Cushing syndrome is caused by prolonged exposure to high levels of cortisol, the body’s primary stress hormone. The most common cause, by far, is taking corticosteroid medications like prednisone or dexamethasone for other health conditions. When the cause isn’t medication, it’s typically a tumor in the pituitary gland, adrenal gland, or elsewhere in the body that disrupts normal cortisol regulation. The condition is rare, affecting roughly 2 to 5 people per million each year, with women diagnosed about four times more often than men, usually around age 44.
Corticosteroid Medications: The Most Common Cause
The majority of Cushing syndrome cases are “exogenous,” meaning the excess cortisol comes from outside the body in the form of prescribed medications. Synthetic glucocorticoids like prednisone, prednisolone, and dexamethasone mimic cortisol’s effects and are widely used to treat conditions like asthma, rheumatoid arthritis, lupus, and inflammatory bowel disease. These medications come in many forms: pills, intravenous infusions, joint injections, inhalers, skin creams, enemas, and eye drops.
What surprises many people is that even topical steroids, the creams you apply to your skin, can cause Cushing syndrome if enough of the drug is absorbed into the bloodstream. Nearly 40% of patients using topical glucocorticoids show signs that the medication has suppressed their body’s normal cortisol regulation. The risk depends on the potency of the cream, how much skin it covers, where it’s applied (eyelids and groin absorb far more than the soles of your feet), whether the skin is broken, and how long you use it. Infants are especially vulnerable. Cases have been reported in babies treated with high-potency creams for diaper rash. Even some nonprescription skin-bleaching creams contain glucocorticoids and have triggered the condition.
Relatively low amounts of high-potency topical steroids, used for as little as two weeks, have been linked to adrenal suppression. And stopping corticosteroids suddenly after using them for more than about two weeks can trigger a life-threatening adrenal crisis, because the body’s own cortisol production has shut down in response to the external supply.
How Your Body Normally Controls Cortisol
To understand why tumors cause Cushing syndrome, it helps to know how cortisol is normally regulated. Your brain and adrenal glands communicate through what’s called the HPA axis. The hypothalamus (a region deep in the brain) sends a signal to the pituitary gland (a pea-sized gland at the base of the brain), which then releases a hormone called ACTH. ACTH travels through the bloodstream to the adrenal glands, which sit on top of the kidneys, telling them to produce cortisol.
When cortisol levels rise high enough, the hypothalamus and pituitary detect this and dial back their signaling. It’s a built-in braking system. Cushing syndrome develops when something breaks this feedback loop, allowing cortisol to climb unchecked.
Pituitary Tumors: Cushing’s Disease
When Cushing syndrome is caused by a tumor in the pituitary gland, it’s specifically called Cushing’s disease (with the possessive). This is the most common cause of endogenous Cushing syndrome, meaning cases where the body itself is overproducing cortisol rather than getting it from medications.
These pituitary tumors are almost always benign (noncancerous) and small, often just a few millimeters across. They pump out ACTH on their own, ignoring the brain’s signals to stop. The tumors still have some ability to sense cortisol levels, but their sensitivity is blunted compared to normal pituitary cells. Several molecular glitches explain why. The tumor cells convert active cortisol into an inactive form at higher rates than normal, essentially making themselves partially blind to cortisol. In about 55% of these tumors, a protein that normally helps cells respond to cortisol’s “stop” signal is missing entirely. The result is a pituitary gland that keeps ordering cortisol production even when levels are already dangerously high.
Ectopic ACTH: Tumors Outside the Pituitary
About 12% of ACTH-dependent Cushing syndrome cases come from tumors located outside the pituitary that independently produce ACTH. These tumors are even more resistant to cortisol feedback than pituitary adenomas, so ACTH levels tend to be markedly elevated.
The most common culprits, by a wide margin, are lung tumors. Pulmonary neuroendocrine tumors account for roughly 48% of ectopic ACTH cases, and small cell lung cancer adds another 5%. Thymic tumors (from the thymus gland in the chest) contribute 6% to 10% more. In total, most of these ACTH-producing tumors sit somewhere in the chest. Beyond the lungs, the breakdown looks like this:
- Pancreatic neuroendocrine tumors: about 13% of cases
- Tumors of unknown origin (found as lymph node metastases): about 12%
- Pheochromocytoma (adrenal gland tumors): about 4%
- Medullary thyroid cancer: about 3%
- Gastrinoma (stomach-acid-producing tumors): about 3%
Because the source tumor can be small and slow-growing, it sometimes takes months or years to locate where the ACTH is coming from.
Adrenal Gland Tumors
Sometimes the adrenal glands themselves are the problem. A benign adrenal adenoma can autonomously churn out cortisol without waiting for ACTH signals from the pituitary. When this happens, the excess cortisol actually suppresses the pituitary through normal feedback, so ACTH levels drop and the healthy adrenal tissue on both sides gradually shrinks from disuse.
Adrenal carcinomas (cancerous adrenal tumors) can also cause Cushing syndrome, but they behave differently. These aggressive tumors often lose normal steroid-producing machinery as they become less differentiated, leading to a buildup of cortisol precursors rather than a clean excess of cortisol alone. This biochemical signature, particularly elevated levels of certain precursor hormones, can actually help distinguish a malignant adrenal tumor from a benign one.
Genetic and Hereditary Causes
A small number of Cushing syndrome cases trace back to inherited genetic mutations. Two conditions stand out. Carney complex is an autosomal dominant syndrome, meaning a child needs only one copy of the mutated gene from one parent to be affected. It’s caused by mutations in the PRKAR1A gene, found in roughly 40% to 50% of affected families. This gene normally helps regulate cell growth in the adrenal glands, and when it’s disabled, small pigmented nodules develop across both adrenal glands and overproduce cortisol.
McCune-Albright syndrome takes a different path. It involves a mutation that permanently switches on a signaling pathway inside adrenal cells, causing them to produce cortisol independently. Unlike Carney complex, this mutation arises spontaneously during early development rather than being passed down from a parent.
Conditions That Mimic Cushing Syndrome
Some people develop elevated cortisol levels and physical features resembling Cushing syndrome without actually having a tumor or taking steroids. This is called pseudo-Cushing syndrome, and it’s caused by chronic overactivation of the stress hormone system. Conditions linked to it include severe depression, eating disorders, chronic alcoholism, extreme physical stress, obesity, and insulin resistance.
Pseudo-Cushing’s can produce cortisol levels high enough to cause real confusion during diagnostic testing. The distinction matters because the treatment is entirely different: addressing the underlying condition (treating depression, stopping heavy drinking, managing metabolic health) typically brings cortisol back to normal without the surgical or medical interventions that true Cushing syndrome requires.
How Cushing Syndrome Is Confirmed
Because cortisol levels fluctuate naturally throughout the day, diagnosis relies on tests that capture abnormal patterns. One of the most practical is the late-night salivary cortisol test. Cortisol normally drops to its lowest point late in the evening, so an elevated reading at that time is a red flag. Recent research has identified optimal cutoff values: a salivary cortisol level above 2.25 nmol/L at night catches about 92% of true cases while correctly ruling out 89% of people who don’t have the condition. Measuring cortisone in saliva (a breakdown product of cortisol) performs even better, with 94% accuracy in both directions.
Once excess cortisol is confirmed, additional testing determines whether it’s ACTH-dependent (coming from a pituitary or ectopic tumor) or ACTH-independent (coming from the adrenal glands). This distinction drives every treatment decision that follows, from imaging scans to surgery planning.