Summary objective In the present study we have performed a grey-scale quantitative analysis of thyroid echogenicity in the patients affected by Hashimoto's thyroiditis (HT), obtaining a numerical estimate of the degree of hypoecogenicity associated with the appearance of thyroid dysfunction. materials and methods The study group included 89 patients with serum positivity for thyroglobulin (TgAb) and/or peroxidase (TPOAb) antibodies. Ultrasound (US) evaluation of thyroid gland and biochemical assay of serum thyrotropin (TSH), free-thyroxine (FT4) and free-triiodiothyronyne (FT3) were performed in all patients, and in 40 healthy subjects enrolled as control group. Thyroid echogenicity was compared with that of the surrounding neck muscles, using the grey-scale histogram analysis. The echogenicity was expressed in grey-scales (GWE). results In HT patients, the mean of thyroid echogenicity was lower when compared to the normal thyroid (61·9 ± 8·3 GWE vs. 71·9 ± 3·1 GWE; P = 0·01). In all HT patients the lowest limit of thyroid echo distribution was in the echogenicity range of the surrounding muscle, the overlapping ranging between 3·4% and 95·0% (mean ± SD 48·4 ± 20·9%). The extension of like-muscle hypoechogenicity into the thyroid gland was significantly correlated with serum TSH values (r = 0·37; P < 0·001), serum FT4 values (r = –0·60; P < 0·001), and serum TPOAb values (r = 0·31; P = 0·004). Nobody was hypothyroid when the hypoechogenicity was less than 38·0%, whereas hypothyrodism occurred in all cases with hypoechogenicity of more than 68·9%. The receiving operating characteristic curve demonstrated that 48·3% was the best cut-off for identifying hypothyroid patients with sensitivity, specificity and diagnostic accuracy of 88·9%, 86·3% and 87·6%, respectively. conclusions In conclusion, the grey-scale quantitative analysis has... [ABSTRACT FROM AUTHOR]