Spontaneous abortion (SA), defined as unintended pregnancy loss before 20 weeks of gestation, is a common adverse pregnancy outcome. It occurs in approximately 12-24% of clinically recognized pregnancies (1975), with recurrent spontaneous abortion (RSA) affecting 1-5% of couples (Quenby et al., 2021, Stephenson, 2001). The causes of SA are multifactorial, involving genetic, autoimmune, thrombophilic, endocrine, and metabolic disorders. Nevertheless, the underlying pathophysiology remains unknown in nearly 50% of cases (Sonehara et al., 2024). Among known risk factors, the genetic polymorphism such as a homozygous mutation of methylenetetrahydrofolate reductase (MTHFR) C677T has been implicated in SA (Li et al., 2023, Liu et al., 2019). This genotype is associated with a higher risk of RSA compared to the MTHFR A1298C genotype (Zhao et al., 2020). Furthermore, allele-specific methylation of the MTHFR gene significantly increases RSA risk (Zhu et al., 2023).

The MTHFR enzyme is essential for folate metabolism and catalyzes the remethylation of homocysteine (HCY) to methionine. The homozygous C677T (TT) genotype reduces the MTHFR enzyme activity, leading to hyperhomocysteinemia (HHCY) (Wang et al., 2025). Elevated HCY levels promote endothelial dysfunction, prothrombotic states, oxidative stress, and impaired uteroplacental circulation (Cheng et al., 2021, Cheng et al., 2011), all of which are recognized mechanisms contributing to SA (Dai et al., 2021a). Paternal MTHFR C677T polymorphism may also influence pregnancy outcomes in their partners. The paternal 677 T allele is associated with an increased risk of RSA in their partners (Lin et al., 2019). The homozygous C677T mutation is more prevalent in men from RSA couples than the A1298C homozygous variant (Yenicesu et al., 2010). Moreover, the TT genotype may compromise sperm DNA integrity by affecting DNA methylation in men, thereby increasing the risk of SA (Liu et al., 2019). Parental HHCY is another recognized risk factor for RSA (Govindaiah et al., 2009), and elevated HCY levels are consistently observed in men whose partners have experienced adverse pregnancy outcomes (Feng et al., 2024).

Thyroid function plays a crucial role in reproductive health, and the MTHFR C677T polymorphism may also increase the risk of hypothyroidism (Yang et al., 2022), which is often driven by autoimmune thyroid disease (AITD) (Wei et al., 2024). AITD, characterized by the presence of antibodies against thyroid peroxidase (TPO-Ab), thyroglobulin (Tg-Ab), or the thyroid-stimulating hormone (TSH) receptor (TR-Ab), is strongly associated with SA (Huisman et al., 2023, Knøsgaard et al., 2023, Zhang et al., 2023a). Thyroid disorders also adversely affect male fertility and sperm morphology (Krassas et al., 2010). The presence of thyroid antibodies is significantly correlated with pathozoospermia and asthenozoospermia (Trummer et al., 2001), which are linked to adverse pregnancy outcomes in their partners (Dai et al., 2021b). These findings indicate that autoimmune thyroid disease in men may also influence reproductive success. A recent study (Kolanis et al., 2025) has reported a higher frequency of T allele sequences (CT and TT variants) and total T alleles of the MTHFR C677T polymorphism among children and adolescents with AITD, further supporting a genetic link between this variant and thyroid autoimmunity.

While both MTHFR C677T polymorphisms and AITD are well-established risk factors for SA in women, and C677T variants and thyroid antibodies in men have been associated with impaired sperm quality and poorer pregnancy outcomes, the relationships between MTHFR C677T polymorphisms and thyroid autoimmunity in both genders of SA patients remain insufficiently explored. Therefore, in the present study, we investigated potential sex-specific associations between MTHFR C677T polymorphisms and thyroid autoantibodies in both women and men. Stratified analyses were further conducted based on the history of SA in study population. These findings may provide new insights into probable sex differences in SA cohorts.