Autoimmune
thyroid diseases (AITD) are common autoimmune disorders. A few studies have analyzed the association between serum
vitamin D levels and AITD, and available data remain inconclusive. The aim of this study was to evaluate the association between serum
vitamin D levels and 3 types of AITD, that is
Graves' disease (GD), Hashimoto's
thyroiditis (HT), and
postpartum thyroiditis (PPT). Two independent case-control studies were designed. The first is a cross-sectional case-control study in which we examined the levels of 25(
OH)D in patients with newly diagnosed GD or HT and in controls; the second is a nested case-control study in which we compared 25(
OH)D levels in 610 women who developed PPT during the follow-up after delivery and those who did not. Compared with the controls, GD patients and HT patients had significantly lower 25(
OH)D levels. PPT cases also had a lower serum 25(
OH)D concentration than controls. Serum 25(
OH)D levels were associated with neither antithyroid
peroxidase antibody nor
antithyroglobulin antibody in GD and HT. There was no significant relationship between
thyroid-stimulating hormone and 25(
OH)D levels. Every 5 nmol/L increase in serum 25(
OH)D concentrations was associated with a 1.55-, 1.62-, and 1.51-fold reduction in GD, HT, and PPT risk, respectively. We observed a lower serum
vitamin D levels in AITD patients compared with controls. The lower the
vitamin D level is, not
vitamin D deficiency per se, the higher the risk for developing AITD will be. However,
vitamin D does not have strong association with the titers of thyroid
antibodies or the levels of
thyroid hormones.