The short answer
The Bethesda system is a standard way to report thyroid fine-needle aspiration results. It helps estimate concern and guide follow-up, but the next step depends on ultrasound findings, nodule size, symptoms, and specialist review.
A Bethesda category is a probability estimate for one nodule, not a diagnosis; each of the six categories carries a published range for the chance that surgery would find cancer.
Categories III and IV are indeterminate. Centers differ on what comes next: repeat aspiration, molecular testing, or a diagnostic lobectomy.
Published risk tables show two figures per indeterminate category depending on whether NIFTP is counted as cancer, so check which convention your numbers use.
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The full explanation.
What the number is actually reporting
A thyroid fine-needle aspiration (FNA) uses a thin needle, usually guided by ultrasound, to draw cells out of a thyroid nodule. A cytopathologist looks at those cells on a slide and assigns one of six Bethesda categories.
The category is not a diagnosis. It is a probability statement. Each category carries an estimated risk of malignancy, meaning the chance that surgery would find cancer in that nodule. That is why two people can get the same category and different plans: the nodule size, the ultrasound features, the patient's age, and their preferences all still count.
The system is formally called The Bethesda System for Reporting Thyroid Cytopathology. It has been in use since 2010 and was revised in 2017 and again in 2023.
The six categories
Cancer risk estimates below come from a published summary of the system. Two figures are given for some categories, depending on whether a borderline tumor called NIFTP is counted as cancer. More on that below.
I. Nondiagnostic. The sample was not usable. Too few cells, only cyst fluid, or blood obscuring the slide. Estimated cancer risk 5 to 10 percent. Usual next step: repeat the FNA. A nondiagnostic result is a sampling problem, not a finding.
II. Benign. Cells look like a benign follicular nodule, chronic lymphocytic thyroiditis (Hashimoto disease), or granulomatous thyroiditis. Estimated cancer risk 0 to 3 percent. Usual next step: clinical and ultrasound follow-up, not surgery. One caveat: if the ultrasound looked highly suspicious, guidance is to repeat the FNA within 12 months even after a benign result.
III. Atypia of undetermined significance (AUS). Some cells look off, but not enough to call anything. Older reports use AUS or FLUS, follicular lesion of undetermined significance. Estimated cancer risk 6 to 18 percent, or 10 to 30 percent when NIFTP is counted as cancer. Usual next step: repeat FNA, molecular testing, or a diagnostic lobectomy.
IV. Follicular neoplasm. The cells are crowded into microfollicles with little or no colloid. Estimated cancer risk 10 to 40 percent, or 25 to 40 percent counting NIFTP. Usual next step: molecular testing or diagnostic lobectomy.
V. Suspicious for malignancy. Features point toward papillary thyroid carcinoma, medullary thyroid cancer, lymphoma, or another cancer, but fall short of certainty. Estimated cancer risk 45 to 60 percent, or 50 to 75 percent counting NIFTP. Usual next step: surgery, either lobectomy or total thyroidectomy. Molecular markers are not recommended for making the decision here.
VI. Malignant. The findings are conclusive. Papillary cancer shows true papillae, psammoma bodies (tiny calcium spheres), and nuclear pseudo-inclusions. Estimated cancer risk 94 to 96 percent, or 97 to 99 percent counting NIFTP. Next step: surgery.
Why category IV cannot be settled by the needle
This is the part that frustrates people, and it is a real limitation, not a failure of your pathologist.
A follicular adenoma (benign) and a follicular carcinoma (cancer) contain cells that look identical under the microscope. The only difference is whether tumor cells have pushed through the capsule around the nodule or into blood vessels. A needle samples cells from the middle. It cannot see the edge.
So category IV usually means the whole lobe has to come out to answer the question. That is why the operation is called a diagnostic lobectomy: the surgery is the test.
The NIFTP wrinkle behind the two sets of numbers
NIFTP stands for noninvasive follicular thyroid neoplasm with papillary-like nuclear features. In 2016 a group of pathologists reclassified this tumor. It had been called a cancer, and it is now treated as a borderline tumor that does not spread.
That reclassification moved a chunk of cases out of the cancer column. It is why risk-of-malignancy tables now show two figures for the same category. When you compare numbers from different sources, check which convention is being used, or the same category will appear to have two different risks.
What the 2023 revision changed
The third edition of the system, published in 2023, made several changes worth knowing if your report and your online reading do not match.
Each category now has one official name instead of two alternatives. Category III is simply atypia of undetermined significance, dropping FLUS. Category IV is simply follicular neoplasm, dropping "suspicious for a follicular neoplasm." The risks were recalculated using newer data, and each category now carries an average risk as well as a range. AUS was simplified into two subgroups based on risk and molecular profile. Pediatric risks and management were added, and the naming was aligned with the 2022 World Health Organization classification of thyroid tumors.
If your report uses older wording, that does not make it wrong. It means the lab is on an earlier edition. It is fair to ask which edition your lab uses.
Molecular testing, and when it helps
For indeterminate results, categories III and IV, many centers send an extra sample for molecular testing. These panels look for gene changes such as BRAF, RAS, and RET fusions. The aim is to reclassify a nodule as low enough risk to watch, and so avoid an operation.
Practice varies. Some centers use molecular testing. Others repeat the FNA after 6 to 12 weeks. A radionuclide scan may be used if a repeat sample shows only architectural atypia. Ask which route your center takes and why.
Questions worth asking about your report
- Which Bethesda category, and which edition of the system did the lab use?
- What cancer risk does my center attach to that category?
- Was the sample adequate, and how many passes were taken?
- What did the ultrasound show, and does it agree with the cytology?
- If this is category III or IV, is molecular testing available to me?
- If surgery is suggested, is it a lobectomy or the whole gland, and why?
Call sooner, do not wait for the follow-up appointment, if
- A nodule grows noticeably over weeks rather than months
- New hoarseness or a voice change lasting more than 2 weeks
- Trouble swallowing solids, or a choking sensation when lying flat
- Any shortness of breath or noisy breathing
- A hard, fixed lump in the neck, or new swollen neck nodes
Related pages
Thyroid Nodule on Ultrasound: What Now?, What Does TI-RADS Mean?, Thyroid Cancer, and Pathology Reports.
Sources
Words to know
Tap any term to see what it means.
Common questions
Does a benign Bethesda II result mean no more testing?
Not always. Benign nodules are usually watched with ultrasound rather than operated on, but if the ultrasound looks highly suspicious, guidance is to repeat the aspiration within 12 months despite the benign biopsy.
Why can a follicular neoplasm result only be settled by surgery?
A follicular adenoma and a follicular carcinoma look the same under the microscope. The difference is whether tumor cells have invaded the capsule or blood vessels, which a needle sampling the middle of the nodule cannot show.
My report uses wording I cannot find online. Is it wrong?
Probably not. The system was first published in 2010 and revised in 2017 and 2023, and the 2023 edition gave each category a single name. An older report may simply reflect the edition your lab uses. It is fair to ask which one.
Questions to ask your doctor
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Sources last checked: 2026-07-21 what this meansLast updated: 2026-08-13Next planned review: 2027-07-21
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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