Risk of subclinical hypothyroidism in pregnant women with asymptomatic autoimmune thyroid disorders.

A prospective study was undertaken in 87 healthy pregnant women with thyroid antibodies and normal thyroid function at initial presentation [asymptomatic autoimmune thyroid disorders (AITD)]. The aims of the study were to assess whether women with AITD constitute a group at risk of developing subclinical hypothyroidism during pregnancy, and whether a mild thyroid function impairment may be associated with obstetrical repercussions. The women investigated were selected among a cohort of 1660 consecutive pregnancies on the basis of 1) no previous history of thyroid disease, 2) euthyroidism at initial presentation, and 3) positive thyroglobulin antibodies and/or thyroid peroxidase antibodies (TPO-Ab). Women with AITD had a basal TSH value significantly higher, albeit still normal, in the first trimester (1.6 vs. 0.9 mU/L; P < 0.001) than that in women with healthy pregnancies used as controls. Despite a 60% average reduction in TPO-Ab titers during gestation, serum TSH remained higher in women with AITD than in controls throughout gestation: at delivery, 40% of the cases had serum TSH levels above 3 mU/L, and 16% had serum TSH levels above 4 mU/L. A TRH test carried out in the days after parturition showed an exaggerated response in 50% of the cases. Furthermore, free T4 concentrations were in the range of hypothyroid values in 42% of the women. Obstetrical repercussions were observed, namely increased rates of spontaneous miscarriage and premature deliveries. In conclusion, women with asymptomatic AITD who are euthyroid in early pregnancy carry a significant risk of developing hypothyroidism progressively during gestation, despite a marked reduction in antibody titers. Hypothyroidism results from the reduced ability of the gland to adjust to the changes in thyroidal economy associated with pregnancy. At the individual level, progression to subclinical hypothyroidism was broadly predictable on the basis of serum TSH levels and TPO-Ab titers in the first trimester. Hence, these parameters provide useful markers to identify women who carry a higher risk, allowing for a close monitoring of thyroid function during pregnancy and the administration of L-T4 in specific cases. Taken together with the known incidences of postpartum thyroiditis and hypothyroidism in women with AITD, the present observations in our opinion justify systematic screening of thyroid autoimmunity during pregnancy.