Sex Differences in Autoimmune Multimorbidity Across Eleven Disorders: A Real-World Primary Care Study in Germany

<i>Background and Objectives</i>: While most studies have focused on the incidence, pathogenesis, and severity of individual autoimmune diseases (AIDs), limited attention has been given to <i>autoimmune multimorbidity</i>-the co-occurrence of multiple AIDs in a single individual. This study aims to examine sex differences in autoimmune multimorbidity across eleven AIDs in a real-world setting. <i>Materials and Methods</i>: This retrospective cross-sectional study analyzed data from the Disease Analyzer database (IQVIA) and included 164,596 individuals who visited one of 1037 primary care physicians in 2024. All patients had been diagnosed with at least one of eleven predefined AIDs between 2020 and 2024. For each AID, the prevalence of autoimmune multimorbidity was compared descriptively between female and male patients. <i>Results</i>: The total number of patients varied substantially across conditions. The highest numbers were observed for autoimmune thyroiditis (<i>n</i> = 51,765), psoriasis (<i>n</i> = 39,063), and rheumatoid arthritis (<i>n</i> = 33,182), while the lowest numbers were observed for systemic lupus erythematosus (<i>n</i> = 897) and Sjögren's syndrome (<i>n</i> = 2728). A notable sex disparity was present in several conditions. For instance, 30.2% of women with systemic lupus erythematosus had at least one additional AID, compared to 25.4% of men; 31.6% of women with Sjögren's syndrome, compared to 20.7% of men; 28.5% of women with ankylosing spondylitis, compared to 18.6% of men; and 20.7% of women with celiac disease, compared to 12.5% of men. In contrast, autoimmune thyroiditis and rheumatoid arthritis exhibited smaller sex-related differences in autoimmune multimorbidity. Adjusted analyses confirmed these differences after accounting for age and clustering by practice. <i>Conclusions</i>: This study reveals significant sex differences in autoimmune multimorbidity among individuals with predefined AIDs in a real-world primary care setting. The findings support the hypothesis that women may be more prone to coexisting autoimmune conditions due to underlying hormonal, genetic, or immunological factors.