Quick answer
Thyroid fine-needle aspiration samples cells from the nodule with a fine needle under ultrasound guidance for analysis. The result is classified using the Bethesda system (I to VI), which guides management: surveillance, repeat aspiration or active treatment.
What is thyroid fine-needle aspiration?
Fine-needle aspiration consists of sampling, with a fine needle under ultrasound guidance, a few cells from the thyroid nodule to analyse them under the microscope. It is offered when ultrasound identifies a nodule requiring more precise characterisation.
The Bethesda classification system
Results are classified into 6 categories, from Bethesda I (non-diagnostic) to Bethesda VI (malignant), including Bethesda II (benign, the most frequent), III and IV (indeterminate), and V (suspicious for malignancy).
What management follows each result?
A Bethesda II (benign) result most often allows simple surveillance, or even treatment by radiofrequency if the nodule is symptomatic. Indeterminate or suspicious categories warrant specialist discussion.
Frequently asked questions
Does a Bethesda II result rule out all risk?
What happens with an indeterminate result (Bethesda III/IV)?
Treatment of benign thyroid nodules by radiofrequency, without surgery and without a scar.
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Written by the Radiology team at Cochin Hospital AP-HP, Université Paris Cité. Medically reviewed by a radiologist of the department. Last updated: August 2026.