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Purpose To investigate the diagnostic value of ultrasound gray scale ratio (UGSR) for differentiating papillary thyroid microcarcinomas (PTMCs) and benign micronodules (BMNs) in patients with HT.Methods The ultrasound images of 285 PTMCs (in 247 patients) and 173 BMNs (in 140 patients) in the HT group, as well as 461 PTMCs (in 417 patients) and 234 BMNs (in 197 patients) in the non-HT group were retrospectively analyzed. All cases were confirmed by histological examinations. The gray scale values of the nodules and surrounding thyroid tissues were measured and subsequently the UGSR was calculated. Receiver operating characteristic curve was used for determining the area under the curve (AUC), optimal UGSR threshold, sensitivity and specificity in differentiating PTMCs and BMNs in the two groups.Results The UGSRs of PTMCs and BMNs were 0.52±0.12 and 0.85±0.24 (P<0.001) in the HT group and 0.57±0.13 and 0.87±0.20 (P<0.001) in the non-HT group, respectively. The differences in the UGSRs of PTMCs were significantly different between the two groups (P<0.001), whereas the difference in the UGSRs of BMNs was not significantly different between the two groups (P=0.416). The AUC, optimal UGSR threshold, sensitivity and specificity of UGSR for differentiating PTMCs and BMNs in the HT and non-HT groups were 0.901 and 0.890, 0.727 and 0.687, 82.05% and 77.46% and 90.67% and 91.23%, respectively.Conclusions UGSR exhibits important diagnostic value for differentiating PTMCs from BMNs in both HT and non-HT groups, and the USGR was lower in the HT group compared with that in the non-HT group.