Abstract: Purpose: Our purpose was to determine the risk of malignancy of the incidentally discovered thyroid
nodule by assessment of the sonographic and color duplex features. Patients and Methods: One hundred fifty eight
non-palpable thyroid nodules in one hundred and seventeen patients underwent ultrasonographic and color duplex
examinations and ultrasound guided fine needle aspiration biopsy with adequate cytological material. Each nodule
was evaluated by nine specific sonographic and color duplex features: echogenicity, echostructure, size, shape,
borders, microcalcifications, halo sign, vascularity and resistive index of the flow. All patients with malignant or
suspicious lesions underwent surgery which was also done for non-cytological indications. Results: Thyroid
malignancies were observed in 11 of the 158 nodules. The prevalence of malignancy in our study was estimated to
be 7%. Intergroup comparison of the sonographic and color duplex features among the benign and malignant
nodules resulted in indication of RI of more than 0.75 and presence of microcalcifications are both sensitive and
specific predictor of malignancy (81.8% and 63.6% sensitivity & 98.6% and 87.1% specificity). Solid echotexture is
sensitive (81.8%) but not a specific (28.8%) predictor. Marked nodal hypoechogenicity, tall nodal shape and halo
sign showed high specificity to predict malignancy (95.9%, 91.8%, and 82.3% respectively), nevertheless, all
showed low sensitivity of 45.4%, 36.4% and 27.3%. Nodal hypoechogenicity, nodule size  10 mm and nodules
with blurred margins were non-reliable sonographic features for prediction of malignancy. Conclusion: Our study
indicated that the presence of RI more than 0.75 is the only both most sensitive (81.8%) and most specific (98.6%)
feature predicting malignancy of the thyroid nodules.