Thyroid Cytology Reports

Bethesda Category 3: — What AUS/FLUS Actually Means

A Category 3 result on your thyroid FNAC report means the cells were neither clearly benign nor clearly malignant. This is a genuinely uncertain result — not a finding of cancer, but not a simple reassurance either. The next step is more testing, not immediate surgery.

Medically reviewed by Dr. Muralidhar Muddusetty, Surgical Oncologist, MBBS (AIIMS) · MS Surgery (AIIMS) · DNB Surg Onc · MRCS (Edinburgh) · Last reviewed September 2026

Not a cancer diagnosis — Category 3 means the sample was inconclusive, not that cancer has been found.
A real range of risk — The malignancy risk is neither negligible nor very high — further testing is designed to narrow it.
Most turn out benign — On further investigation, the majority of Category 3 nodules are found to be non-malignant.
Two options before surgery — Repeat FNAC and molecular testing both exist precisely to avoid unnecessary operations.

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Bethesda Category 3 means the cells sampled were neither clearly benign nor clearly malignant — the result was inconclusive. The 2023 Bethesda System assigns this category a malignancy risk range of approximately 13 to 30 per cent. Most people with this result are asked to repeat the FNAC or consider molecular testing before any surgery is discussed.

Where does Category 3 sit among the Bethesda categories?

	Category 2 — Benign	Category 3 — AUS/FLUS	Category 4 — Follicular Neoplasm
What the report means	Cells look normal under the microscope	Some unusual features — not enough to classify clearly	Cells suggest a follicular growth that needs further assessment
Malignancy risk range (2023 Bethesda System)	0–3%	13–30%	25–40%
Typical next step	Ultrasound monitoring	Repeat FNAC or molecular testing	Diagnostic thyroid lobectomy

What should you do now?

Ask your team whether your result is AUS or FLUS and what drove that classification
Ask whether a repeat FNAC or molecular testing is the recommended next step for your nodule
Find out the ultrasound risk category of the nodule — this affects which option your team recommends
Do not arrange surgery on this result alone; a second test is usually needed first
Return for your follow-up on the date your team gives you — do not delay it
Tell your team immediately about any new symptoms: a growing lump, voice changes, or difficulty swallowing

Why does a thyroid FNAC come back inconclusive?

A cytologist reports what they can see under the microscope. In Category 3, the cells have some unusual features — but not enough, or not clearly enough, to be called cancerous.

This can happen when the sample has too few cells to be certain, when cells show features that overlap between benign and malignant conditions, or when the nodule is a type of growth that genuinely sits between the two.

Category 3 exists to avoid two mistakes: unnecessary surgery on the majority who are benign, and false reassurance for the minority who need treatment. Further testing resolves the uncertainty in most cases.

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What do these terms on your report mean?

AUS — Atypia of Undetermined Significance: Cells have some abnormal features that could suggest malignancy, but not enough to make a definitive call. The concern is about how individual cells look.
FLUS — Follicular Lesion of Undetermined Significance: Similar uncertainty, but the concern is about how cells are arranged in groups rather than individual cell features. Often grouped with AUS in current reporting practice.
NIFTP: A thyroid growth reclassified in 2016 from cancer to a non-malignant lesion. Its existence is one reason Category 3 risk estimates have been revised across successive editions of the Bethesda System.
Molecular testing: Laboratory analysis of the FNAC sample looking for genetic changes linked to malignancy. It can help reclassify some Category 3 results as more or less likely to be cancer.
Diagnostic lobectomy: Surgical removal of the half of the thyroid containing the nodule, so the tissue can be examined in full. For Category 3, this is the surgery discussed — not removal of the whole thyroid.

What do families ask most about a Category 3 result?

What is the difference between AUS and FLUS?

AUS flags unusual features in individual cells — the nucleus shape or chromatin texture. FLUS flags an unusual group arrangement suggesting a follicular pattern. The 2023 Bethesda System groups both under Category 3 for management purposes. Most centres now use AUS as the single term. If your report uses either label, the next steps are the same.

What does molecular testing actually look for?

Molecular tests examine genetic material in the FNAC sample for mutations and gene fusions linked to thyroid malignancy. A low-risk result supports continued monitoring; a high-risk result moves management toward surgery. These tests shift probability rather than giving a yes or no. Availability varies across centres in India; your endocrinologist will advise whether molecular testing is indicated for your nodule.

If I need surgery, will the whole thyroid be removed?

For Category 3, the standard surgical approach — when surgery is needed — is a diagnostic lobectomy: removal of the lobe containing the nodule, not the whole thyroid. If the tissue confirms malignancy, a decision about completing the thyroidectomy follows. Many Category 3 lobectomies find benign disease, and the remaining thyroid lobe continues to function normally.

Can I ask for a second opinion on the cytology report?

Yes, and it is reasonable to do so. Requesting that the same slides be reviewed by a second pathologist does not change the cells but may change the classification — some results move to Category 2 or 4, which clarifies the next step. Your treating team can arrange this, or you can request it directly from the laboratory that issued the report.

What ultrasound features matter alongside a Category 3 result?

The ultrasound risk category of the nodule — assessed using systems such as ACR-TIRADS or EU-TIRADS — is interpreted alongside the Bethesda category. Low-risk ultrasound with Category 3 cytology often supports repeat FNAC with monitoring. High-risk features may prompt earlier molecular testing or surgery. Bring both your FNAC and ultrasound reports to the same appointment for a coordinated assessment.

Did you know?

The 2016 reclassification of a common thyroid growth — NIFTP — from cancer to a non-malignant lesion immediately reduced the implied malignancy risk for Category 3 nodules in centres that adopted the new definition.

No patient's treatment changed overnight. The risk figure changed because the science did.

Source: Bethesda System for Reporting Thyroid Cytopathology, 3rd edition, National Cancer Institute, 2023

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Common questions

Frequently asked questions

What does Bethesda Category 3 mean on my thyroid report?

Category 3 means the cells in your thyroid sample had unusual features that could not be classified as clearly benign or clearly malignant. It is not a finding of cancer — the majority of Category 3 nodules turn out to be benign on further investigation. It signals that more information is needed, which is why a repeat FNAC, molecular testing, or occasionally surgery is the recommended next step.

What is the actual risk that my nodule is cancer?

The 2023 Bethesda System for Reporting Thyroid Cytopathology gives Category 3 a malignancy risk range of approximately 13 to 30 per cent. This range exists partly because of how a reclassified lesion called NIFTP is counted at each centre. Your individual probability sits within that range, and further testing is designed to narrow it. Your endocrinologist will also factor in the nodule's size and ultrasound features.

Should I have a repeat FNAC or molecular testing next?

The choice depends on your nodule's ultrasound features, what is available at your centre, and the degree of certainty your team needs before deciding. Repeat FNAC resolves the result for a proportion of patients; molecular testing analyses genetic material without another needle. Both are supported by the 2023 Bethesda System and ATA guidance for Category 3. Your endocrinologist will recommend which fits your situation.

What is the difference between AUS and FLUS?

AUS (Atypia of Undetermined Significance) flags unusual features in individual cells; FLUS (Follicular Lesion of Undetermined Significance) flags an unusual pattern in how cells group together. Both sit in the same zone of uncertainty. The 2023 Bethesda System groups them under Category 3 for management purposes, and most centres now use AUS as the single umbrella term. If your report uses either label, the management approach is the same.

Do I need surgery after a Category 3 result?

Surgery is not the automatic next step. ATA and 2023 Bethesda System guidance recommends gathering more information — through repeat FNAC or molecular testing — before surgery is considered. If those tests suggest malignancy, or if the nodule has high-risk ultrasound features, diagnostic lobectomy of one lobe may then be recommended. The aim is to avoid operating on nodules that would turn out to be benign, which would happen in the majority of untested Category 3 cases.

How long can I safely wait before deciding on next steps?

There is no universal timeline. ATA guidance generally supports a three-to-six-month interval for repeat FNAC in lower-risk nodules. Waiting longer than your team recommends is not advisable — earlier confirmation of malignancy preserves more management options. The date your team gives you reflects their assessment of your individual risk. If anything changes before that date — a growing lump, voice changes, or difficulty swallowing — contact your team sooner.

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