Background: Thyroid-stimulating hormone (TSH) receptor (TSHR) antibodies (TRAb) can be present in chronic autoimmune thyroiditis. Transplacental TRAb transfer can lead to fetal thyroid dysfunction and serious complications. Patient Findings: We report the case of a woman with autoimmune hypothyroidism and extremely high TRAb levels, with blocking and stimulating activities (biological activities characterized with Chinese hamster ovary cells expressing TSHR). At week 22 of her first pregnancy, sonography detected fetal growth retardation and cardiac abnormalities (extreme tachycardia, right ventricular dilatation, pericardial effusion). The mother's TRAb level, assayed later, was 4030 IU/L (n < 10). Delivered via caesarean section gestational week 30, the newborn girl had several malformations, signs of malnutrition, goiter and hyperthyroidism associated with elevated TRAb (1200 IU/L). The newborn died 26 days after delivery. Faced with persistently high TRAb levels and a desire to become pregnant again, the woman was treated with three consecutive 740-MBq activities of iodine-131, which resulted in a decrease in TRAb to 640 IU/L. The patient had two subsequent pregnancies 16 and 72 months after the radioiodine administration. During the close follow-ups, fetal development was normal, and initial TRAb levels during the two pregnancies were 680 and 260 IU/L, respectively, which initially decreased but then increased in late pregnancy. In both cases, labor was induced at 34 weeks. The newborns, mildly hyperthyroid at birth, required carbimazole treatment at days 5 and 2, respectively. The mild hyperthyroidism despite high TRAb levels was likely due to the concomitant presence of stimulating and blocking TRAb. The two girls, now aged 12 and 8 years, are in good health. The mother has no detectable thyroid gland tissue and is euthyroid on levothyroxine (175 μg/d). Her TRAb level gradually decreased to 136 IU/L. Summary and Conclusions: This remarkable case illustrates the severe consequences of untreated fetal hyperthyroidism and the need to assay and follow-up TRAb levels in women of reproductive age with autoimmune thyroiditis.