Thyroid FNA - Bethesda Reporting and Follow Up

Key Points

• Bethesda and TI-RADS answer complementary rather than interchangeable questions.
• Bethesda I is nondiagnostic and usually requires repeat sampling if the nodule still warrants biopsy.
• Bethesda II is benign and usually returns to ultrasound-based follow-up.
• Bethesda III and IV are indeterminate categories in which molecular testing can be useful.
• Follicular carcinoma cannot be diagnosed by FNA solely from a follicular pattern because capsular or vascular invasion requires histology.
• Bethesda V and VI generally prompt surgical or oncology referral.
• Molecular-test predictive value depends on pretest risk and local cancer prevalence.
• Always assess radiologic-pathologic concordance.

Algorithm

1. Confirm that the nodule met an appropriate indication for FNA.
2. Perform ultrasound-guided sampling and provide relevant imaging and clinical history to cytopathology.
3. Assign one Bethesda category and describe significant qualifiers.
4. For Bethesda I, repeat FNA or pursue alternative evaluation based on ultrasound risk.
5. For Bethesda III or IV, choose among repeat FNA, molecular testing, surveillance, or diagnostic surgery according to risk.
6. For Bethesda V or VI, refer for definitive thyroid cancer management.
7. Reconcile cytology with TI-RADS findings and cervical lymph-node assessment.
8. Continue long-term surveillance or postoperative care according to the final diagnosis.

Clinical Synopsis & Reasoning

Thyroid FNA answers a different question from TI-RADS. Ultrasound estimates pretest malignancy risk and determines whether a nodule should be sampled; cytology then classifies the sampled cells. The Bethesda system uses six categories: I nondiagnostic, II benign, III atypia of undetermined significance, IV follicular neoplasm or suspicious for follicular neoplasm, V suspicious for malignancy, and VI malignant. The 2023 third edition updates terminology and risk-of-malignancy estimates in the era of reclassification of noninvasive follicular thyroid neoplasm with papillary-like nuclear features and widespread molecular testing.

Bethesda I reflects inadequate cellularity or preservation and usually requires repeat ultrasound-guided FNA after the reason for inadequacy is considered. Bethesda II includes benign follicular nodules, thyroiditis, and other benign processes and generally returns to ultrasound-based follow-up. Bethesda III is an indeterminate category for limited atypia that is insufficient for a higher category. Overuse of Bethesda III reduces its clinical value, so the report should explain the atypical feature when possible.

Bethesda IV captures follicular-patterned neoplasms for which cytology cannot determine capsular or vascular invasion. Bethesda V has features strongly concerning for malignancy but short of definitive criteria. Bethesda VI includes cytologically malignant tumors such as papillary thyroid carcinoma, medullary carcinoma, anaplastic carcinoma, or metastatic malignancy when appropriately identified.

Treatment Strategy & Disposition

Management depends on category and clinical context. Repeat FNA is common for Bethesda I and can be considered for Bethesda III. Molecular testing may help risk-stratify Bethesda III and IV nodules when results would change the choice between surveillance and surgery. Molecular tests vary in whether they are best used to rule out or rule in malignancy, and predictive values depend on the baseline cancer prevalence in the practice population.

Bethesda IV often leads to diagnostic lobectomy or molecular testing, while Bethesda V and VI usually prompt surgical or oncology referral. The extent of surgery depends on tumor type, nodule size, ultrasound findings, nodal disease, contralateral nodules, patient age, comorbidity, and preference. A diagnosis of medullary thyroid carcinoma triggers a different workup, including calcitonin-related evaluation and consideration of RET testing, than papillary carcinoma.

Radiologic-pathologic concordance is critical. A highly suspicious TR5 nodule with repeatedly benign or nondiagnostic cytology warrants reassessment, possibly repeat biopsy or surgical consultation. Conversely, a benign cytology result from a low-risk nodule should not lead to serial biopsy solely because the lesion persists. FNA technique, on-site adequacy assessment, and experienced cytopathology can reduce nondiagnostic rates.

Epidemiology / Risk Factors

• Thyroid nodules are common but most are benign.
• A minority of FNAs fall into indeterminate Bethesda III or IV categories.
• Malignancy risk rises stepwise from Bethesda II through VI.
• Widespread molecular testing has reduced diagnostic surgery for some indeterminate nodules.

Investigations

Bethesda Overview

Pharmacology

Prognosis / Complications

• Benign cytology carries a low malignancy risk when imaging is concordant.
• Most differentiated thyroid cancers have an excellent prognosis.
• Medullary and anaplastic carcinoma have different biology and require distinct management.
• Indeterminate cytology prognosis depends on the final histology rather than the Bethesda label itself.

Patient Education / Counseling

• Explain the category in plain language and whether it means benign, indeterminate, suspicious, or malignant.
• For indeterminate results, discuss the purpose and limitations of molecular testing before ordering it.
• Tell patients that a follicular neoplasm category is not the same as a diagnosis of follicular cancer.
• Provide a specific plan for repeat FNA, ultrasound, molecular testing, or surgical consultation.
• Encourage patients to retain the ultrasound and pathology reports because concordance matters over time.

Notes

Bethesda risk estimates are ranges, not guarantees. They vary with case mix, diagnostic thresholds, and whether noninvasive follicular thyroid neoplasm with papillary-like nuclear features is counted as malignant in a given dataset.

The most important quality check is concordance. Cytology should not be interpreted in isolation from the ultrasound target that generated the biopsy.

References

1. The 2023 Bethesda System for Reporting Thyroid Cytopathology — Link
2. American Thyroid Association: Thyroid Nodules and Differentiated Thyroid Cancer Guideline — Link