Hashimoto’s Thyroiditis is an autoimmune thyroid disease characterized by the continuous attack upon cells of the body’s own thyroid gland. Although the cause of Hashimoto’s Thyroiditis is not entirely clear, it is generally understood to result from a combination of genetic and environmental influences. According to research, Vitamin D deficiency may play a role as one of the environmental factors that contributes to the occurrence of Hashimoto’s Thyroiditis. Vitamin D, increasingly viewed as a hormone, is involved in many processes in the human body, including so-called “immunomodulatory” functions. Vitamin D increases the body’s ability to regulate immune activities. There is a clear correlation between low Vitamin D levels and the presence of Hashimoto’s Thyroiditis, that indicates Vitamin D deficiency could contribute to developing the disease. In a complex series of processes, Vitamin D has been shown to contribute to both the protection of the gland from autoimmune attacks, and the lowering of antibodies that react to the gland. In most people, Vitamin D is produced in adequate quantities by the skin in response to sunlight exposure, with additional amounts ingested via foods such as milk, eggs, and fish. Vitamin D levels can be affected by influences such as exposure to sunlight, skin conditions and pigmentation, diet, exercise, body fat, age, and some medications. The relationship of Hashimoto’s Thyroiditis with conditions such as obesity further complicate the issue, as some of these disorders are capable of triggering a Vitamin D deficiency on their own. Incorporating a quality Vitamin D supplement, and monitoring Vitamin D levels appears to be an important component in the care of patients with Hashimoto’s Thyroiditis. Thyroid patients need to pay more attention to keeping their Vitamin D level in an optimal range to reduce autoimmunity.