An adrenal incidentaloma is an adrenal gland mass found by accident during imaging performed for an unrelated reason. You go in for a CT scan after a car accident or to investigate abdominal pain, and the radiologist spots a lump on one of your adrenal glands that nobody was looking for. Most of these masses turn out to be harmless, nonfunctioning adenomas, but a small fraction produce excess hormones or, rarely, harbor cancer. The challenge is figuring out which ones need attention and which can be safely left alone.
Why They Keep Turning Up
The simplest explanation for why adrenal incidentalomas seem so common is that we scan people far more often than we used to. A UK teaching hospital study tracking five years of imaging found that the total number of scans rose about 8% per year, while abdominal CT scans specifically climbed by more than 55% over the same period. The number of scans flagging adrenal lesions grew even faster, increasing roughly 70% from 2015 to 2019.1PubMed Central. Adrenal Incidentaloma: Prevalence and Referral Patterns From Routine Practice in a Large UK University Teaching Hospital Better scanner resolution also plays a role. Modern CT machines pick up smaller lesions that older equipment would have missed, and the detection rate on imaging has been climbing toward what pathologists have traditionally found at autopsy.2PubMed. Prevalence of adrenal incidentaloma in a contemporary computerized tomography series Autopsy studies have long reported adrenal masses in anywhere from 1% to 8% of the general population, with higher rates in older adults. As scanning catches up to that range, more and more people are hearing the words “adrenal incidentaloma” for the first time.
How Imaging Separates the Harmless from the Worrisome
The first clue radiologists look at is the density of the mass on a noncontrast CT scan, measured in Hounsfield units (HU). Fat-rich adenomas tend to register at or below 10 HU because intracellular fat absorbs X-rays differently than solid tissue. The classic threshold of 10 HU or less is highly specific for a benign adenoma, but it misses a substantial number of them. One study found that this cutoff caught fewer than half of confirmed adenomas, though it was right about 93% of the time when it did flag one. Raising the cutoff to 16 HU improved detection to about two-thirds of adenomas without sacrificing that specificity.3PubMed Central. Utility of the 10 Hounsfield Unit Threshold for Identifying Adrenal Adenomas: Can We Improve?
When the density is above that threshold, the mass is called “lipid-poor” or “indeterminate,” and further imaging steps in. A washout CT involves giving contrast dye and then scanning again after a delay. Benign adenomas release the contrast quickly, while malignant lesions and pheochromocytomas tend to hold onto it. Using a relative washout threshold of 50%, one study correctly categorized 99 out of 101 lesions as benign or malignant.4PubMed. Characterization of indeterminate (lipid-poor) adrenal masses: use of washout characteristics at contrast-enhanced CT Washout CT works well for most situations, though it has a known blind spot for certain fast-growing cancers and pheochromocytomas, which can mimic adenoma washout patterns.5PubMed. Update on CT and MRI of Adrenal Nodules
MRI offers an alternative, especially a technique called chemical-shift imaging. This method exploits the fact that fat and water molecules behave differently in a magnetic field. Adenomas, which contain intracellular lipid, lose signal on one set of images compared to the other. A meta-analysis of chemical-shift MRI found pooled sensitivity around 94% and specificity around 95% for identifying adenomas.6PubMed. Chemical shift imaging for evaluation of adrenal masses: a systematic review and meta-analysis In a head-to-head comparison of lipid-poor adenomas, washout CT actually outperformed chemical-shift MRI, achieving 100% sensitivity versus about 76% for MRI.7PubMed. Characterization of lipid-poor adrenal adenoma: chemical-shift MRI and washout CT Still, MRI avoids radiation exposure, which can matter for younger patients or those needing repeated follow-up imaging.
For truly difficult cases where neither CT nor MRI gives a clear answer, a subtraction technique on MRI can help. In one small study, subtracting opposed-phase images from in-phase images produced zero overlap between adenomas and metastases, perfectly separating the two groups.8PubMed. Value of chemical shift subtraction MRI in characterization of adrenal masses Chemical-shift MRI also performs similarly at both common scanner strengths (1.5 Tesla and 3 Tesla), so the specific machine used at your hospital should not substantially change the result.9PubMed. Characterization of adrenal lesions at chemical-shift MRI: a direct intraindividual comparison of in- and opposed-phase imaging at 1.5 T and 3 T
The Hormonal Workup Everyone Should Get
Even if imaging makes a mass look completely benign, guidelines recommend checking whether it is quietly producing hormones. The American Association of Endocrine Surgeons advises biochemical testing for autonomous cortisol secretion in all patients with an adrenal incidentaloma at least 1 cm in size. Patients with high blood pressure or low potassium should also be checked for excess aldosterone, and anyone whose mass measures above 10 HU on noncontrast CT should be screened for pheochromocytoma.10JAMA Surgery. American Association of Endocrine Surgeons Guidelines for Adrenalectomy: Executive Summary
Pheochromocytoma screening matters more than many patients realize. These tumors produce adrenaline-like hormones and can trigger dangerous blood pressure spikes, especially during surgery or anesthesia. The standard test measures metanephrines in blood or urine. Mildly elevated results are common and often turn out to be false alarms, but in one series, about 30% of patients with borderline-elevated metanephrines (one to two times the upper limit of normal) actually had a pheochromocytoma on final pathology.11JAMA Surgery. Adrenal Incidentaloma, Borderline Elevations of Urine or Plasma Metanephrine Levels, and the “Subclinical” Pheochromocytoma Given that an undiagnosed pheochromocytoma can cause a life-threatening crisis, even borderline results warrant careful follow-up.
Screening for primary aldosteronism is relevant mainly in incidentaloma patients who also have high blood pressure. Among hypertensive patients with adrenal incidentalomas, confirmed primary aldosteronism has been found in about 4%.12PubMed. Determination of the aldosterone/renin ratio in 269 patients with adrenal incidentaloma A blood test comparing aldosterone to renin levels is a reliable way to screen for this condition, and studies have identified ratio thresholds that combine high sensitivity with high specificity for catching or ruling out aldosterone-producing adenomas.13PubMed. The aldosterone to renin ratio in the evaluation of patients with incidentally detected adrenal masses
Mild Autonomous Cortisol Secretion and Its Quiet Damage
The most common hormonal abnormality in adrenal incidentalomas is not dramatic Cushing syndrome with the classic moon face and purple stretch marks. Instead, it is a subtler condition called mild autonomous cortisol secretion, or MACS, in which the adrenal mass produces a small amount of cortisol on its own, bypassing the brain’s normal feedback loop. This is detected through an overnight dexamethasone suppression test: you take a low dose of a synthetic steroid at bedtime, and if your morning cortisol level fails to drop below a certain threshold, it suggests autonomous production.
The threshold that defines MACS has been debated for years. Recent evidence suggests that even cortisol levels sitting in what was previously considered a “gray zone” carry real metabolic consequences. In one large analysis, patients whose post-suppression cortisol fell between 1.2 and 1.8 micrograms per deciliter had significantly higher rates of high blood pressure, diabetes, and cardiovascular events compared to those whose cortisol suppressed below 1.2. After adjusting for age, sex, obesity, and cholesterol levels, the association held, with roughly 55% higher odds of hypertension and 60% higher odds of diabetes in this intermediate group.14PubMed Central. Dexamethasone Suppression Testing in Patients with Adrenal Incidentalomas with/Without Mild Autonomous Cortisol Secretion: Spectrum of Cortisol Cutoffs and Additional Assays (An Updated Analysis)
Across broader reviews, hypertension shows up in as many as 80% of MACS patients and diabetes in up to 40%.15PubMed Central. Epidemiology and Management of Hypertension and Diabetes Mellitus in Patients with Mild Autonomous Cortisol Secretion: A Review MACS is also associated with higher heart muscle mass and more metabolic syndrome features than found in patients whose adrenal nodules are hormonally silent.16PubMed. Impact of mild autonomous cortisol secretion on metabolism & cardiac morphology in adrenal incidentaloma There is growing evidence that this excess cortisol, even in modest amounts, accelerates cardiovascular remodeling and raises mortality risk.17PubMed. Cardiovascular complications of mild autonomous cortisol secretion The comorbidities linked to MACS extend further still, including insulin resistance, abnormal lipid profiles, fatty liver disease, and vertebral fractures.18PubMed. Comorbidities in Mild Autonomous Cortisol Secretion – A Clinical Review of Literature
Interestingly, one study looking specifically at remnant cholesterol (a lipid marker that captures certain atherogenic particles not measured by standard cholesterol tests) found that elevated remnant cholesterol in adrenal incidentaloma patients was tied to higher cardiovascular event risk, but this association was driven by the incidentaloma population overall rather than MACS specifically. In that cohort, MACS itself did not independently predict remnant cholesterol levels or major cardiovascular events during follow-up.19PubMed Central. The Influence of Remnant Cholesterol on Cardiovascular Risk and Mortality in Patients with Non-Functional Adrenal Incidentalomas and Mild Autonomous Cortisol Secretion: A Retrospective Cohort Study This suggests that metabolic risk in these patients is multifactorial, and cortisol excess is one contributor among several.
The Malignancy Question
The fear that drives much of the workup is cancer. The two main concerns are adrenocortical carcinoma, a rare primary adrenal cancer, and metastatic disease from a cancer elsewhere in the body, most commonly lung, kidney, or gastrointestinal cancers.20PubMed Central. Presentation, disease progression and outcomes of adrenal gland metastases Among true incidentalomas in patients with no known malignancy, the rate of adrenocortical carcinoma is low, but size is the most reliable warning sign. Masses 4 cm or larger, especially with a density above 20 HU on noncontrast CT, warrant concern.21JAMA Surgery. American Association of Endocrine Surgeons Guidelines for Adrenalectomy: Executive Summary Adrenocortical carcinomas tend to be large at diagnosis, with 4 cm commonly used as the threshold separating low-risk from higher-risk lesions.22PubMed Central. Computed tomography of adrenocortical carcinoma containing macroscopic fat
For masses that remain indeterminate after CT and standard MRI, a multiparametric MRI approach combining several imaging features alongside a 4 cm size cutoff can push sensitivity for adrenocortical carcinoma above 95%.23PubMed. Differentiation between adrenocortical carcinoma and lipid-poor adrenal adenoma using a multiparametric MRI-based diagnostic algorithm The goal is to catch the rare cancer without subjecting everyone else to unnecessary surgery.
Why Biopsy Is Usually the Wrong Move
If a mass looks uncertain on imaging, the intuitive next step might seem like sticking a needle in it. In practice, adrenal biopsy is discouraged for most incidentalomas. One study of patients with incidental adrenal masses found that 80% of biopsies came back nondiagnostic, and complications included bleeding in the liver, chest, and intestine. Worse, five patients in that series turned out to have pheochromocytomas, and none of them had been screened for it before the needle went in.24PubMed. Needle biopsy of incidentally discovered adrenal masses is rarely informative and potentially hazardous Biopsying a pheochromocytoma without first ruling it out biochemically can trigger a hypertensive crisis. Beyond that, biopsy cannot reliably distinguish a benign adrenal adenoma from adrenocortical carcinoma, and there is a risk of tumor seeding along the needle track.25The Journal of Clinical Endocrinology & Metabolism. Approach to the Patient With Adrenal Incidentaloma – Section: Diagnosing Adrenal Malignancy The one scenario where biopsy can be useful is when a patient has a known cancer elsewhere and the adrenal mass might be a metastasis, because that distinction changes treatment planning and biopsy can differentiate metastatic tissue from normal adrenal.
When Surgery Makes Sense
European and American guidelines agree on the broad indications for surgery. Adrenalectomy is recommended for tumors causing significant hormone excess, including pheochromocytoma, primary aldosteronism, and overt Cushing syndrome. For MACS, surgery can be considered when the patient has worsening hypertension, diabetes, or osteoporosis that might be driven by the cortisol excess, though this decision is individualized rather than automatic.26European Journal of Endocrinology. European Society of Endocrinology clinical practice guidelines on the management of adrenal incidentalomas, in collaboration with the European Network for the Study of Adrenal Tumors – Section: 1.4. Surgical treatment
Size is the other main trigger. A mass over 4 cm with imaging features that do not clearly indicate a benign adenoma raises the possibility of adrenocortical carcinoma and generally warrants removal. When surgery is indicated for a suspected benign mass, a minimally invasive (laparoscopic or robotic) approach is preferred, and guidelines suggest that masses with suspicious imaging features up to 6 cm without local invasion should still be handled by an experienced high-volume adrenal surgeon using minimally invasive techniques.27European Journal of Endocrinology. European Society of Endocrinology clinical practice guidelines on the management of adrenal incidentalomas, in collaboration with the European Network for the Study of Adrenal Tumors – Section: 1.4. Surgical treatment In practice, the size threshold at which surgeons recommend removal varies widely, ranging from under 4 cm to as large as 12 cm depending on the institution and the clinical context.28PubMed Central. Adrenal Incidentaloma Controversial Size Recommendations
Importantly, guidelines recommend against surgery for asymptomatic, nonfunctioning masses that look clearly benign on imaging. Operating on every incidentaloma would subject a huge number of people to unnecessary surgical risk for a condition that, in most cases, will never cause a problem.
What Happens If You Just Watch
For the majority of people with a small, benign-looking, hormonally inactive adrenal mass, the natural history is reassuring. A 10-year follow-up study found that the average growth of nonfunctioning incidentalomas was essentially zero millimeters over a decade. About 9% of tumors grew by 10 mm or more, but none of those developed imaging characteristics suggestive of cancer.29Endocrine Connections. Natural history of nonfunctioning adrenal incidentalomas: a 10-year longitudinal follow-up study A Portuguese single-center study echoed these findings: about 82% of tumors stayed stable in size, density, and appearance over years of follow-up, with significant growth in only about 11% of patients.30Endocrinology Insights. Long-Term Follow-Up of Adrenal Incidentalomas: A Portuguese Single-Center Study
Current European guidelines reflect this low-risk trajectory. For masses with clearly benign imaging features, they recommend against any further imaging follow-up at all. For indeterminate masses where the patient opts against surgery, one repeat scan at 6 to 12 months is suggested, with surgery recommended only if the mass grows by more than 20% in its largest dimension (and at least 5 mm in absolute terms). Hormonally, if the initial workup is normal, repeated testing is unnecessary unless new symptoms appear or existing conditions like hypertension or diabetes get worse.31European Journal of Endocrinology. European Society of Endocrinology clinical practice guidelines on the management of adrenal incidentalomas, in collaboration with the European Network for the Study of Adrenal Tumors For patients with MACS who do not undergo surgery, annual monitoring of comorbidities is recommended, and this monitoring can often happen through a primary care physician rather than requiring ongoing specialist visits.
The Cost of Watching Too Closely
Surveillance itself has a cost, both financial and in terms of unnecessary procedures. A cost-effectiveness analysis found that for small, benign-looking, nonfunctioning incidentalomas, extensive surveillance (multiple rounds of imaging) was actually less effective and more expensive than either no surveillance or a single follow-up scan. One-time surveillance was the sweet spot, costing about $158 more than no surveillance at all while providing the best balance of outcomes.32PubMed. Less is more: cost-effectiveness analysis of surveillance strategies for small, nonfunctional, radiographically benign adrenal incidentalomas A separate model found that for masses over 4 cm, adrenalectomy was cost-effective, primarily because of how aggressive adrenocortical carcinoma is when caught late.33PubMed. A cost-effectiveness analysis of adrenalectomy for nonfunctional adrenal incidentalomas: is there a size threshold for resection? Even the type of initial imaging affects cost-effectiveness: a recent analysis concluded that using noncontrast CT alone at the time of diagnosis was the only strategy that met standard cost-effectiveness thresholds, while adding washout CT to the initial assessment increased costs without improving outcomes.34PubMed. Cost-Effectiveness of Follow-Up Imaging for Incidental Adrenal Nodules to Rule Out Adrenocortical Carcinoma
The Emotional Weight of an Unexpected Finding
An often-overlooked aspect of incidentalomas is how the diagnosis affects patients psychologically. About three-quarters of patients in one study reported that learning about the mass made them worried, though most did not think about it much during follow-up, and only a small percentage felt it had a large impact on daily life. Still, roughly 29% screened positive for possible anxiety and 30% for possible depression, with anxiety linked to a more negative experience of the surveillance process itself.35PubMed Central. Patient-reported impacts of a conservative management programme for the clinically inapparent adrenal mass
The cortisol connection runs deeper than just metabolic effects. Patients with MACS show higher levels of perceived stress and disability related to mental health compared to those with nonfunctioning adenomas. In one analysis, the association between MACS and psychological distress persisted even after accounting for age, sex, and body weight, suggesting it may be a direct effect of the excess cortisol on the brain rather than just a reaction to the diagnosis.36The Journal of Clinical Endocrinology & Metabolism. Mental Health in Patients With Adrenal Incidentalomas: Is There a Relation With Different Degrees of Cortisol Secretion? This is worth raising with your doctor if you have MACS and notice mood changes, sleep problems, or heightened anxiety.
Bilateral Masses and Genetic Connections
Most adrenal incidentalomas are one-sided, but about 10% to 15% show up on both adrenal glands. Bilateral masses broaden the differential diagnosis. While two benign adenomas are still the most common explanation, bilateral findings raise the question of inherited conditions. Germline mutations in the ARMC5 gene, for example, have been identified as a cause of bilateral macronodular adrenal hyperplasia.37PubMed Central. Genetic Alterations in Benign Adrenal Tumors And bilateral pheochromocytomas, though uncommon, usually point to a hereditary syndrome, meaning genetic testing and screening of family members may be appropriate.38Journal of Clinical and Translational Endocrinology: Case Reports. Management of the patient with incidental bilateral adrenal nodules Familial adenomatous polyposis, a hereditary colon cancer syndrome, also carries an elevated rate of adrenal incidentalomas, though long-term follow-up suggests that these rarely behave aggressively.39Diseases of the Colon & Rectum. Adrenal Incidentaloma in Familial Adenomatous Polyposis: A Long-Term Follow-Up Study and Schema for Management
Artificial Intelligence and the Future of Adrenal Imaging
Radiomics, the extraction of quantitative features from imaging that the human eye cannot assess, is being paired with machine learning to push diagnostic accuracy further. In PET-CT studies, combining standard metabolic measurements with texture features (patterns in how the image signal varies across a lesion) achieved sensitivity of 100% and specificity of 85% for telling benign from metastatic adrenal lesions.40PubMed Central. Artificial intelligence and radiomics applications in adrenal lesions: a systematic review These tools are still mostly in the research phase, but they point toward a future where imaging alone can more confidently characterize adrenal masses, potentially reducing the need for repeated scans, biopsies, and surgeries that turn out to be unnecessary. For now, the clinical reality is that a well-performed CT scan, a thorough hormonal workup, and an experienced endocrinologist can manage the vast majority of adrenal incidentalomas safely and efficiently.