To the best of our knowledge, this is the first MR study to assess the possible causal relationship and underlying mechanisms between GD and eight IMDs. The principal outcomes provide compelling evidence supporting that genetically predicted GD increased the risk of SLE and CD, and might be a risk factor for RA, but a protective factor for AD and asthma. In contrast, we found that genetically predicted RA was linked to a higher risk of GD, whereas UC and AD might be protective factors for GD, and CD was the opposite. High PP.H4 of colocalization analysis determined the shared SNPs between GD and IMDs.
Consistent with our findings, Kristien Boelaert and colleagues' study confirmed that RA is the most prevalent autoimmune disease that coexists with GD[13]. A systematic review conducted in 2020 provided support for a 1–3-fold increase in the prevalence of RA among patients with AITD, and conversely, a 1–6-fold increase in the prevalence of AITD among patients with RA[31]. The MR analysis indicated that patients with GD exhibit a 14% higher risk of developing RA, whereas those with RA have a 26% increased likelihood of developing GD. A cohort study of 8779 GD cases and 8779 controls in Taiwan showed that GD patients suffer an increased risk of SLE (hazard ratio [HR]: 5.45, 95%CI: 1.74-17.0) [32]. Based on our research, patients with GD have a higher chance of developing SLE as shown by a significant OR value of 1.31. And the outcomes of this study were in line with previous reports, supporting a causal relationship between GD and inflammatory bowel disease, including CD and UC[33]. In contrast to our findings, the results of observational studies indicated that AITD is the most prevalent comorbid autoimmune condition in T1DM[34]. Further, GD increases the rate of neuropathy[35]. The association between GD and AD has shown inconsistent findings in recent case-control studies[36, 37]. However, our findings indicated no association between GD and T1DM, while AD and GD were protective factors for each other. Besides, the available literature does not provide any large observational studies on asthma and ALS, however, there is evidence of co-occurrence with GD[38, 39]. And GWAS identifies shared genetic components between asthma and GD, while the MR results suggested that GD was a protective factor for asthma[40]. Confounding variables in observational studies have the potential to introduce bias into conclusions drawn from such research. Additionally, the functional implications of certain SNPs remain unclear and may impact MR results. These two factors could account for the inconsistent findings across studies.
We performed colocalization analyses to explore the potential shared genetic structure. According to the outcomes, the SNP rs12612769 of gene STAT4, located in chromosome 2, mediates the relationship between GD and SLE, RA. STAT4, activated by IL12, IFN-1, and IL23 cytokines, belongs to the STAT family[41]. It affects the differentiation of immune cells and the production of cytotoxicity and is associated with immune disorders and autoimmunity, including SLE and RA[42]. A recent study demonstrated that miR-363 could inhibit Treg cells through the target gene STAT4[43]. These findings suggest that STAT4 may play a significant role in the comorbidity of GD with RA and SLE. Similarly, the SNP rs9380340 of gene HLA-DPA1 may be a causal genetic variant between GD and CD. Previous studies indicated a strong association between HLA-DPA1 and GD[44]. And the research conducted in 2022 revealed a significant association between DPA1 and CD[45].
GD is a prevalent autoimmune disorder primarily affecting the thyroid gland, with its development influenced by both genetic and environmental factors[46]. Similarly, these factors also affect other IMDs[47–52]. Environment and shared genetic variants seem to be the potential mechanisms of the frequent co-occurrence of GD with other IMDs. As previously discussed, STAT4 is believed to be involved in the pathogenesis of GD, RA, and SLE. This correlation suggests that STAT4 may serve as an intermediary between these three conditions. Additionally, the impact of smoking on the development and progression of RA and GD, particularly in GD patients with ocular involvement, is significant. Smoking can significantly contribute to or exacerbate ocular symptoms. Therefore, smoking cessation is crucial for managing both diseases. Recently, the role of gut microbiota in the pathogenesis of these diseases has been revealed[53–55]. Variations in gut microbiota among individuals may impact the pharmacokinetics of anti-rheumatic drugs, resulting in divergent treatment outcomes at an individual level[56]. In addition, the gut microbiota and its metabolites can influence the bioavailability of intestinal microelements as well as immune regulation, which mediates the development of thyroid disease[57]. Microbial-targeted therapy may be a common strategy for the treatment of IMDs in the future.
From a clinical perspective, the presence of comorbidity can significantly impact patients' quality of life; however, it may also indicate shared etiology and pathological mechanisms among certain diseases. Consequently, drugs used to treat one condition could potentially be effective in treating another. For example, the traditional antirheumatic drugs chloroquine and hydroxychloroquine have shown great potential in the treatment of GD by inhibiting autophagy in orbital fibroblasts[58].
In summary, GD exhibits intricate associations with various IMDs through multifaceted mechanisms. Further research is imperative to elucidate the underlying mechanisms and develop effective treatments for these diseases.
Our study had several limitations. First, the limited number of cases may affect the genetic representation of the disease from this GWAS data. Second, there may be some overlap in the GWAS population, which may affect the reliability of the study conclusions. Finally, the GWAS data is derived from the East Asian population, necessitating further investigation to ascertain its generalizability across diverse ethnic groups. Therefore, a larger sample size and the involvement of different ethnic groups are needed for further research.