TPO and autoimmune thyroid disease: Se is primarily stored in the body as selenoprotein that plays a crucial role in the immune system and activation of thyroid hormones.[13,14] In patients with autoimmune thyroid diseases, the Se levels are significantly low, leading to weakened resistance against oxidative stress.[15] The synthesis of thyroid hormone requires the enzyme thyroid peroxidase (TPO) to convert iodine into active iodine, which then iodinates tyrosine residues under the oxidation of hydrogen peroxide (H2O2).