Ricci D, Brancatella A, Marinò M, et al. - Since the role of serum IgM in autoimmune thyroid diseases is not known, the authors endeavored for IgM to thyroglobulin (Tg) in individuals with subacute thyroiditis (SAT), which is described by high serum Tg levels, the possible de-novo appearance of IgG to Tg (TgAb-IgG) and no autoimmune sequelae. The upper reference limit of TgAb-IgM and TgAb-IgG was established in 40 normal individuals. TgAb-IgM have been explored for in 16 SAT, 11 Hashimoto’s Thyroiditis (HT) and 8 Graves’ Disease (GD), all positive for TgAb-IgG. Findings revealed that in 10/16 SAT, 2/11 HT and 1/8 GD patients, TgAb-IgM were positive. Tg leak linked to thyroid injury induces the production of specific TgAb-IgM, which in turn, may prevent the establishment of a persistent thyroid autoimmune response by increasing the clearance of Tg.