ACTH (adrenocorticotropic hormone) is a hormone produced by the pituitary gland that tells your adrenal glands to release cortisol, your body’s primary stress hormone. It’s the middle link in a three-step chain that connects your brain to your stress response, and it plays a central role in how your body handles everything from waking up in the morning to surviving a crisis.
How ACTH Triggers Cortisol Production
The process starts in your hypothalamus, a small region at the base of your brain that monitors threats and stress. When it detects a need for cortisol, it releases a signaling molecule called CRH into the pituitary gland. CRH prompts specialized pituitary cells to release ACTH into the bloodstream.
ACTH then travels to the adrenal glands, which sit on top of your kidneys. There, it locks onto highly specific receptors on the outer layer of the adrenal gland (the zona fasciculata). Once bound, it kicks off a chain of events inside the cell: cholesterol gets converted into a precursor molecule, which then gets processed through several steps until it becomes cortisol. ACTH controls the very first, rate-limiting step in this conversion, essentially acting as the on switch for cortisol production. It also increases the cell’s uptake of cholesterol, the raw material cortisol is made from, so the adrenals have more fuel to work with.
The result is a rapid rise in cortisol. Cortisol then raises blood sugar, suppresses inflammation, and shifts your body’s resources toward dealing with whatever triggered the response in the first place.
The Built-In Off Switch
Your body doesn’t want cortisol flowing indefinitely. Rising cortisol levels feed back to both the hypothalamus and the pituitary, signaling them to stop releasing CRH and ACTH. This negative feedback loop is one of the most important regulatory systems in the body. After an acute stressor, ACTH release shuts off quickly, followed by a more gradual decline in cortisol back to baseline.
Part of this fast shutdown involves an interesting mechanism: cortisol binds to receptors in the hypothalamus that trigger the release of endocannabinoids, your body’s own cannabis-like molecules. These endocannabinoids block the excitatory signals that drive CRH neurons, effectively silencing the alarm.
ACTH Follows a Daily Rhythm
Even without stress, ACTH levels rise and fall on a predictable 24-hour cycle. Levels peak in the early morning hours, typically between about 6 a.m. and 8 a.m., which is why cortisol is also highest when you wake up. This morning surge helps mobilize energy for the day ahead. ACTH then drops throughout the afternoon and reaches its lowest point around midnight.
This rhythm is why blood tests for ACTH are drawn in the morning, usually between 7 a.m. and 10 a.m. The normal range for a morning draw is roughly 7.2 to 63 pg/mL, though labs may use slightly different cutoffs. A value taken in the afternoon can’t be meaningfully compared to morning reference ranges.
What Happens When ACTH Is Too High
Chronically elevated ACTH drives excess cortisol production, leading to a condition called Cushing’s syndrome. The most common cause of ACTH-driven Cushing’s is a small, benign tumor on the pituitary gland that pumps out ACTH without responding normally to feedback signals. This specific scenario is called Cushing’s disease.
Less commonly, tumors elsewhere in the body can produce ACTH on their own. This is called ectopic ACTH syndrome, and the lung is the most frequent site, accounting for over 25% of cases. Pancreatic tumors, thyroid cancers, and tumors in the thymus gland can also be responsible. In up to 20% of these cases, the source of the excess ACTH is never found despite extensive testing.
Too much ACTH also causes a distinctive symptom that has nothing to do with cortisol: skin darkening. ACTH is structurally similar to a pigment-stimulating hormone called alpha-MSH, and it binds to the same receptor on skin cells with equal strength. When ACTH is very high, it activates these pigment receptors, increasing melanin production. This hyperpigmentation tends to appear in skin creases, gums, and areas exposed to friction. It’s a clinical clue that the excess cortisol is being driven by ACTH rather than by the adrenal glands acting on their own.
What Happens When ACTH Is Too Low
When the pituitary gland can’t produce enough ACTH, the adrenal glands don’t get the signal to make cortisol. This is called secondary adrenal insufficiency, and it can be serious. Without adequate cortisol, you may experience persistent fatigue, weakness, weight loss, low blood pressure, nausea, and dizziness, especially during illness or physical stress when cortisol demand is highest.
Several things can suppress ACTH production. The most common, by far, is long-term use of corticosteroid medications like prednisone. These drugs mimic cortisol, so the pituitary gradually dials down its own ACTH output. Over time, the adrenal glands shrink from disuse. If the medication is stopped abruptly, the pituitary can’t ramp up ACTH production fast enough, leaving you dangerously low on cortisol. This is why corticosteroids are always tapered slowly rather than stopped all at once. Even non-oral forms like inhaled, topical, or injected corticosteroids can suppress the system, particularly at high doses or with prolonged use.
Other causes of low ACTH include pituitary tumors, autoimmune disease affecting the pituitary, pituitary surgery, bleeding into the pituitary gland, and traumatic brain injury. Because the pituitary produces many hormones, ACTH deficiency from pituitary damage often comes alongside deficiencies in other hormones as well.
How ACTH Levels Are Tested
A simple blood draw measures ACTH directly, but the more informative test is the ACTH stimulation test. In this test, a synthetic version of ACTH is injected, and your cortisol levels are measured 30 and 60 minutes later. If your adrenal glands are healthy and responsive, cortisol should rise to at least 18 µg/dL (500 nmol/L) by the 60-minute mark. A lower response suggests the adrenals aren’t functioning properly, either because they’ve been damaged directly or because they’ve atrophied from chronic ACTH deprivation.
For people tapering off corticosteroid therapy, the initial screening test is simpler: a morning blood cortisol level. A value above 10 µg/dL (300 nmol/L) generally indicates the system has recovered enough to stop replacement therapy. Values below 5 µg/dL (150 nmol/L) mean the system is still suppressed, and the physiologic replacement dose should continue with retesting in a few months.
ACTH Beyond Cortisol
While cortisol is ACTH’s primary target, ACTH also stimulates the adrenal glands to produce other steroid hormones, including aldosterone (which regulates salt and water balance) and adrenal androgens (which contribute to body hair and, in women, are a meaningful source of certain sex hormones). This is why disruptions in ACTH can have effects that extend well beyond the classic stress response, affecting blood pressure regulation, electrolyte balance, and even aspects of sexual development and function.