Abstract
A hypothyroid mother, due to thyroid stimulation-blocking antibody (TSBAb), gave birth three times. Although her TSBAb levels remained almost 100%, her thyrotropin receptor antibody (TRAb) levels before delivery varied as follows: 315.0, 88.2 and 34.9 IU/L in the first, second, and third pregnancies, respectively. The first baby had a hypoplastic thyroid gland and developed severe hypothyroidism three days after birth. The second and third babies only showed latent hypothyroidism. Apparent neonatal hypothyroidism due to maternal TSBAb occurs when TRAb levels are very high, and extremely high TSBAb activity might impair not only the fetal thyroid function, but also its development.