Association between serum 25-hydroxyvitamin D levels and thyroid hormone profiles in Graves’ disease: a retrospective cross-sectional stratified analysis

AIM: Graves' disease is an autoimmune thyroid disorder mediated by thyroid-stimulating hormone receptor antibodies (TRAb). Vitamin D has been proposed as a modulator of autoimmunity; however, its relationship with thyroid hormone profiles in Graves' disease remains unclear. METHODS: We retrospectively analyzed 182 patients with Graves' disease and 172 healthy controls. Serum 25-hydroxyvitamin D [25(OH)D] levels were stratified into four categories (<10, 10-20, 20-30, and ≥30 ng/ml). Thyroid parameters [thyroid-stimulating hormone (TSH), free T3 (fT3), free T4 (fT4)] were compared across groups using Kruskal-Wallis and Mann-Whitney U tests. Multivariate linear regression analyses were performed in the Graves' disease group to evaluate independent associations between 25(OH)D levels and thyroid hormone parameters, adjusting for age and sex. RESULTS: Severe vitamin D deficiency (<10 ng/ml) was associated with lower TSH levels compared with the other vitamin D groups (all P < 0.05). FT3 levels were highest in Group I and significantly lower in Groups II-IV (all P < 0.001). Similarly, fT4 levels were higher in Group I compared with Groups II and III (P < 0.001) and Group IV (P = 0.002). No statistically significant differences were observed among higher vitamin D categories. In multivariate analysis, serum 25(OH)D levels were independently and inversely associated with both fT3 (β = -0.260, P < 0.001) and fT4 levels (β = -0.253, P = 0.001), whereas no independent association was observed with TSH (β = 0.107, P = 0.158). CONCLUSION: Serum 25(OH)D levels were inversely associated with peripheral thyroid hormone levels in patients with Graves' disease, while no independent association was observed with TSH. These findings suggest a potential relationship between vitamin D status and thyroid hormone dynamics; however, given the cross-sectional design, no causal inference can be made. Further prospective studies are required to clarify the clinical significance of this association.