Fine-Needle Aspiration Biopsy of Thyroid Nodules

Fine-needle aspiration biopsy is the diagnostic standard for evaluating thyroid nodules sized 1 cm or larger, or smaller nodules with high-risk sonographic features. The procedure is performed under ultrasound guidance with a 25-27 gauge needle, and 2-4 passes are typically obtained to ensure adequate cellularity. Indications follow ultrasound risk stratification systems such as ACR TI-RADS or ATA categories, where nodules with hypoechogenicity, irregular margins, microcalcifications, or taller-than-wide shape are prioritized for sampling.

Cytology results are reported using the Bethesda System for Reporting Thyroid Cytopathology, which classifies findings into six categories: I-nondiagnostic (5-10% malignancy risk, repeat FNA), II-benign (less than 3% risk, surveillance), III-atypia of undetermined significance (10-30%, molecular testing or repeat), IV-follicular neoplasm (25-40%, lobectomy), V-suspicious for malignancy (60-75%, surgery), and VI-malignant (97-99%, surgery). Bethesda III and IV are indeterminate categories where molecular tests such as Afirma GSC, ThyroSeq v3, or ThyGeNEXT/ThyraMIR can refine cancer risk and reduce diagnostic surgery.

Complications are rare and include minor bleeding, hematoma, transient pain, and very rarely needle tract seeding. The procedure is well tolerated and can be repeated for nondiagnostic samples. After cytologic diagnosis, management ranges from continued ultrasound surveillance for benign nodules, to thyroid lobectomy or total thyroidectomy for indeterminate or malignant lesions, sometimes with central neck dissection for documented cancer. Active surveillance has emerged as an option for low-risk papillary microcarcinomas in select patients.