Abstract: Objective To investigate the accuracy of fine needle aspiration cytology(FNAC), multimodal ultrasound and the combination of the two in the diagnosis of thyroid nodules of different sizes. 
Methods We retrospectively selected 152 patients(160 nodules) with thyroid nodules. All patients underwent FNAC, multimodal ultrasound and histopathological examinations, and divided into different groups according to the size of the nodules: group A(≤10 mm, 64 nodules), group B(10-30 mm, 61 nodules), and group C(≥30 mm, 35 nodules). Histopathology was used as the gold standard, to analyze and compare the diagnostic accuracy of FNAC, multimodal ultrasound and the combination of the two. 
Results There were 75 malignant nodules detected in 160 thyroid nodules by histopathology, and 5 grade I nodules, 45 grade Ⅱ nodules, 25 grade Ⅲ nodules, 23 grade Ⅳ nodules, 27 grade Ⅴnodules and 35 grade VI nodules detected by FNAC examination. The accuracy and sensitivity of FNAC in groups B and C were significantly higher than those of group A, and the specificity in group B was significantly higher than that of groups A and C(P＜0.05). The accuracy, specificity, and sensitivity of the three groups of multimodal ultrasound were not statistically different(P＞0.05). The accuracy, specificity, and sensitivity of combined detection in group A were significantly higher than those of FNAC(P＜0.05); The accuracy and specificity of combined detection in group C were significantly higher than those of FNAC(P＜0.05). 
Conclusion Thyroid nodules ≤10 mm and ≥30 mm in size will reduce the accuracy of FNAC in the diagnosis of benign and malignant thyroid nodules. Combination with multimodal ultrasound diagnosis is beneficial to improve the accuracy of FNAC in the diagnosis of benign and malignant thyroid nodules ≤10 mm and ≥30 mm in size.

Key words: thyroid nodule, biopsy, fine-needle, multimodal ultrasound