Thyroid nodule found by chance on CT or MRI

The main guidance

ACR Incidental Thyroid Findings Committee white paper (US radiologists, 2015): a flow chart for nodules seen on CT or MRI, based on suspicious features, age, nodule size and the person's other illnesses and life expectancy. (ACR 2015)(Lyuman 2025)
American Thyroid Association adult thyroid nodule guideline (US, 2015): covers nodules found on other imaging and on PET scans. (ATA 2015)
NICE thyroid disease guideline NG145 (England, 2019): covers when an ultrasound of an incidental finding is worth doing. (NICE NG145)

Who it applies to

Adults whose CT, MRI, PET or other scan, done for another reason, mentions a thyroid nodule. The American Thyroid Association says about half of people have a thyroid nodule by age 60, and over 90% are benign. (ATA patient page) In a Scottish study that looked again at 1,499 consecutive CT scans that included the thyroid, 11% showed a nodule of 1 cm or more; the average age was 70 and only 30% of these nodules were mentioned in the report. (Lyuman 2025) If you have no scan finding and want to know about checking the thyroid for cancer, see thyroid cancer screening.

Which nodules get an ultrasound next

When an incidental thyroid nodule on CT or MRI leads to a thyroid ultrasound (simplified)
Guidance	Ultrasound suggested	Usually nothing more
ACR 2015 (Lyuman 2025)	Suspicious features on the scan, such as abnormal lymph nodes or the nodule growing into nearby tissue; or a nodule of 1 cm or more under age 35, or 1.5 cm or more from age 35	Smaller nodules without suspicious features, and people whose other illnesses or life expectancy make it not worthwhile
ATA 2015 (ATA 2015)	An ultrasound with a check of the neck lymph nodes for a nodule suggested by CT, MRI or PET	Diffuse PET uptake across the gland with ultrasound signs of thyroiditis
NICE 2019 (NICE NG145)	Consider an ultrasound if clinical factors suggest cancer is possible	Ultrasound should not be the default for incidental findings (as quoted in Lyuman 2025) (Lyuman 2025)

The British Thyroid Association 2014 guideline, as quoted in the 2025 review, says that in most cases no further assessment is needed, and that an ultrasound is recommended when the CT shows suspicious findings (spread outside the gland, growth into the windpipe, suspicious lymph nodes), the person is in a high-risk group, or there is significant clinical concern. (Lyuman 2025)
The review notes that CT and MRI are not recommended for sorting nodules into benign or cancer, though they can show worrying features such as spread outside the gland and abnormal lymph nodes. (Lyuman 2025)
If an ultrasound is done, it is graded with the ACR TI-RADS or ATA patterns explained on thyroid cancer screening.

If the thyroid lit up on a PET scan

Focal uptake (one bright spot): the ATA says focal FDG-PET uptake within a nodule confirmed on ultrasound means an increased risk of thyroid cancer and should be looked at further. (ATA 2015) A review of 61 studies found focal uptake in 2.22% of PET-CT scans, and 30.8% of those that were sampled were cancer, mostly papillary thyroid cancer. (de Leijer 2021)
Diffuse uptake (the whole gland): the ATA says that with ultrasound and clinical signs of chronic lymphocytic thyroiditis it needs no further imaging, and that checking thyroid function is appropriate. (ATA 2015) See thyroid test results.

What longer follow-up has shown

In the Scottish CT study, 150 people with a nodule of 1 cm or more were followed for 13 years: 11% had a thyroid ultrasound, none developed thyroid cancer that caused problems, and the 80% who died during follow-up died of other causes. The authors say this supports the UK approach of not testing further unless the scan looks suspicious or there is real clinical concern. (Lyuman 2025) The ACR white paper notes that testing every incidental nodule adds procedures and, in some cases, risk. (ACR 2015) See false positives and overdiagnosis.

Symptoms a thyroid nodule can occasionally cause, from the American Thyroid Association patient page: (ATA patient page)

Pain in the neck, jaw or ear
Difficulty breathing or swallowing, or a "tickle in the throat", if a nodule is large enough to press on the windpipe or food pipe
Hoarseness, less commonly

Tell your doctor if you notice any of these.

Routine age-based screening

A finding on a scan does not replace the routine screenings for your age; see screenings by age and gender.

Where they differ:

Who gets an ultrasound. The ATA recommends an ultrasound for a nodule suggested by CT, MRI or PET. (ATA 2015) The ACR white paper limits it by size, age, suspicious features and life expectancy. (Lyuman 2025) NICE and the BTA say most incidental nodules need no further test unless the scan or the clinical picture is worrying. (NICE NG145)(Lyuman 2025)
How many people that means. The 2025 review notes that the ACR size limits would send a large share of people with incidental nodules for ultrasound, while the UK approach depends on advanced features or known risk factors. (Lyuman 2025)

Your doctor decides what your result means for you.

Questions people ask

How common is a thyroid nodule on a CT scan?

Very common. In a Scottish study that looked again at 1,499 CT scans that included the neck, 11% showed a nodule of 1 cm or more, and most were not mentioned in the report. (Lyuman 2025)

Is a nodule found on a scan likely to be cancer?

Usually not. The ATA guideline says cancer is found in 7% to 15% of thyroid nodules, depending on age, sex, radiation exposure, family history and other factors, and the ACR white paper says most incidental nodules are benign and small cancers usually grow slowly. (ATA 2015) (ACR 2015)

Will I need a thyroid ultrasound?

It depends on the guidance your team follows and on the scan. The ATA recommends an ultrasound for nodules found on CT, MRI or PET; the ACR white paper uses size, age and suspicious features; NICE says an ultrasound should be considered when clinical factors suggest cancer is possible. Your doctor decides. (ATA 2015) (ACR 2015) (NICE NG145)

What if the nodule lit up on a PET scan?

The ATA says focal uptake within a nodule confirmed on ultrasound raises the chance of cancer, while diffuse uptake across the gland with signs of thyroiditis needs no further imaging. In a large review, about 3 in 10 focal PET findings that were sampled were cancer. (ATA 2015) (de Leijer 2021)

Do I need a blood test?

The ATA guideline says checking thyroid function is appropriate when a PET scan shows diffuse uptake. For a nodule, your doctor decides which tests fit your case; see thyroid test results for what the numbers mean. (ATA 2015)