Association of Vitamin D Levels and Systemic Inflammation in Patients with Hashimoto's Thyroiditis.

Background/Objectives: Vitamin D is an important regulator of immune and inflammatory processes, while its deficiency is frequently observed in Hashimoto's thyroiditis (HT). The relationship between Vitamin D and systemic inflammation in HT remains insufficiently understood, particularly across different disease stages. Methods: We used the Olink Target 96 Inflammation panel to investigate this association. Inflammatory proteins and serum 25(OH)D levels were measured in 257 HT patients and 173 controls from the CROHT biobank. Participants were categorized as Vitamin D deficient (<20 ng/mL) or non-deficient (≥20 ng/mL), with HT patients further stratified as euthyroid, hypothyroid, or levothyroxine-treated. Associations between 25(OH)D and 92 inflammatory proteins were assessed using multivariable linear regression adjusted for age, sex, BMI, smoking, and season of blood sampling, followed by interaction testing and meta-analysis. Results: Vitamin D deficiency was prevalent in both HT patients and controls. Among Vitamin D-deficient HT patients, higher 25(OH)D levels showed nominal inverse associations with the previously reported HT-associated inflammatory proteins IL-17C, CCL20, and CCL11, as well as with GDNF. Among non-deficient HT patients, a nominal positive association with CD6 was observed. None of these associations or interaction terms remained statistically significant after false discovery rate correction. Conclusions: These findings should therefore be considered exploratory and hypothesis-generating. Nevertheless, the observed Vitamin D status-specific patterns suggest that the relationship with inflammatory profiles in HT may differ according to Vitamin D status and warrant validation in prospective and mechanistic studies.