Vitamin D Status and Its Association With Disease Severity in Hashimoto's Thyroiditis.

Introduction Hashimoto's thyroiditis (HT), the most common autoimmune thyroid disorder, involves chronic lymphocytic infiltration and thyroid dysfunction. Vitamin D deficiency has been linked to autoimmune thyroid diseases, suggesting an immunomodulatory role. This study examines the association between serum vitamin D levels and HT severity in patients at a secondary-level hospital in Mexico. Methods A retrospective observational study was conducted on adult patients diagnosed with HT from January 2022 to January 2024. Clinical and laboratory data, including serum 25-hydroxyvitamin D (25(OH)D), thyroid-stimulating hormone (TSH), free thyroxine (FT4), and anti-thyroid peroxidase (anti-TPO) antibodies, were collected. Patients were classified by thyroid function and vitamin D status. Statistical analyses were performed to evaluate correlations and group associations to determine the relationship between vitamin D deficiency and HT severity. Results A total of 114 patients were included in the study: 1.8% (n=2) were classified as euthyroid, 18.4% (n=21) as having subclinical hypothyroidism, and 79.8% (n=91) as having overt hypothyroidism. Vitamin D deficiency, defined as serum 25(OH)D levels below 20 ng/mL, was highly prevalent, affecting 49.5% (n=45) of patients with overt hypothyroidism and 42.9% (n=9) of those with subclinical hypothyroidism. A significant inverse correlation was observed between 25(OH)D levels and anti-TPO antibody titers (p=0.01), suggesting an association between lower vitamin D concentrations and increased autoimmune activity. Additionally, vitamin D-deficient patients exhibited significantly higher TSH levels compared to those with sufficient vitamin D (p=0.01). No significant differences were found in FT4 levels across the different vitamin D categories. Conclusions This study suggests an inverse correlation between vitamin D deficiency and the severity of HT, supporting the potential immunomodulatory role of vitamin D. However, due to the observational nature of the study, causal relationships cannot be inferred. Given the high prevalence of vitamin D deficiency among patients with HT, further research is warranted to explore the therapeutic implications of vitamin D supplementation on disease progression and management.