Endogenous Cushing’s syndrome is caused by the body’s own overproduction of cortisol, most often driven by a small, benign tumor in the pituitary gland or, less commonly, in the adrenal glands or elsewhere. The condition is more common in women than men and affects nearly every organ system, producing a recognizable pattern of weight gain, muscle wasting, mood changes, and metabolic disruption. Because the symptoms overlap with many everyday health complaints, diagnosis is frequently delayed by years, and treatment depends entirely on identifying where the excess cortisol is coming from.
Where the Excess Cortisol Comes From
Doctors split endogenous Cushing’s syndrome into two broad categories based on what is driving the cortisol overload. About 80% of cases are ACTH-dependent, meaning a tumor somewhere in the body is pumping out too much adrenocorticotropic hormone (ACTH), which in turn forces the adrenal glands to churn out cortisol.1Medicine. Pituitary disorders Cushing’s syndrome The remaining roughly 20% are ACTH-independent: the adrenal glands themselves have gone rogue, producing cortisol on their own without being told to by ACTH.
Within the ACTH-dependent group, about four out of five patients have a pituitary adenoma, a usually tiny benign growth on the pituitary gland at the base of the brain. When the source is specifically the pituitary, the condition is called Cushing’s disease, a subset of the broader syndrome. The pituitary adenoma secretes ACTH, which drives both adrenal glands to enlarge and overproduce cortisol.2PubMed Central. Cushing’s syndrome: from physiological principles to diagnosis and clinical care Researchers have identified somatic mutations in a gene called USP8 as one molecular cause of these pituitary tumors.3PubMed Central. USP8 Mutations in Pituitary Cushing Adenomas—Targeted Analysis by Next-Generation Sequencing
The other ACTH-dependent cause is ectopic ACTH secretion, where a tumor outside the pituitary produces ACTH. This accounts for roughly 10% to 20% of endogenous cases. The most common culprits are lung neuroendocrine (carcinoid) tumors, responsible for about half of all ectopic ACTH cases.4PubMed Central. Ectopic ACTH syndrome caused by pulmonary carcinoid tumor mimicking long-standing sclerosing hemangioma These tumors can be small and slow-growing, sometimes hiding for years on imaging.
In ACTH-independent Cushing’s syndrome, the problem sits in the adrenal glands. Benign adrenal adenomas account for about 60% of these cases and adrenal carcinomas for the remaining 40%.1Medicine. Pituitary disorders Cushing’s syndrome A rarer adrenal cause is primary macronodular adrenal hyperplasia (PMAH), in which both adrenal glands develop multiple large nodules. Mutations in the ARMC5 gene account for more than 80% of familial forms and about 30% of apparently sporadic cases of PMAH.5PubMed Central. An Overview of the Heterogeneous Causes of Cushing Syndrome Resulting From Primary Macronodular Adrenal Hyperplasia (PMAH) In an unusual twist, some of these patients develop cortisol spikes after eating, a phenomenon called food-dependent Cushing’s syndrome, which is linked to a different gene (KDM1A) rather than ARMC5.
Recognizing the Signs
Cushing’s syndrome reshapes the body in ways that, taken individually, could be mistaken for aging, weight gain, or stress. The combination, though, is distinctive. Cortisol promotes fat storage in the trunk, face, and upper back while breaking down muscle in the arms and legs. The result is central obesity with thin limbs, a rounded “moon” face, and a fat pad between the shoulder blades sometimes called a buffalo hump.
Cortisol also weakens collagen, the structural protein that holds skin together. This leads to skin that bruises easily, heals slowly, and develops wide, reddish-purple stretch marks, particularly on the abdomen, hips, and thighs. In children, the combination of weight gain and slowed growth is nearly universal and should raise immediate concern.6PubMed Central. Dermatologic manifestations of endocrine disorders Facial plethora (a ruddy, flushed face), proximal muscle weakness in the thighs and shoulders, and thinning of the skin are among the features that most reliably point toward Cushing’s rather than simple obesity.7PubMed Central. Screening for Cushing Syndrome at the Primary Care Level: What Every General Practitioner Must Know
Beyond appearance, excess cortisol raises blood sugar, blood pressure, and cholesterol. Many patients develop type 2 diabetes or prediabetes. Hypertension is common, and recent research suggests it may partly involve local activation of the renin-angiotensin system in the fat tissue surrounding the adrenal glands, not just the systemic effects of cortisol on blood vessels.8PubMed Central. Unique Gene Expression Signature in Periadrenal Adipose Tissue Identifies a High Blood Pressure Group in Patients With Cushing Syndrome Very high cortisol levels also suppress the immune system, leaving patients vulnerable to unusual or severe infections.9PubMed Central. Cushing’s syndrome complicated by multiple opportunistic infections
What Cortisol Does to the Brain
The neuropsychiatric toll of Cushing’s syndrome is often underappreciated. Depression is one of the most consistent findings, with patients showing depressive symptoms roughly three standard deviations more severe than healthy controls in one meta-analysis.10The Journal of Clinical Endocrinology & Metabolism. Hippocampal Volume, Cognitive Functions, Depression, Anxiety, and Quality of Life in Patients With Cushing Syndrome Anxiety, irritability, and emotional instability are also common, and these changes are not simply a response to feeling ill; cortisol physically alters brain structure.
Imaging studies show reductions in the volume of the hippocampus, amygdala, and prefrontal cortex in people with active Cushing’s syndrome.11PubMed Central. Functional brain alterations in Cushing’s syndrome These regions govern memory, emotional regulation, and decision-making, and the cognitive effects follow predictably: large deficits in verbal and visual memory, learning, and language, with moderate impairments in attention, processing speed, and executive function.10The Journal of Clinical Endocrinology & Metabolism. Hippocampal Volume, Cognitive Functions, Depression, Anxiety, and Quality of Life in Patients With Cushing Syndrome
Perhaps the most troubling finding is that these brain changes do not always reverse after successful treatment. Overall brain volume tends to recover once cortisol normalizes, but frontal and temporal lobe volumes may not return to normal. Higher pre-treatment cortisol levels and longer duration of disease are both associated with more persistent cognitive problems.12PubMed. Cognitive decline in Cushing’s syndrome: A systematic review This is one reason early diagnosis and treatment matter so much.
Bone Loss and Fractures
Cortisol is deeply hostile to bone. It suppresses the cells that build new bone (osteoblasts) while leaving the cells that break it down relatively unopposed. At a molecular level, genes responsible for osteoblast maturation, collagen synthesis, and key growth factors are all turned down in patients with active Cushing’s, while signals that inhibit bone formation are ramped up.13PubMed Central. Cushing’s disease and bone The result is accelerated osteoporosis, often affecting the spine first. Vertebral fractures can occur even in young adults and sometimes with no memory of injury.
The strongest predictor of fracture in endogenous Cushing’s syndrome is the degree of cortisol excess itself, as measured by 24-hour urinary free cortisol levels.14PubMed. The risk factors for fractures and trabecular bone-score value in patients with endogenous Cushing’s syndrome Standard bone density scans sometimes underestimate fracture risk in Cushing’s patients because cortisol degrades bone quality (the internal architecture) even before overall density drops to the osteoporosis threshold.
How Cushing’s Syndrome Is Diagnosed
No single test confirms the diagnosis. Doctors follow a two-stage process: first prove that cortisol is genuinely elevated, then figure out why. For screening, clinicians rely on three first-line tests: a 24-hour urinary free cortisol collection, a late-night salivary cortisol measurement, and an overnight dexamethasone suppression test, in which the patient takes a low dose of a synthetic steroid at bedtime and has blood drawn the next morning to see if cortisol appropriately drops.7PubMed Central. Screening for Cushing Syndrome at the Primary Care Level: What Every General Practitioner Must Know Random cortisol measurements are unreliable because cortisol naturally fluctuates throughout the day.
Late-night salivary cortisol is particularly practical because the patient collects the sample at home. Research suggests it performs at least as well as the other tests, and in one study achieved perfect sensitivity and specificity at an appropriate cutoff.15The Journal of Clinical Endocrinology & Metabolism. Reproducibility of Nighttime Salivary Cortisol and Its Use in the Diagnosis of Hypercortisolism Compared with Urinary Free Cortisol and Overnight Dexamethasone Suppression Test In practice, clinicians usually want at least two abnormal screening tests before moving forward, because conditions like severe depression, chronic alcoholism, and extreme obesity can produce mildly elevated cortisol results, a situation sometimes called pseudo-Cushing’s.16Endocrine Abstracts. Differentiating Cushing from Pseudo-Cushing
Pinpointing the Source
Once hypercortisolism is confirmed, the next step is determining whether it is ACTH-dependent or ACTH-independent, which a blood test for ACTH levels can usually answer. If ACTH is suppressed, the adrenals are acting on their own and imaging of the adrenal glands typically reveals the culprit. If ACTH is elevated, the challenge is distinguishing a pituitary adenoma from an ectopic ACTH source.
Pituitary MRI is the obvious first step, but many pituitary adenomas causing Cushing’s disease are tiny microadenomas that MRI can miss. One study found pituitary MRI had only 50% accuracy for detecting the pituitary source, compared to 88% for bilateral inferior petrosal sinus sampling (BIPSS).17PubMed. Bilateral inferior petrosal sinuses sampling in the routine investigation of Cushing’s syndrome: a comparison with MRI BIPSS is an invasive procedure in which catheters are threaded through a vein in the groin up to the veins draining the pituitary. By measuring ACTH levels in these veins versus a peripheral vein, doctors can determine whether the ACTH is coming from the pituitary. After stimulation with corticotropin-releasing hormone, BIPSS achieved 100% sensitivity and 100% specificity for identifying the pituitary as the source in a landmark study.18PubMed. Petrosal sinus sampling with and without corticotropin-releasing hormone for the differential diagnosis of Cushing’s syndrome
Surgical Treatment
For Cushing’s disease (the pituitary form), the first-line treatment is transsphenoidal surgery, in which a surgeon reaches the pituitary through the nose and sphenoid sinus to remove the adenoma. At experienced centers, long-term remission rates average about 80% overall, rising to roughly 90% for well-defined microadenomas that have not invaded surrounding tissue.19PubMed. Transsphenoidal surgery for Cushing’s disease: a review of success rates, remission predictors, management of failed surgery, and Nelson’s Syndrome Surgical mortality is typically below 1%.
A meta-analysis of studies comparing endoscopic and microscopic approaches found overall remission rates around 80% with no clear difference between the two techniques for microadenomas. For larger tumors (macroadenomas), the endoscopic approach showed a higher remission rate (about 76% versus 60%) and lower recurrence.20PubMed Central. Endoscopic vs. microscopic transsphenoidal surgery for Cushing’s disease: a systematic review and meta-analysis Even after initial surgical remission, recurrence happens in roughly 18% of patients after a first operation and in about 28% after revision surgery.21PubMed. Outcomes of pituitary surgery for Cushing’s disease: a systematic review and meta-analysis
For adrenal causes, the approach depends on the specific lesion. A unilateral adenoma is typically cured by removing the affected adrenal gland. Adrenal carcinoma requires more aggressive surgery, often combined with other therapies. In ectopic ACTH syndrome, cure depends on whether the responsible tumor can be found and completely removed.
Medications When Surgery Is Not Enough
When surgery fails, is not possible, or the disease recurs, several drug classes can help control cortisol. These medications work through different mechanisms: some block cortisol production in the adrenal glands, some target the pituitary tumor itself, and one blocks cortisol’s action at the receptor level.
Osilodrostat, a steroidogenesis inhibitor, brought urinary free cortisol down to normal in about 77% of patients by 12 weeks in a randomized trial, compared to 8% on placebo. After open-label treatment, about 81% had normal cortisol by 36 weeks.22PubMed Central. Randomized Trial of Osilodrostat for the Treatment of Cushing Disease Pasireotide, a somatostatin analogue that acts on the pituitary tumor, is another option, though it normalizes cortisol in a smaller fraction of patients and commonly causes hyperglycemia as a side effect. In a phase 3 trial, the median urinary cortisol level dropped by about half within two months, with clinical signs improving over time.23PubMed. A 12-month phase 3 study of pasireotide in Cushing’s disease Other agents in use include ketoconazole, metyrapone, and mifepristone, each with its own tradeoffs in terms of effectiveness and side effects.
Bilateral Adrenalectomy and the Risk of Nelson’s Syndrome
When all else fails for Cushing’s disease, removing both adrenal glands guarantees that cortisol production stops. The tradeoff is significant: the patient becomes permanently dependent on replacement hormones for both cortisol and aldosterone. There is also a specific risk unique to this approach. Without adrenal glands to produce cortisol, the negative feedback loop that kept the pituitary tumor in check disappears. The pituitary adenoma, freed from cortisol’s suppressive effect, can grow aggressively and secrete massive amounts of ACTH, leading to darkening of the skin and an expanding tumor. This complication is called Nelson’s syndrome.
In one series, 11 patients who did not receive preventive radiation to the pituitary developed Nelson’s syndrome over a follow-up period averaging about four and a half years, while none of the 17 who received prophylactic pituitary radiation did.24PubMed. Nelson’s syndrome after bilateral adrenalectomy for Cushing’s disease However, a systematic review and meta-analysis found that the overall risk of developing Nelson’s syndrome was not significantly reduced by prior pituitary radiotherapy or prior pituitary surgery when pooling data across studies.25PubMed Central. Prevalence of Nelson’s syndrome after bilateral adrenalectomy in patients with cushing’s disease: a systematic review and meta-analysis The question of whether prophylactic radiation actually prevents Nelson’s syndrome remains, frankly, unresolved. Patients considering bilateral adrenalectomy need to understand that lifelong monitoring of the pituitary is non-negotiable.
The Glucocorticoid Withdrawal Phase
Patients are often surprised to learn that successful surgery does not mean immediate relief. When a cortisol-producing tumor is removed, cortisol levels crash, and the rest of the body’s cortisol-producing machinery, which has been shut down by negative feedback, takes months to wake back up. During this window, patients need to take replacement hydrocortisone and taper slowly, sometimes over six months to a year or longer.
This recovery period, called glucocorticoid withdrawal syndrome, can feel worse than the disease itself in the short term. The most common complaints are muscle and joint pain (about 50% of patients), fatigue (45%), weakness (34%), sleep disruption (29%), and mood changes (19%). Many of these symptoms persisted or even worsened during the first three months after surgery. Physical function as measured by grip strength actually declined in the weeks after surgery, even as disease-specific quality of life began to improve.26European Journal of Endocrinology. Glucocorticoid withdrawal syndrome following surgical remission of endogenous hypercortisolism: a longitudinal observational study The severity of the withdrawal phase tracks with how severe the Cushing’s syndrome was before surgery, so patients who were sickest before tend to have the roughest recovery.
Diagnosing Cushing’s Syndrome in Pregnancy
Pregnancy creates a unique diagnostic blind spot for Cushing’s syndrome. Normal pregnancy raises cortisol, ACTH, and urinary free cortisol levels on its own. Dexamethasone does not fully suppress cortisol in healthy pregnant women, which undercuts one of the standard screening tests.27PubMed Central. The diagnosis and management of Cushing’s syndrome in pregnancy On top of that, symptoms like weight gain, high blood pressure, glucose intolerance, and mood swings overlap heavily with those of a complicated pregnancy, including preeclampsia and gestational diabetes.28PubMed Central. A Case of Cushing’s Syndrome in Pregnancy
Even localization is harder. MRI of the pituitary during pregnancy avoids radiation but is done without gadolinium contrast, making small adenomas harder to spot. Inferior petrosal sinus sampling is generally not recommended in pregnancy because of the radiation exposure and blood clot risk.27PubMed Central. The diagnosis and management of Cushing’s syndrome in pregnancy Adding to the confusion, a large proportion of pregnant women with adrenal Cushing’s syndrome have ACTH levels that are not fully suppressed, which can misdirect the diagnostic workup. The condition is rare enough in pregnancy that no large trials exist, and management relies heavily on expert judgment and case series.
Pseudo-Cushing’s and Cyclic Disease
Not everyone with elevated cortisol on a screening test has Cushing’s syndrome. Conditions that chronically activate the stress response, including severe depression, alcohol use disorder, poorly controlled diabetes, and extreme obesity, can produce enough cortisol to trip screening tests. This is called pseudo-Cushing’s, and telling it apart from genuine Cushing’s syndrome is one of the trickiest problems in endocrinology.
Late-night salivary cortisol and midnight serum cortisol tend to have the highest diagnostic accuracy for making this distinction, because true Cushing’s syndrome disrupts the normal cortisol rhythm (which should bottom out at night), while pseudo-Cushing’s states often do not fully abolish it.16Endocrine Abstracts. Differentiating Cushing from Pseudo-Cushing A combined test using dexamethasone suppression followed by CRH stimulation can also help, because patients with true pituitary Cushing’s disease tend to respond to CRH with a cortisol surge even after dexamethasone, while patients with pseudo-Cushing’s generally do not.
A separate diagnostic headache is cyclic Cushing’s syndrome, where cortisol production swings between normal and excessive in an unpredictable pattern. Standard 24-hour urine collections can miss the diagnosis entirely if they happen to catch a normal phase. Repeated testing over time, sometimes guided by symptoms, is often the only way to pin it down. Patients with cyclic disease can spend years in diagnostic limbo, with symptoms that come and go and test results that seem contradictory.