Risk of Functional Disorders and/or Thyroid Autoimmunity and Its Association with 25OH Vitamin D and Magnesium Levels: A Population-Based Case-Control Study

Background/Objectives: Vitamin D (Vit-D) and magnesium (Mg) levels have been associated with an increased risk of developing functional thyroid disorders or autoimmune thyroid diseases (AITD). In this study, our objective was to evaluate if 25-hydroxyvitamin D (25OH Vit-D) and/or Mg levels are associated with an increased risk of functional thyroid disorders and/or AITD. Methods: A population-based case-control study was conducted, with a total of 1028 participants (514 cases and 514 controls). Blood concentrations of 25OH Vit-D, Mg, TSH, FT4, FT3, and thyroid autoantibodies (TPOAb, TgAb, and TRAb) were determined in the study participants. Results: Among the cases (in women), the prevalence of goiter, hypothyroidism, and thyroid autoantibody positivity was significantly higher. No differences were found in the prevalence of functional thyroid disorders or in thyroid antibody positivity (among cases) according to sex or age. The prevalence of thyroid antibody positivity (specifically TPOAb and/or TgAb) was significantly higher in cases with 25OH Vit-D and/or Mg deficiency. The 25OH Vit-D level that best discriminated the highest frequency of AITD was 23.5 ng/mL [AUC: 0.665 (95% CI: 0.636–0.694, p < 0.001)]; while for Mg it was 1.8 mg/dL [AUC: 0.697 (95% CI: 0.668–0.725, p < 0.001)], indicating that the model has weak discrimination (although better than chance), with good sensitivity and low specificity, being able to identify the vast majority of positive cases (with AITDs), at the cost of including a significant proportion of false positives. Conclusions: Overall, we found that low serum levels of 25OH Vit-D and/or Mg appear to be associated with a significantly increased risk of goiter, functional thyroid disorders (specifically hypothyroidism), and with greater positivity of thyroid antibodies.