Background <p>Gilbert syndrome (GS) is an inherited disorder characterized by unconjugated hyperbilirubinemia as a result of the reduced transcriptional activity of proteins of UGT1A1. The roles and mechanisms of promoter polymorphism, gene expression, and inflammation are not fully understood, particularly in Middle Eastern populations, including Iraq.</p> Methods <p>A case-control study with 50 GS patients and 50 healthy controls was conducted for identifying the UGT1A1 rs8175347 polymorphism using PCR and Sanger sequencing. RT-qPCR was used to evaluate gene expression, whereas ELISA was used to detect IL-1β levels. Multiple inheritance models were applied to assess genetic association, and regression analyses were used to identify independent predictors.</p> Result <p>GS patients demonstrated much more prevalently expanded TA-repeat alleles (TA₇/TA₈) than healthy controls (74% versus 6%, <i>p</i> &lt; 0.001). UGT1A1 expression was significantly reduced in patients (<i>p</i> &lt; 0.001). Serum levels of IL-1β were increased and showed good diagnostic accuracy (AUC = 0.90). Although genotype did not correlate with gene expression in univariate analysis, it appeared as an independent predictor after multivariate analysis (β = -0.583, <i>p</i> = 0.011). The UGT1A1 promoter genotype was an independent predictor of GS susceptibility.</p> Conclusion <p>UGT1A1 promoter polymorphism appears to be strongly associated with GS susceptibility and transcriptional downregulation, whereas IL-1β may reflect a secondary, non-regulatory inflammatory response.</p>