Findings explained
What an “adrenal incidentaloma” is on a CT report
Read Your Scan Editorial Team·Last updated ·5 min read
Written from published radiology references and reviewed for plain language. Not written or signed by a physician, and not a diagnosis. How we write these pages →
The short answer
An adrenal incidentaloma is a lump on one of the two adrenal glands that was found by accident, on a scan ordered for an unrelated reason. They are common — found in several percent of abdominal CT scans — and the large majority are benign, non-functioning adenomas that will never cause a problem. Two questions decide what happens next: does it look benign on imaging, and is it producing hormones.
- Found by accident is part of the definition. The scan was looking for something else.
- Most are benign adenomas. Prevalence rises with age and they are commoner in people scanned for other abdominal problems.
- CT density measured in Hounsfield units does most of the work: a low-density lesion under 10 HU is very likely a benign fat-containing adenoma.
- Even a benign-looking lump gets hormone blood tests, because a minority quietly overproduce cortisol, aldosterone or adrenaline-type hormones.
- Size matters. Larger lesions get taken more seriously, and 4 cm is the usual point where the conversation changes.
What the adrenal glands are, and what was found
You have two adrenal glands, one perched on top of each kidney. Each is small — a few centimetres, and thin. They make hormones: cortisol, aldosterone, and the adrenaline family.
An incidentaloma is a lump on one of them that turned up on a scan that was not looking for it. Someone has a CT for abdominal pain, for kidney stones, for chest symptoms that included the upper abdomen in the field, and the report comes back mentioning a nodule on the left adrenal gland.
The word tells you nothing about what the lump is. It records how it was found. A great deal of the anxiety this finding creates comes from reading "-oma" as a verdict, when the informative half of the word is "incidental".
How common
Common enough to be considered a routine problem in radiology. Adrenal nodules are found on a few percent of abdominal CT scans, and the rate climbs with age — by the seventh decade it is substantially higher than in young adults.
The large majority are non-functioning adenomas: benign growths of adrenal tissue that make no excess hormone and do nothing. They do not become cancer, and in someone with no history of cancer elsewhere the odds that an incidental adrenal nodule is malignant are low.
Why the report talks about density
CT does not just show shape, it measures how much each spot absorbs X-rays, on a scale in Hounsfield units. Water sits at 0, fat is negative, dense bone is in the high hundreds.
This matters here because the commonest benign adrenal adenoma contains fat, and fat drags the measured density down. So a radiologist puts a measurement circle on the nodule and reports the number.
| What the report says | What it means in practice |
|---|---|
| Under 10 HU on unenhanced CT | Very likely a benign lipid-rich adenoma. This is usually the end of the imaging question. |
| Above 10 HU | Not settled by density alone. A dedicated adrenal protocol CT, or MRI, is often the next step. |
| Washout percentages | From a scan taking images before, shortly after and later after contrast. Benign adenomas release contrast faster than most other things. |
| Chemical shift MRI / signal drop-out | The MRI way of detecting the same fat. Often used when CT is inconclusive or to avoid more radiation. |
If your report gives a number under 10 and calls the lesion consistent with an adenoma, that number is the reason.
The half that imaging cannot answer
A scan can suggest that a nodule is benign. It cannot say whether it is making hormones, and a minority of benign adrenal adenomas do — quietly, for years.
- Excess cortisol, often at a low level that produces no obvious appearance but does contribute to blood pressure, blood sugar and bone loss.
- Excess aldosterone, which shows up as high blood pressure that is hard to control, sometimes with a low potassium level.
- A phaeochromocytoma, rare, producing adrenaline-type hormones, classically with episodes of palpitations, sweating and headaches.
This is why guidelines recommend biochemical testing for essentially every adrenal incidentaloma, even one that looks entirely benign on the images. It is blood and sometimes urine tests, and it is answering a different question from the one the scan answered.
What usually happens next
The common pathway: hormone tests, and either a confident imaging diagnosis or one further dedicated scan. If the lesion looks benign and the hormones are normal, most people are discharged from further imaging.
Surgery is reserved for lesions that are producing hormones, lesions with imaging features that are worrying, and larger lesions — 4 cm is the size usually quoted as the threshold where the balance shifts, because the small risk of malignancy rises with size.
One situation is different: someone with a known cancer elsewhere. There, an adrenal nodule is not treated as incidental in the same way, because the adrenal glands are a place cancers spread to. The workup is more active, and the report usually says so.
Frequently asked questions
- Is an adrenal incidentaloma cancer?
- Usually not. In someone with no known cancer elsewhere, the large majority are benign non-functioning adenomas, and primary adrenal cancer is rare. Low density on CT, small size and a stable appearance over time all point away from malignancy. In someone who does have a cancer history, the finding is taken more seriously because the adrenals are a common site of spread.
- Why do I need blood tests if the scan says it is benign?
- Because the scan and the blood tests answer different questions. Imaging addresses what the lump is made of; the blood tests address whether it is producing hormones. A benign adenoma can still overproduce cortisol or aldosterone, and that is treatable and worth finding.
- What does "less than 10 Hounsfield units" mean?
- Hounsfield units measure density on CT. Fat measures below zero, water at zero. The commonest benign adrenal adenoma contains fat, which pulls its measured density below 10. That number is strong evidence for a benign adenoma and often ends the imaging investigation.
- Will it need to be removed?
- Most are not. Surgery is considered when the lesion is producing hormones, when imaging features are suspicious, or when it is large — around 4 cm is the commonly quoted point where removal is discussed.
- Do I need repeat scans forever?
- No. Current guidance has moved away from prolonged repeat imaging of small lesions that look clearly benign and are hormonally silent. Your team decides based on the size, the imaging features and the hormone results.
When you want a second read of the images themselves
Understanding your radiologist’s report is free. A Second Opinion is the other thing: an independent AI reading of your images, done from the pictures rather than from the report, with the differences called out and a Q&A on the result. It is informational — not a diagnosis, and not a substitute for your doctor.
Words on this page
Each of these has its own page in the report glossary.
Report glossary, A–Z →Sources
- Patient-facing procedure descriptions (MRI, CT, X-ray, ultrasound)RadiologyInfo.org — Radiological Society of North America & American College of Radiology
- CT scan — overview, how it is performed and resultsNHS (United Kingdom)
- ACR Appropriateness Criteria and practice parameters for reportingAmerican College of Radiology
- ACR Manual on Contrast MediaAmerican College of Radiology
Keep reading
Read Your Scan is informational only — not a medical diagnosis, and not a substitute for a licensed radiologist or your doctor. This page explains what a report says and what a radiologist looks at; it cannot tell you what your own images show. If you have urgent symptoms, seek care.