Autoimmune thyroiditis (AIT) is one of the most prevalent autoimmune endocrine disorders, affecting approximately 7.5% of the adult population (Hu et al., 2022). The MTHFR gene, which encodes methylenetetrahydrofolate reductase, plays a crucial role in folate metabolism and homocysteine remethylation (Antoni F. Araszkiewicz, 2025). The MTHFR C677T polymorphism reduces enzyme activity, leading to elevated homocysteine, oxidative stress, and activation of autoimmune inflammation (Cui et al., 2023; Chen et al., 2025). Objective. To assess the prevalence of the MTHFR C677T polymorphism and its association with folate metabolism parameters in patients with AIT.

Nineteen patients with AIT were examined at the Turakulov Endocrinology Center. Serum levels of TSH, free T₄, free T₃, anti-TPO, anti-TG, homocysteine, folic acid, vitamin B₁₂, total cholesterol, and triglycerides were measured using chemiluminescent immunoassay (CLIA). The MTHFR C677T polymorphism was genotyped by PCR-RFLP.

The MTHFR C677T polymorphism was detected in 9 of 19 patients (47.4%), corresponding to the frequency of T-allele carriers (CT + TT). Genotype distribution was CC – 10 (52.6%), CT – 7 (36.8%), TT – 2 (10.5%); allele frequencies were C – 27 (71.1%) and T – 11 (28.9%). The distribution conformed to Hardy–Weinberg equilibrium (p > 0.05). Carriers of heterozygous and mutant genotypes showed a tendency toward higher homocysteine levels (15.2 ± 2.8 µmol/L) and lower folate (8.0 ± 1.9 ng/mL) and vitamin B₁₂ (294.4 ± 43.1 pg/mL) compared with wild-type (CC) carriers. Thyroid parameters were characterized by elevated TSH (6.5 ± 1.9 mIU/mL), anti-TPO (283.7 ± 24.8 IU/mL), and anti-TG (310.7 ± 45.6 IU/mL) levels, along with reduced free T₄ (1.1 ± 0.3 ng/dL) and free T₃ (2.6 ± 0.8 pg/mL). Concomitant dyslipidemia was observed, with increased total cholesterol (5.7 ± 0.3 mmol/L) and triglycerides (2.3 ± 0.1 mmol/L), indicating metabolic dysfunction associated with chronic inflammation.

The MTHFR C677T polymorphism was found in 47.4% of AIT patients, predominantly in the heterozygous CT form. The presence of the T allele was associated with elevated homocysteine and decreased folate and vitamin B₁₂ levels, reflecting genetically determined remethylation defects that promote oxidative stress and autoimmune inflammation. The MTHFR C677T polymorphism may serve as a marker of metabolic dysregulation in autoimmune thyroiditis.