Low ACTH is most commonly caused by long-term use of steroid medications, which suppress the pituitary gland’s ability to produce this hormone. Other causes include pituitary tumors, autoimmune inflammation, head trauma, and conditions affecting the hypothalamus. A normal morning ACTH level falls between 7.2 and 63 pg/mL, measured from a blood draw taken between 7 and 10 a.m.
ACTH (adrenocorticotropic hormone) is made by the pituitary gland, a pea-sized structure at the base of the brain. Its job is to signal the adrenal glands to produce cortisol, the hormone your body needs to manage stress, blood sugar, blood pressure, and inflammation. When ACTH drops too low, cortisol production falls with it, leading to a condition called secondary adrenal insufficiency.
Steroid Medications: The Most Common Cause
Taking corticosteroid medications for an extended period is by far the most frequent reason for low ACTH. When you take steroids like prednisone, dexamethasone, or even high-dose inhaled steroids, your body registers the incoming cortisol-like drug and tells the pituitary to stop making ACTH. Over time, the ACTH-producing cells essentially go dormant.
The risk depends on the dose, duration, and type of steroid. Doses above 20 to 30 mg of prednisone (or equivalent) taken for more than three weeks carry a meaningful risk of suppressing the pituitary-adrenal axis. Adrenal suppression is extremely rare in patients treated for one week or less, but it has been documented in courses as short as 14 days. Dexamethasone is the most suppressive because it stays active in tissues the longest, while hydrocortisone and cortisone are the least.
How you take steroids also matters. Splitting doses throughout the day or taking them in the evening creates more suppression than a single morning dose, because evening steroids blunt the natural early-morning surge of ACTH. Injected or oral steroids suppress the axis more than inhaled ones, though high-dose inhaled fluticasone has been linked to adrenal insufficiency even at relatively low total exposure. People who develop a Cushingoid appearance (round face, weight gain around the midsection) while on steroids are especially likely to have significant ACTH suppression.
Pituitary Gland Problems
The pituitary is the direct source of ACTH, so anything that damages or compresses it can reduce production. Pituitary tumors, both hormone-secreting and non-functioning, are one of the more common structural causes. Even benign tumors can press on the ACTH-producing cells (called corticotrophs) and impair their function. Surgical removal of a pituitary tumor can also leave ACTH levels low afterward, sometimes temporarily and sometimes permanently.
Autoimmune inflammation of the pituitary, known as lymphocytic hypophysitis, destroys pituitary tissue through chronic immune-mediated attack. ACTH deficiency is the earliest and most frequent hormonal loss in this condition, appearing in roughly 65% of cases. In some people, the damage stays limited to ACTH-producing cells. In others, it progresses to affect multiple pituitary hormones.
Other pituitary causes include loss of blood flow to the gland, infection, radiation therapy directed at or near the pituitary, and rare genetic conditions that affect how the pituitary develops.
Sheehan Syndrome
Sheehan syndrome is a specific form of pituitary damage that occurs after severe blood loss during or after childbirth. The pituitary gland enlarges during pregnancy and becomes vulnerable to drops in blood supply. If hemorrhage and shock are severe enough, pituitary cells die.
The resulting hormone losses vary. Some women lose all anterior pituitary function, while others lose only certain hormones. Early signs often include inability to breastfeed (from loss of prolactin) and failure to resume menstrual periods. Symptoms of low ACTH and cortisol, like fatigue, low blood pressure, and weight loss, can develop later. Blood tests may reveal low sodium, low blood sugar, and anemia, all clues pointing toward secondary adrenal insufficiency.
Hypothalamic Dysfunction
The hypothalamus sits just above the pituitary and controls it by releasing corticotropin-releasing hormone (CRH). CRH is the signal that tells the pituitary to make ACTH. When the hypothalamus doesn’t produce enough CRH, ACTH falls and cortisol drops in turn. This is sometimes called tertiary adrenal insufficiency.
Hypothalamic dysfunction can result from tumors, surgery in the area, radiation, or infiltrative diseases. It can also occur after long-term steroid use, because exogenous steroids suppress CRH production as well as ACTH. Traumatic brain injury is another recognized cause: physical trauma can damage the hypothalamus, the pituitary stalk connecting the two structures, or the pituitary itself.
Traumatic Brain Injury
Head injuries, even those classified as moderate, can damage the pituitary or its blood supply. The pituitary sits in a bony pocket at the skull base, and the forces involved in trauma can shear the delicate stalk connecting it to the hypothalamus or cause small bleeds within the gland. ACTH deficiency is one of several hormonal losses that can follow. Symptoms sometimes appear days after the injury but may not become apparent for weeks or months, which makes this cause easy to miss if no one checks for it.
How Low ACTH Feels Different From High ACTH
One detail worth knowing: low ACTH produces a different clinical picture than the adrenal insufficiency caused by direct adrenal gland damage (Addison’s disease). In Addison’s disease, the pituitary ramps up ACTH production trying to stimulate failing adrenal glands, and that excess ACTH causes skin darkening, especially in skin creases, gums, and scars. With low ACTH, that darkening doesn’t happen. Instead, skin may appear unusually pale.
The shared symptoms of both conditions reflect cortisol deficiency: fatigue, weakness, low blood pressure, nausea, weight loss, and difficulty handling physical stress. Low blood sugar episodes and low sodium levels are common lab findings. Because these symptoms overlap with many other conditions, low ACTH often goes undiagnosed for months or years unless a clinician specifically tests for it.
How Low ACTH Is Identified
Diagnosis starts with a morning blood draw for ACTH and cortisol, taken between 7 and 10 a.m. when both hormones are naturally at their peak. If both levels come back low, that pattern points toward a pituitary or hypothalamic problem rather than an adrenal one.
When results are borderline, a stimulation test may be needed. The most definitive is the insulin tolerance test, which works by inducing a brief drop in blood sugar (below 45 mg/dL). Under normal conditions, that stress should trigger a cortisol rise to at least 18 to 20 micrograms per deciliter. A blunted response confirms that the pituitary-adrenal axis isn’t functioning properly. This test requires medical supervision because of the intentional low blood sugar involved.
Once low ACTH is confirmed, imaging of the pituitary with MRI helps identify structural causes like tumors, signs of prior bleeding, or inflammation. A thorough medication history is equally important, since steroid use remains the single most common explanation.