Though E2 mostly suppresses Th17 activity [22,24], several studies reported the stimulatory effects of E2 on Th17 cells: E2 at estrous levels of concentration enhanced the expression of IL-21 and retinoic acid receptor-related orphan nuclear receptor γt (RORγt) in female ovariectomized autoimmune thyroiditis model mice [21]. Here, IL21 is linked to autoimmune thyroid disease.