Isolated Vitamin D Deficiency Is Not Associated with Nonthyroidal Illness Syndrome, but with Thyroid Autoimmunity

Aim. This study aimed to compare thyroid functions, thyroid autoantibodies, and the existence of nonthyroidal illness syndrome (NTIS) according to vitamin D level. Materials and Methods. The study included age- and BMI-matched healthy volunteers with and without vitamin D deficiency. In addition, the nonthyroidal illness syndrome status was evaluated. Results. Anti-TPO positivity was significantly more common in those with severe and moderate vitamin D deficiency, as compared to those with a normal 25(OH)D level. Furthermore, TSH levels were significantly lower in those with severe and moderate vitamin D deficiency than in those with a normal 25(OH)D level. In addition, there was a significant weak inverse correlation between anti-TPO positivity and the 25(OH)D level and a positive correlation between the TSH level and 25(OH)D level. Only 1 thyroid function test result was compatible with NTIS among the participants with moderate vitamin D deficiency; therefore the difference was not significant. Conclusions. The prevalence of thyroid autoantibody positivity was higher in those with severe and moderate vitamin D deficiency than in those with a normal 25(OH)D level. Additional large-scale studies must be conducted to determine if vitamin D deficiency plays a causal role in the pathogenesis of Hashimoto's thyroiditis and NTIS.