Association between serum 25-hydroxyvitamin D levels and thyroid function alterations in hospitalized adults: a retrospective cross-sectional study

Background Thyroid function abnormalities and low vitamin D status are common in hospitalized adults, but evidence on their association remains limited and inconsistent. Methods We conducted a retrospective cross-sectional study of 225 adult inpatients between January 2020 and January 2025. Serum 25-hydroxyvitamin D [25(OH)D] and thyroid function indicators were measured during the same admission. Thyroid function alterations were defined as any TSH, FT3, or FT4 value outside the corresponding laboratory reference range. Associations of 25(OH)D with thyroid-related indicators and thyroid function alterations were evaluated using multivariable linear and logistic regression models. Sensitivity, subgroup, and restricted cubic spline analyses were also performed. Results Among 225 participants, 91 (40.4%) had thyroid function alterations. Median serum 25(OH)D was lower in participants with thyroid function alterations than in those with normal thyroid function (18.00 vs 21.40 ng/mL). In the fully adjusted model, ln[25(OH)D] was positively associated with FT4, FT3, T3, and T4 and inversely associated with TSH. Continuous ln[25(OH)D] was inversely associated with the odds of thyroid function alterations in Models A–C (Model C: OR 0.566, 95% CI 0.317–0.988), but the association was attenuated after further adjustment for TP and CRP in Model D (OR 0.569, 95% CI 0.309–1.021). After excluding participants with isolated low FT3, the inverse association remained similar in direction but was not statistically significant in the fully adjusted model. Significant interactions were observed by both sex and age group, with inverse associations observed in men and participants aged ≥65 years. Restricted cubic spline analysis showed no significant overall or nonlinear association. Conclusions Serum 25(OH)D was associated with multiple thyroid function indicators in hospitalized adults, whereas its inverse association with thyroid function alterations was attenuated after further adjustment for TP and CRP. Prospective studies are needed to clarify the temporal direction and clinical significance of these associations.