ACTH is a hormone, a chemical messenger made by the pituitary gland at the base of the brain. It tells the adrenal glands above the kidneys to make cortisol. An ACTH blood test helps investigate why cortisol levels are abnormal.
Cortisol helps regulate blood pressure, blood sugar and the response to stress. Normally, low cortisol prompts the body to make more ACTH. Higher cortisol reduces that signal.
Comparing the two results helps doctors work out whether a problem may involve the adrenal glands or the signals controlling them. ACTH alone cannot establish the cause. Further tests may be needed.
ACTH changes during the day. Blood is often taken in the morning, when levels are higher. Use the laboratory range for that sample rather than a number from another test. Tell the clinician about your medicines and follow the preparation instructions. Do not stop prescribed medicines on your own.

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