<p>Elevated blood viscosity (BV) is a key factor in vascular diseases; however, its role in thyroid-associated ophthalmopathy (TAO) and its association with autoimmune activity remain unclear. This study examined the relationship between BV, thyroid-stimulating immunoglobulin (TSI) levels, and clinical parameters in TAO. Specifically, we measured the systolic BV (SBV) and diastolic BV (DBV) of 70 patients with TAO and 48 controls using a scanning capillary tube viscometer and examined the correlation between BV and clinical and laboratory markers. Patients with TAO were stratified according to the median BV to assess the differences in TSI and thyroid function markers. Patients with TAO had significantly higher median SBV (<i>p</i> = 0.028) and DBV (<i>p</i> = 0.006) than those in the control group. BV correlated positively with TSI (rho = 0.241, <i>p</i> = 0.044 for SBV; rho = 0.239, <i>p</i> = 0.046 for DBV). T3 showed a weak positive correlation in univariable analysis; however, in multivariable analy`sis, only TSI remained significantly associated with SBV (<i>p</i> = 0.019) and DBV (<i>p</i> = 0.015). TAO was associated with elevated systemic BV, likely related to autoimmune activity and largely explained by hematologic parameters. These findings highlight the interplay between autoimmunity, hematology, and blood rheology.</p>