Cortisol Test Explained

Learn what cortisol measures, why collection time matters, and how high or low results are interpreted with ACTH testing.

Cortisol is a time-sensitive hormone test, so the same number can mean different things in the morning, afternoon, during illness, or after steroid medicine use.

What This Test Measures

Cortisol is a hormone made by the adrenal glands under control of ACTH from the pituitary. It helps the body respond to stress, maintain blood pressure, influence glucose metabolism, and coordinate daily energy patterns. Blood cortisol is commonly reported in micrograms/dL or nmol/L, with micrograms/dL x 27.6 approximately equal to nmol/L.

The key feature is rhythm. Cortisol usually peaks around waking, especially near 8 a.m., then falls through the afternoon and reaches its lowest point toward evening and midnight. Because of this rhythm, a cortisol result without collection time is hard to interpret.

Single cortisol values rarely answer the full endocrine question. Doctors often use dynamic tests when they suspect cortisol excess or cortisol deficiency.

Normal Range

Use the range printed on your own lab report. A commonly cited morning range is about 6-23 micrograms/dL, but ranges differ by assay and laboratory. Afternoon values are expected to be much lower than morning values, and some evaluations use a second draw around 16:00.

The facts around variation matter: about 17% of people may not show a clear daily rhythm, and about 30% show day-to-day variation. Healthy adults secrete roughly 8-30 mg of cortisol per day. None of these numbers replace your lab's reference range or a clinician's interpretation of the collection time.

What A High Result May Mean

A high cortisol may be reversible or physiologic. It can rise with acute stress, severe illness, pain, morning collection, pregnancy, estrogen use, oral contraceptives, and external glucocorticoid medication. Long-term high-dose glucocorticoid use is the most common cause of Cushing syndrome patterns in clinical materials.

Pathologic cortisol excess can come from Cushing disease due to a pituitary ACTH adenoma, autonomous adrenal cortisol secretion from an adrenal adenoma or cancer, or ectopic ACTH production from some tumors. Symptoms that may prompt evaluation include central weight gain, muscle loss, high blood pressure, impaired glucose tolerance, and purple stretch marks.

A single high result does not diagnose Cushing syndrome. Doctors may use a 1 mg overnight dexamethasone suppression test, where next-morning cortisol <=1.8 micrograms/dL, or 50 nmol/L, is described as normal suppression; lack of suppression can prompt further evaluation. They may also use 24-hour urine free cortisol or late-night salivary cortisol.

What A Low Result May Mean

Low cortisol can reflect timing if the sample was collected later in the day. It can also suggest adrenal insufficiency. Primary adrenal insufficiency, also called Addison disease, involves adrenal gland damage and tends to have high ACTH. Causes listed in endocrine materials include autoimmune disease and infections such as tuberculosis or HIV.

Secondary adrenal insufficiency comes from low or inappropriately normal ACTH due to pituitary or hypothalamic problems. Long-term external glucocorticoid use followed by dose reduction or stopping can suppress the HPA axis and lead to low cortisol. Symptoms may include fatigue, low blood pressure, low sodium, and skin darkening in primary disease.

Related Lab Tests To Check Together

ACTH is central because it helps separate adrenal gland causes from pituitary or hypothalamic causes. For possible cortisol excess, doctors may use 24-hour urine free cortisol, late-night salivary cortisol, and dexamethasone suppression testing. For possible cortisol deficiency, ACTH stimulation testing may be used.

Sodium, potassium, glucose, and sometimes aldosterone help complete the adrenal picture. Medication history is also part of the workup, especially steroid pills, injections, inhalers, creams used over large areas, estrogen, and oral contraceptives.

Single Result vs Long-Term Trend

Cortisol is less about a long trend line than about the right test at the right time. A morning baseline, an afternoon value, a late-night saliva sample, a 24-hour urine collection, and a post-dexamethasone result answer different questions.

If results are being followed over time, keep the collection time, test type, medication context, illness status, and laboratory method attached to each value. Without those details, comparing cortisol numbers can be misleading.

When To Talk With A Doctor

Talk with a doctor if cortisol is outside your lab's range, if the sample timing was wrong or unclear, if you use or recently stopped glucocorticoid medication, or if symptoms fit cortisol excess or deficiency. Do not stop prescribed steroid medication suddenly without medical guidance.

Seek urgent medical care for severe weakness, fainting, very low blood pressure, confusion, severe vomiting, or symptoms your clinician has warned could signal adrenal crisis. Routine abnormal cortisol results should be reviewed with the ordering clinician or an endocrinologist.

Frequently Asked Questions

Why does cortisol collection time matter?

Cortisol has a strong daily rhythm. It is usually highest around 8 a.m. and much lower later in the day, so the report must be interpreted with the collection time.

What is a common morning cortisol range?

Use your own lab range. A common morning range is about 6-23 micrograms/dL, but methods vary.

Can stress raise cortisol?

Yes. Acute stress, severe illness, pain, and morning collection can raise cortisol temporarily.

Does one high cortisol result diagnose Cushing syndrome?

No. Evaluation often uses dynamic tests such as overnight dexamethasone suppression, 24-hour urine free cortisol, or late-night salivary cortisol.

What does low cortisol suggest?

Low cortisol may suggest adrenal insufficiency, pituitary ACTH deficiency, or suppression after long-term glucocorticoid use, but timing and confirmatory testing matter.

What is the dexamethasone suppression cutoff listed in clinical materials?

For the 1 mg overnight dexamethasone suppression test, next-morning cortisol <=1.8 micrograms/dL, or 50 nmol/L, is described as normal suppression.

What tests are checked with cortisol?

ACTH, 24-hour urine free cortisol, late-night salivary cortisol, dexamethasone suppression, ACTH stimulation, sodium, potassium, glucose, and sometimes aldosterone may be used.

Can oral contraceptives affect cortisol?

Pregnancy and estrogen or oral contraceptive use can raise cortisol-binding globulin and increase total cortisol, so clinicians consider medication context.

How MediLens Helps Track This Over Time

MediLens can store cortisol results with the sample time, units, lab range, and test type. That is essential for cortisol because a morning blood result and a late-night salivary result should not be compared as if they were the same test.

You can add notes about steroid medication, oral contraceptives, illness, stress, sleep schedule, and dynamic testing. MediLens helps keep these details near the numbers so your doctor can review a cleaner timeline.

Key Takeaways

Cortisol has a strong daily rhythm, so collection time is part of the result.
A common morning range is about 6-23 micrograms/dL, but your lab report range comes first.
Stress, illness, pregnancy, estrogen use, oral contraceptives, and steroid medicines can affect total cortisol.
One cortisol value usually cannot diagnose Cushing syndrome or adrenal insufficiency.
ACTH and dynamic testing often matter more than a single standalone number.

This article is for general education, based on Endocrine Society guidance and public endocrine testing materials. It is not a diagnosis or treatment advice and does not replace your doctor. Interpret results using the reference ranges on your own lab report and your physician's guidance.

A single lab result only tells part of the story. MediLens helps you scan lab reports, organize your results, compare changes over time, and better understand your long-term health trends.