A 27-year-old gravid 1 at 27 weeks 6 days with a history of hypothyroidism had an ultrasound that demonstrated a 3.9 × 3.2 × 3.3-cm well-circumscribed anterior neck mass, an extended fetal head, and polyhydramnios. Further characterization by magnetic resonance imaging (MRI) showed a fetal goiter. During her evaluation for the underlying cause of the fetal goiter, the patient revealed she was t...