Hashimoto's thyroiditis (HT), also known as chronic lymphocytic or autoimmune thyroiditis (AITD), is a type of chronic autoimmune thyroid disease that is associated with varying degrees of hypothyroidism, thyroid autoantibody production, and lymphocytic infiltration. In China, the incidence rate showed a rising trend year by year. For the majority of HT hypothyroidism patients, synthetic levothyroxine (LT4) (first crystallized by Kendall in 1915, commercialized in the 1930s, and mass-produced in the 1960s) remains the only effective drug for HT patients. The treatment works for most patients, but is still controversial. Dietary vitamin D supplementation has been evaluated as a way to protect the thyroid from autoimmune damage, but results from randomized clinical trials have been inconclusive. Clinical studies have shown that vitamin D3 supplementation has a unique advantage in reducing TPO antibody titers. In 2014, AACE (American Association of Clinical Endocrinologists) guidelines indicated that vitamin D can be used as a complementary treatment for Hashimoto's thyroiditis. The prevalence of vitamin D deficiency in HT patients is as high as 50%-90%. Studies have shown that VD is widely used in vivo. VD can be used as an immune factor to participate in the process of immune regulation. Cells of the immune system (B cells, T cells and antigen presenting cells) can synthesize active metabolites of vitamin D due to the expression of 1α-hydroxylase (CYP27B1), which shows immunomodulatory properties. In addition, studies have shown that Vitamin D receptor (VDR) is found not only in bone, kidneys, and intestine, but also in the immune system (T and B cells, macrophages, and monocytes), reproductive system, endocrine system, muscle, brain, skin, and liver. The expression of the vitamin D receptor in these cells suggests a local role for vitamin D in immune responses. Existing genetic studies have found that most VDR can be restricted by CYP1α and vitamin D binding proteins, showing the relationship between VDR, DBP and CYP1α gene polymorphisms and thyroid autoimmune system, which affect T and B lymphocytes, dendritic cells and macrophages, as well as enzymes with CYP1α and VDR. Therefore, Vitamin D is thought to be an immune modulator.

Clinical studies have confirmed that vitamin D deficiency is more common in patients with autoimmune thyroid disease (AITD), both in children and the elderly, and in both low and high latitudes. Combining vitamin D with anti-thyroid drugs or thyroid hormones can help treat AITD by suppressing autoimmune responses and lowering serum levels of thyroid autoantibodies. However, studies have shown that vitamin D deficiency is not associated with AITD, especially early AITD. However, a study based on Asian Indian communities found only a weak inverse association between serum 25-hydroxyvitamin D values and TPO antibody titers. A 2015 meta-analysis demonstrated that vitamin D deficiency is prevalent in AITD subjects and that these subjects have low serum 25-hydroxyvitamin D levels, suggesting that lower serum 25-hydroxyvitamin D is associated with AITD disease.

There is a complex relationship between HT and serum 25-hydroxyvitamin D. It is not clear whether the decreased serum 25-hydroxyvitamin D level is one of the causes of HT hypothyroidism or a consequence of HT.