The potential role of thyroid microsomal (Mic)
antibodies in the development of postpartum
hypothyroidism was investigated in 34 euthyroid women, whose sera were found to contain Mic
antibodies in pregnancy. Additional serum samples were obtained 2. 5 and 10-12 months after delivery and analysed for
IgG class and
IgG subclass levels of Mic
antibodies by ELISA techniques. Characteristically, Mic
antibodies decreased from early pregnancy to 2 months postpartum, increased two-fold 5 months postpartum and had returned 10-12 months postpartum to the early pregnancy level. Mic
antibodies were predominantly subclass IgG-1 or IgG-4 with only minor contributions from IgG-2 and IgG-3. In each individual the percentage contribution made by each
IgG subclass to Mic antibody was essentially similar in early pregnancy and the postpartum period despite changes in total
IgG class Mic antibody. During the year following delivery,
thyrotoxicosis alone (
Graves' disease) developed in 5 women. In the remaining 29 patients the absolute levels of Mic
antibodies of IgG-4 subclass were similar 5 months postpartum in women with maximal serum
thyrotropin (TSH) greater than 20 mU/1 (mean optical density in ELISA +/- s.d.; 0.84 +/- 0.538; n = 13) and in women with maximal TSH less than 10 mU/l (0.69 +/- 0.457; n = 16). In contrast, significantly higher values were observed for Mic antibody of IgG-1 subclass in patients with TSH greater than 20 mU/l (1.14 +/- 0.440) compared with women with maximal TSH less than 10 mU/l (0.65 +/- 0.289) (P less than 0.001 by t-test for groups). These results imply that the magnitude of Mic antibody levels of subclass IgG-1 but not IgG-4 is associated with the development of postpartum
hypothyroidism and possibly with tissue destruction in autoimmune
thyroid disease in general.