Our MR analysis concluded a positive causal correlation between autoimmune hyperthyroidism and KOA, while no significant correlation was found with HOA. Conversely, no genetic causality was found between autoimmune hypothyroidism and knee osteoarthritis or hip osteoarthritis.
This study marks the pioneering endeavor to investigate the genetic causality between autoimmune hyperthyroidism and OA. Interestingly, our findings align with a recent comprehensive review indicating that elevated levels of FT3 and FT4 are linked to an increased risk of OA, consistent with our MR results [33]. And previously there is a genetic relationship between thyroid function status and osteoarthritis, it has been shown that DIO2 is a gene that can make inactive T4 to active T3, and upregulation of DIO2 can promote the increase of T3([34] [35]), and in the cartilage, the intracellular bioavailability of T3 stimulates the differentiation of chondrocytes, inhibits the proliferation of chondrocytes and induces chondrocytes to hypertrophy, and initiates terminal differentiation and calcification. Chondrocyte hypertrophy impairs cartilage viability by altering the expression of bone-specific collagen, which triggers calcification of the cartilage matrix and upregulation of cartilage-specific protein hydrolases. It is well known that most of the cartilage matrix consists of collagen and proteoglycans, and that chondrocyte-specific protein hydrolases hydrolyze collagen and proteoglycans, which in turn damage articular cartilage, leading to osteoarthritis༈[34] [36] [37]༉, which is also elevated by autoimmune hyperthyroidism T3, which likewise leads to osteoarthritis by the same mechanism. And further research is needed as to why it leads to KOA but not HOA. Although many epidemiologic studies had found an association between arthritis incidence and hypothyroidism[15], our study's genetic perspective did not establish a causality between autoimmune hypothyroidism and OA. However, this does not exclude the possibility of other non-genetic factors potentially influencing the association between autoimmune hypothyroidism and OA. Several studies have proposed that the reduced TSH in patients with autoimmune hypothyroidism may be a contributing factor to knee osteoarthritis [15].
Our study has several strengths. First, in contrast to observational research, we employed MR for causal inference, utilizing genetic variations as IVs. This approach mitigates the influence of confounding factors and reverse causality, enhancing the robustness of our findings. Second, the exclusive inclusion of samples from European populations bolsters the internal consistency of our results by minimizing the impact of demographic variations. Most notably, we unveil, for the first time, a causality between AITD and OA in genetical, potentially opening new avenues for OA treatment.
On the other hand, we should also recognize certain limitations. First, we did not use SNP data for various thyroid hormone subtypes (e.g., T3, T4) and thyroid autoantibodies (e.g., TPOAb and TgAb) due to the study conditions. Therefore, we were unable to determine whether thyroid hormone subtypes or thyroid autoantibodies caused osteoarthritis of the knee. Secondly our study concluded that AITD is not genetically correlated with hip osteoarthritis, and due to conditioning we did not use thyroid autoantibodies and thyroid hormone subtypes as exposures to determine if there was a correlation between them and hip osteoarthritis. Third, this MR study only examined the relationship between AITD and OA; the relationship between other thyroid diseases and OA will be discussed in further studies. Fourth, because all participants in our study were of European descent, it remains uncertain whether the conclusions can be generalized to other population groups.
Moreover, although some of the methods used in our study yielded results that deviated from the IVW method, we dominated the results derived from IVW. Nevertheless, all SNPs used in our MR analysis followed the assumption of valid instrumental variables, and the MR-presso findings supported the IVW outcomes., which affirms the reliability of our findings.