An association between the development of postpartum
hypothyroidism and high levels of
IgG1 subclass microsomal (M)
antibodies has been reported. Using an assay designed to detect reasonable levels of all the four
IgG subclasses, we found no differences in the proportion of each
IgG subclass in M
antibodies of patients with postpartum
hyperthyroidism or
hypothyroidism compared with control postpartum patients with M
antibodies but no thyroid dysfunction. However the total amount of M antibody of each
IgG subclass was elevated above the controls in the patients with thyroid dysfunction. The relative functional affinity of M
antibodies did not differ between controls and patients with
hypothyroidism but declined 5 and 10-12 months after delivery compared to values at 2 months. These results do not support the suggestion that the amount of
IgG1 subclass M
antibodies particularly determines the course of
postpartum thyroiditis. Rather, the total M antibody level, in all four subclasses, is associated with clinical outcome. Resolution of the disease, despite persisting M
antibodies, may occur in part because the relative functional affinity of these
antibodies declines after delivery.