ACTH Blood Test Explained
An ACTH blood test measures adrenocorticotropic hormone, a pituitary hormone that tells the adrenal glands to make cortisol. Because ACTH and cortisol work as a pair, the test is almost always interpreted together with a cortisol level.
What the test measures
Adrenocorticotropic hormone (ACTH) is released by the pituitary gland at the base of the brain. It signals the adrenal glands to produce cortisol, a hormone involved in stress, metabolism, and many daily functions. Cortisol in turn feeds back to the pituitary, forming a self-regulating loop.
Like cortisol, ACTH follows a daily rhythm, typically highest in the morning and lower at night. Because ACTH and cortisol are linked, measuring both helps clinicians tell whether a problem lies in the pituitary, the adrenal glands, or elsewhere.
Why a clinician might order it
A clinician may order ACTH when cortisol is too high or too low, to help locate the source of the imbalance. It is used when evaluating conditions of cortisol excess or deficiency and to distinguish problems originating in the pituitary from those in the adrenal glands. ACTH is often part of a broader workup that includes cortisol and sometimes dynamic stimulation or suppression tests.
How to prepare
Because ACTH follows a daily rhythm, the draw is usually timed, frequently early in the morning. The sample often needs special handling because ACTH is fragile. Stress, illness, sleep patterns, and several medications can affect levels, so it helps to follow timing instructions and share relevant details with the testing team.
How results are generally interpreted
Interpretation depends on the matching cortisol level and the timing of the draw.
- High ACTH with low cortisol may suggest the adrenal glands are not responding normally.
- Low ACTH with low cortisol may point toward reduced pituitary signaling.
- ACTH with high cortisol helps distinguish whether cortisol excess is being driven by ACTH or is occurring independently.
Illustrative reference ranges
The values below are illustrative only and vary by laboratory, assay, and time of day. Always use the range and timing notes printed on your own report.
| Time of day | Illustrative ACTH (pg/mL) |
|---|---|
| Morning | ~7-50 |
| Evening | typically lower than morning |
| Interpretation | read together with cortisol |
Related tests
ACTH is interpreted alongside cortisol and is sometimes evaluated with other adrenal hormones such as those covered with the aldosterone test. See the blood tests index or explore related conditions and hormones.
Frequently asked questions
Why is cortisol measured with ACTH?
ACTH and cortisol form a feedback loop, so reading them together helps show whether a problem is in the pituitary or the adrenal glands.
Why is the draw usually in the morning?
ACTH follows a daily rhythm that is typically highest in the morning, so a timed draw makes the result easier to interpret.
Does the sample need special handling?
Often yes. ACTH is fragile, so laboratories may use specific collection and processing steps to keep the sample reliable.
Can ACTH be interpreted on its own?
Generally no. The pattern of ACTH together with cortisol is what points toward the source of a problem, not ACTH alone.
Sources
- MedlinePlus. Cortisol Test. https://medlineplus.gov/lab-tests/cortisol-test/
- MedlinePlus. Endocrine Diseases. https://medlineplus.gov/endocrinediseases.html
- Endocrine Society. https://www.endocrine.org/