Background: To evaluate how to correctly manage thyroid nodules showing inadequate cytology after ultrasound (US)-guided fine-needle aspiration biopsy (US-FNAB). Materials and Methods: A total of 393 thyroid nodules with inadequate cytology in 351 patients (M:F = 40:311, mean age: 49.3 years) with surgery or follow-up US-FNAB for at least 1 year were included in this study. Among them, 293 (74.6%) were benign and 100 (25.4%) were malignant on final reference results. Clinical characteristics and US features were reviewed and compared. Results: Malignancy rates (39.5%) of nodules having suspicious US feature were significantly higher than those (10.9%) of nodules without any suspicious US feature (P < .001). Malignancy rates of solid nodules, mainly solid nodules, and mainly cystic nodules were 29.2, 16.7, and 9.5%, respectively, with significant differences (P = .016). Malignancy rates of nodules assessed as suspicious malignant to probably benign in composition are: 39.1-12.8% (P < .001) in solid nodules, 42.1-9.2% (P = .001) in mainly solid nodules, and 50.0-5.3% (P = .04) in mainly cystic nodules. Conclusions: In nodules with inadequate cytology, follow-up US can be considered over repeat aspiration if there are no suspicious US features present, especially in mainly cystic nodules.
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