Association of Dietary Inflammatory Index and Thyroid Function in Patients with Hashimoto's Thyroiditis: An Observational Cross-Sectional Multicenter Study

Background and Objectives: The available research suggests that dietary patterns with high inflammatory potential, as indicated by a high DII score, may exacerbate inflammation and potentially influence thyroid function. Therefore, the aim of this study was to investigate the associations between the inflammatory potential of a diet and thyroid function in adults with Hashimoto's thyroiditis (HT). Materials and Methods: A total of 149 adults diagnosed with Hashimoto's thyroiditis were enrolled in this observational, cross-sectional, multicenter study. The Dietary Inflammatory Index (DII^®) was calculated using a 141-item food frequency questionnaire (FFQ). The serum levels of the thyroid-stimulating hormone (TSH), free thyroxine (fT4), thyroid peroxidase antibodies (TPO-Ab), and high-sensitivity C-reactive protein (hsCRP) were determined. Results: The DII^® scores ranged from -3.49 (most anti-inflammatory) to +4.68 (most pro-inflammatory), whereas three DII^® tertile ranges were defined as <-1.4, -1.39 to +1.20, and >+1.21, respectively. Participants in tertile 1 (more anti-inflammatory diet) had significantly higher levels of fT4 than those adhering to a more pro-inflammatory diet (p = 0.007). The levels of hsCRP and TSH appeared to increase with increasing the DII^® score, but without statistical significance. A significant association was found between the DII^® and TSH (β = 0.42, p < 0.001) and between DII^® and free thyroxine (β = 0.19, p < 0.001). After adjustment for age, gender, energy intake, and physical activity, a significant positive correlation remained between the DII^® and TSH (β = 0.33, p = 0.002) and between the DII^® and body mass index (BMI) (β = 0.14, p = 0.04). Conclusions: Adherence to an anti-inflammatory diet appears to be beneficial in patients with Hashimoto's thyroiditis, suggesting that dietary modification aimed at lowering DII^® levels may be a valuable strategy to improve clinical outcomes in these patients.