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Procedures · 2 min read

Ultrasound-guided thyroid fine-needle aspiration

How the needle looks under ultrasound during thyroid FNA, what a study comparing needle sizes found, and the six Bethesda reporting categories.

Key points

• Ultrasound guidance is the standard way to perform thyroid fine-needle aspiration.
• On ultrasound the needle is a bright straight line held in the plane of the probe, with its tip at the target.
• In a prospective study of 480 thyroid nodules, 25 G needles gave higher smear quality scores than 23 G needles; nondiagnostic rates did not differ significantly.
• Results are reported in six Bethesda categories.

Seeing the needle

Ultrasound guidance is the standard way to sample a thyroid nodule with a fine needle. The reason is simple: you can watch the needle all the way to the target.

In the image above, a straight bright line enters from the upper left and runs down towards a nodule in the isthmus. That line is the needle. It looks straight and continuous because it is held in the plane of the probe. If the needle drifts out of that plane, only part of it, or only a dot, is seen. Keeping the whole shaft and the tip in view is the core skill of an in-plane approach.

Needle size

Thin needles are used for thyroid aspiration. In a prospective study of 480 thyroid nodules, each sampled with needles of different gauges, 25 G needles gave higher smear quality scores than 23 G needles, both with aspiration and with the capillary (non-aspiration) technique. The nondiagnostic rate did not differ significantly between needle sizes.

Two ways to sample

The same study compared two techniques. In aspiration, suction is applied through the needle while it moves within the nodule. In the capillary technique, no suction is used and cells enter the needle as it moves. The thinner 25 G needle gave the better smear quality scores with both techniques, so the finding was not limited to one way of sampling.

How results are reported

Thyroid cytology is reported in six Bethesda categories:

1. nondiagnostic
2. benign
3. atypia of undetermined significance
4. follicular neoplasm
5. suspicious for malignancy
6. malignant

A shared set of categories lets the person who performs the aspiration, the cytopathologist and the clinician talk about the same result in the same words.

What this lesson does not cover

Which nodules need sampling, size thresholds, and what to do after each Bethesda category depend on guidelines and on the clinical context; they are outside the scope of this lesson.

Check yourself

How many diagnostic categories does the Bethesda system use to report thyroid FNA?

1. A. Four
2. B. Five
3. C. Six
4. D. Seven

Show the answer

Sources

1. Gimm O, Ivansson K, Beka E, Rossitti HM, Garvin S, Söderkvist P. Increased diagnostic sensitivity of palpation-guided thyroid nodule fine-needle aspiration cytology by BRAF V600E-mutation analysis. J Pathol Clin Res 2021;7(6):556-564. doi:10.1002/cjp2.231 · PMID 34156770
2. Dong Y, Gao L, Sui Y, Mao M, Zhan W, Zhou J. Comparison of ultrasound-guided fine-needle cytology quality in thyroid nodules with 22-, 23-, and 25-gauge needles. Anal Cell Pathol (Amst) 2021;2021:5544921. doi:10.1155/2021/5544921 · PMID 34211823
3. Ali SZ, Baloch ZW, Cochand-Priollet B, Schmitt FC, Vielh P, VanderLaan PA. The 2023 Bethesda System for Reporting Thyroid Cytopathology. J Am Soc Cytopathol 2023;12(5):319-325. doi:10.1016/j.jasc.2023.05.005 · PMID 37438235