Vitamin D Deficiency Is Associated with Impaired Sensitivity to Thyroid Hormones in Euthyroid Adults

The relationship between vitamin D deficiency and sensitivity to thyroid hormones was unclear. We aimed to explore the association of 25-hydroxyvitamin D (25(OH)D) levels with thyroid hormone sensitivity in euthyroid adults. A total of 3143 subjects were included. The serum 25(OH)D, free thyroxine (FT3), free thyrotropin (FT4), thyroid-stimulating hormone (TSH), and other clinical variables were measured. Vitamin D deficiency was defined as 25(OH)D < 20 ng/mL. Thyroid feedback quantile-based index (TFQI), parametric thyroid feedback quantile-based index (PTFQI), thyroid-stimulating hormone index (TSHI), thyrotrophic thyroxine resistance index (TT4RI), and FT3/FT4 were calculated to assess thyroid hormone sensitivity. Results showed that 58.8% of the participants had vitamin D deficiency. They had significantly higher levels of triglyceride, insulin, FT3, FT4, TSH, TFQI, PTFQI, TSHI, and TT4RI and lower levels of high-density lipoprotein cholesterol than those with sufficient vitamin D (all p < 0.05). Logistic regression analysis showed that the risk of impaired sensitivity to thyroid hormones evaluated by TFIQ, PTFQI, TSHI, and TT4RI increased by 68% (OR: 1.68; 95%CI: 1.45–1.95; and p < 0.001), 70% (OR: 1.70; 95%CI: 1.46–1.97; and p < 0.001), 66% (OR: 1.66; 95%CI: 1.43–1.92; and p < 0.001), and 50% (OR: 1.50; 95%CI: 1.30–1.74; and p < 0.001), respectively, in participants with vitamin D deficiency compared with those with sufficient vitamin D after adjusting for multiple confounders. In conclusion, in euthyroid populations, vitamin D deficiency was associated with impaired sensitivity to thyroid hormones.