In the 22 CH patients with positively detected genetic causes, only one patient was associated with the TD-related gene PAX8, while 21 were associated with the TDH-related genes DUOX2, DUOXA2, TG and TPO (Table 1), suggesting that TDH is more common than TD as the major mechanism for CH development. This evidence concerns the gene PAX8 and thanatophoric dysplasia.