Background <p>Eyelid retraction associated with thyroid eye disease (TED) is a challenging condition, leading to aesthetic concerns and potential ocular surface complications. We aimed to evaluate changes in lower eyelid position before and after Müllerectomy for the correction of unilateral or bilateral upper eyelid retraction in patients with TED.</p> Methods <p>This prospective interventional case series included 32 eyes from 25 patients with TED-related upper eyelid retraction. Eyelid positions were assessed using marginal reflex distance (MRD-1 for the upper eyelid and MRD-2 for the lower eyelid), both preoperatively and postoperatively. Multivariate analysis was performed to determine associations between postoperative MRD-2 changes and various preoperative and intraoperative factors, including history of orbital decompression, demographic characteristics, and eyelid position parameters.</p> Results <p>Six months postoperatively, MRD-1 was significantly reduced compared to preoperative values (4.3 ± 1.5 mm vs. 7.2 ± 1.1 mm, <i>p</i> &lt; 0.001). Conversely, MRD-2 showed a significant increase (6.1 ± 1.5 mm vs. 5.6 ± 1.2 mm, <i>p</i> = 0.001). Although a negative correlation was observed between changes in MRD-1 (−2.9 ± 1.6 mm) and MRD-2 (0.5 ± 0.7 mm), this was not statistically significant (<i>r</i> = −0.344, <i>p</i> = 0.054). Multivariate analysis revealed no significant associations between postoperative MRD-2 changes and any preoperative or intraoperative factors.</p> Conclusion <p>Müllerectomy for upper eyelid retraction correction in TED may result in a statistically significant but clinically negligible inferior shift of the lower eyelid.</p> Level of Evidence IV <p>This journal requires that authors assign a level of evidence to each article. For a full description of these evidence-based medicine ratings, please refer to the Table of Contents or the online Instructions to Authors <a href="http://www.springer.com/00266">www.springer.com/00266</a>.</p>