OBJECTIVE To study whether the presence of antithyroid peroxidase antibodies (TPO-Abs) before gestation in IDDM affects thyroid function and metabolic control during pregnancy and early postpartum as well as neonatal outcome.

RESEARCH DESIGN AND METHODS A prospective study at an outpatient endocrine-obstetric unit was carried out in 20 pregnant IDDM women. Free T4 (thyroxine), thyroid-stimulating hormone (TSH), TPO-Abs, and HbA_1c were assayed before gestation; during the first, second, and third trimester of pregnancy; and 3 months postpartum.

RESULTS HbA_1c was significantly higher in TPO-Ab+ women than in those who were TPO-Ab⁻ during the second (P < 0.01) and third (P < 0.05) trimesters. HbA_1c levels significantly decreased in TPO-Ab⁻ patients when the second (P < 0.01) and third (P < 0.05) trimesters were compared with before the pregnancy and the first trimester. There was a significant increase in the dosage of insulin for TPO-Ab+ versus TPO-Ab⁻ patients during the second (P < 0.05) and third (P < 0.01) trimesters and 3 months postpartum (P < 0.05). TSH was significantly increased in the second (P < 0.001) and third (P < 0.05) trimesters and 3 months postpartum (P < 0.01) when compared with TPO-Ab⁻ patients; 7.6% of the TPO-Ab⁻ group and 29% in the TPO-Ab+ group presented postpartum thyroid dysfunction, and 42% of the TPO-Ab+ women required thyroid treatment.

CONCLUSIONS Pregnant women with IDDM who have a positive test for TPO-Abs before gestation have poorer glucose control and a high prevalence of hypothyroidism. Therefore we recommend that prepregnant IDDM patients be screened for anti-TPO-Abs. Those with a positive result should be followed with serial monitoring of free T4 and TSH levels during each trimester as well as the postpartum period.