['Thyroid ultrasound elastography (US elastography) provides an estimation of tissue stiffness and is helpful to differentiate malignant from benign lesions. Tissue proprieties and molecules causing stiffness are not established. The aim of the study was to correlate US elastography findings with tissue properties in thyroid nodules.', '115 thyroid nodules from 112 patients who underwent surgery for the presence of of Thy 3 (indeterminate) cytology (n=67), Thy 4-5 (suspicious-indicative of carcinoma) cytology (n= 47) or large goiter in the presence of Thy 2 cytology (n= 1) and suspicious US features were examined by US elastography. Tissues obtained after surgery were characterized for cell number, microvessel density, fibrosis, expression of galectin-3 (Gal-3) and fibronectin-1(FN-1).', 'Low elasticity at qualitative US elastography (LoEl) was found in 66 nodules, 1 benign and 65 carcinomas, high elasticity (HiEl) in 49 nodules, 46 benign and 3 carcinomas (p<0.0001). Quantitative analysis, performed in 24 nodules and expressed as elastic ratio between the strain of the nodule and that of the surrounding thyroid parenchyma, was 1.90 (1.18-2.77) (median and IQR) in 14 nodules with LoEl and 1.01-(0.91-1.10) in 10 nodules with HiEl (p=0.002). Stiffness did not correlate with cell number and was inversely correlated with microvessel density. Fibrosis was higher in nodules with LoEl than in those with HiEl (p=0.009) and in carcinomas than in benign nodules (p=0.02). Fibrosis was higher in nodules with high expression of Gal-3 (p<0.001) and FN-1 (p=0.004). Fibrosis and expression of Gal-3 and FN-1 were higher in the classic vs the follicular variant of papillary thyroid carcinoma and lower in follicular adenoma.'] Is low elasticity of thyroid nodules at ultrasound elastography correlated with malignancy , degree of fibrosis and high expression of galectin-3 and fibronectin-1?