**3.6 Vitamin D deficiency and autoimmune Hashimoto's thyroiditis**

Studies have observed an association between autoimmune Hashimoto's thyroiditis and low vitamin D levels [79, 129]. These studies have not observed low vitamin D levels in patients with Graves' disease. A meta-analysis of 26 observational studies confirmed an association between vitamin D deficiency and autoimmune Hashimoto's thyroiditis [130]. The aforementioned meta-analysis found that although there was heterogeneity between the results of the various studies performed all over the globe, studies had similar results in populations from different countries and also in populations in different age ranges, in particular pediatric and adult populations.
