Background: Hashimoto's thyroiditis (HT) is classify within the autoimmune disease of thyroid gland caused by diffuse of special lymphocytes into thyroid, lymphocyte product's special antibody lead to destroy gland tissue . Vitamin D (vit.D) is steroid substance bind with special receptor to have it's functions .Vit.D3 has various function in human body such as non skeletal function of vit.D3 is gene regulation that involve in autoimmune disease.Materials and methods: Present study include collection involved tow groups , HT disease group and healthy individuals , then measured vit.D3 , T4, TSH and anti-TPO levels for all groups.Results: Study result's shown after comparison between HT patients group and control group the reduction of serum vit.D level at HT group with present hypofunction of thyroid gland .Conclustion : Concluded this study to must be serumvit.D3 level within normal rang due to vit.D3 has function to regulation genes that effect on autoimmune diseases such as HT via modulation of immune cells secretions that lead to thyroid cells destroy.