At present, only a small number of TD cases have been explained by molecular defects in TSHR and thyroid transcription factors, such as NKX2-1, FOXE1, PAX8, and HHEX, which are restrictedly expressed in thyroid tissues during the early developmental stages of thyroid budding and migration, and play critical roles in thyroid organogenesis [37]. This evidence concerns the gene PAX8 and thanatophoric dysplasia.