the European College of Sport Science and the American College of Sports Medicine on overtraining syndrome concluded that no single hormonal marker can diagnose the condition. Diagnosis relies on excluding other causes and documenting a prolonged decline in performance despite rest.
The testosterone-to-cortisol ratio was once promoted as a training-stress index. Although it may reflect anabolic and catabolic balance in group studies, individual variability is large, and it has not proven reliable for day-to-day training decisions.
- Do not test cortisol right after a hard session; acute increases are normal.
- Track symptoms, sleep, and performance alongside any lab values.
- Low energy availability can alter cortisol and other hormones; nutrition should be assessed.
- Glucocorticoid medications, including inhaled and injected forms, strongly affect results.
When a cortisol test is medically important
The main clinical reasons for cortisol testing are suspected Cushing's syndrome and suspected adrenal insufficiency. Endocrine Society guidelines describe step-by-step approaches, starting with screening tests and moving to confirmation only when initial results are abnormal.
Athletes and bodybuilders who have used anabolic steroids or high-dose glucocorticoids represent a special group. Glucocorticoid use, including repeated joint injections, can suppress the HPA axis. When the drugs are stopped, the adrenals may not recover immediately, which can cause fatigue, low blood pressure, and in severe cases an adrenal crisis.
Warning signs that justify medical evaluation include unexplained weight gain with purple stretch marks and muscle weakness, or the opposite pattern of persistent exhaustion, dizziness on standing, salt craving, and weight loss. These are not issues to self-manage with supplements.
Conclusions of the editorial team
Cortisol is a vital hormone that rises appropriately with exercise; its acute increase is not something to fear.
Different tests answer different questions, and timing relative to the daily rhythm is critical for meaningful results.
No single cortisol value can diagnose overtraining; it is best interpreted alongside performance, symptoms, sleep, and nutrition.
We suggest continuing with our articles on heart rate variability as a recovery marker, on testosterone testing for athletes, and on relative energy deficiency in sport.
References
- Meeusen R, Duclos M, Foster C, et al. Prevention, diagnosis, and treatment of the overtraining syndrome: joint consensus statement of the European College of Sport Science and the American College of Sports Medicine. Med Sci Sports Exerc. 2013;45(1):186–205.
- 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.
- Nieman LK, Biller BM, 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.
- 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.
- Fry AC, Kraemer WJ. Resistance exercise overtraining and overreaching: neuroendocrine responses. Sports Med. 1997;23(2):106–129.
- Hackney AC. Stress and the neuroendocrine system: the role of exercise as a stressor and modifier of stress. Expert Rev Endocrinol Metab. 2006;1(6):783–792.
