Your Thyroid Biopsy Came Back “Indeterminate”: What Bethesda III (3) and IV (4) Really Mean
Written by John P. Sabra, MD FACS
Updated May 2026
Educational only. This article is not medical advice. Always consult your physician about your individual situation.
You had the biopsy. You waited for the results. And the answer that came back was not the one you were hoping for. Instead, the result says “indeterminate.” Or your physician used the words Bethesda 3, or Bethesda 4, or AUS, or follicular neoplasm — and you left the appointment with more questions than you arrived with.
Roughly 15 to 30% of all thyroid biopsies return an indeterminate result. An indeterminate result is not a cancer diagnosis, and it is not a failed test. It is an honest reflection of what thyroid cytology can and cannot tell us.
What “Indeterminate” Actually Means
The pathologist received an adequate sample of cells, examined them carefully, and found that those cells fall into a category where the visual information available under the microscope is genuinely insufficient to classify the nodule as definitively benign or malignant. This is not an error. It is a fundamental characteristic of thyroid follicular cells — they look remarkably similar whether they are part of a benign adenoma or a follicular carcinoma. The distinction depends on whether the tumor has invaded its own capsule or blood vessels, which can only be assessed by examining the complete removed nodule, not a needle sample.
Bethesda III: Atypia of Undetermined Significance (AUS/FLUS)
AUS means some cells show mild nuclear abnormalities — slightly beyond what would be expected in normal benign cells, but well short of the malignant threshold. FLUS means the architectural arrangement of follicular cells cannot be definitively classified. Estimated malignancy risk is 10 to 30%. The majority of Bethesda III nodules turn out to be benign when definitively evaluated.
Bethesda III is the most varied category in the system. The malignancy risk at a high-volume academic center may be at the lower end of that 10 to 30% range. Ask your physician what the institution-specific malignancy rate is at the practice where your biopsy was read — that number is more informative than the published range.
Bethesda IV: Follicular Neoplasm
Bethesda IV carries a malignancy risk of 25 to 40%. The pathologist found follicular cells in a pattern that could be either a benign follicular adenoma or a follicular carcinoma. The management pathway tends to be more active — molecular testing can reclassify many Bethesda IV nodules as likely benign, potentially avoiding surgery.
Some Bethesda IV results include the designation of Hurthle cell neoplasm (oncocytic neoplasm). Management is similar, though molecular testing performance may differ for this subtype. If your result mentions Hurthle cells, ask your physician specifically how this affects the recommended next step.
Bethesda III vs. Bethesda IV: Key Differences
| Feature | Bethesda III (AUS/FLUS) | Bethesda IV (Follicular Neoplasm) |
|---|---|---|
| Malignancy risk | 10–30% | 25–40% |
| Most common next step | Repeat FNA or molecular testing | Molecular testing or diagnostic lobectomy |
| What benign molecular result means | Can typically avoid surgery with close surveillance | Significantly reduces risk; lobectomy may still be recommended depending on size and clinical factors |
The Three Management Options
| Option | What It Involves | Key Tradeoff |
|---|---|---|
| Repeat FNA | A second needle biopsy, often with technical adjustments | May return another indeterminate result for genuinely difficult follicular lesions |
| Molecular testing | Gene expression analysis of existing biopsy sample (Afirma, ThyroSeq) | Substantially reduces but does not eliminate cancer risk; cost and coverage vary |
| Diagnostic lobectomy | Surgical removal of affected half of thyroid for definitive examination | Involves surgery and recovery; may result in hypothyroidism requiring lifelong medication |
The Emotional Reality of an Indeterminate Result
An indeterminate result is psychologically harder for many patients than a definitive result — even a difficult one. When a result is benign, you can exhale. When it is malignant, you can grieve and move into action. An indeterminate result gives you neither release. That suspended state is genuinely difficult, and the anxiety it produces is real and valid.
What helps most patients is a clear understanding of what the specific next step is and when it will happen. If you leave your physician’s office without a specific timeline and defined next step, ask for one. A concrete timeline transforms an open-ended wait into a defined period with an end point.
The other thing worth holding onto: even at the upper end of the Bethesda IV malignancy range, more than half of indeterminate nodules turn out to be benign when definitively evaluated.
The Bottom Line
An indeterminate thyroid biopsy result is genuinely uncertain. What it is not is a dead end. The three management pathways are well-established and produce definitive answers in the vast majority of cases. Leave your physician’s office with a specific plan and timeline attached to it — that is the version of this situation that is manageable.
References
- Haugen BR, et al. 2015 ATA Management Guidelines for Thyroid Nodules and DTC. Thyroid. 2016.
- American Thyroid Association — Fine-Needle Aspiration Biopsy of Thyroid Nodules.
- ATA Professional Guidelines.
This article was written by John P. Sabra, MD FACS and is intended for patient education only. It does not constitute medical advice and does not replace a consultation with your physician.
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