Suspicious thyroid nodules 4 cm require a diagnostic lobectomy regardless of their benign fine needle aspiration results.

Objective: The diagnostic accuracy of fine needle aspiration biopsy (FNAB) seems limited in large thyroid nodules with Bethesda Cat. 2 result. We aimed to determine the incidence of carcinoma with benign cytology and the reason for the high false-positive rate in thyroid nodules ≥4 cm.

Methods: The records of 103 patients with thyroid nodules ≥4 cm with preoperative cytological diagnosis of Bethesda Cat. 2 who underwent thyroidectomy were consecutively reviewed. Characteristics between patients with malignant vs. benign pathology were compared.

Results: Forty patients (38.8%) had malignancy. Malignancy was subclassified into follicular variant of papillary thyroid carcinoma (43%), minimally invasive follicular thyroid carcinoma (20.0%), and minimally invasive Hurthle cell thyroid carcinoma (10.9%). Patients with malignant cytology had significantly more suspicious ultrasound findings than those with benign cytology (p = 0.001).

Conclusions: Preoperative FNAB showed high false-negative rates in patients with thyroid nodules ≥4 cm with benign cytology. These nodules have a high malignancy rate with suspicious ultrasound findings.