In the post-partum period the immune alterations are associated with the multiple autoimmune diseases relapse. After birth, immune-tolerance variation slowly disappear, and is observed a return to a normal state - after an exacerbation period - of autoimmune reactivity, during which a great increase in T cells and autoantibodies is observed. In this period - 3 to 9 months after birth - the thyroid autoimmune disease relapses or reappears. The reactivation of the immune system in the post-partum period unchains an acute phase of celular destruction which characterizes the post-partum thyroiditis.