This study is the first to investigate the frequency of DUOX2 or DUOXA2 mutations in a borderline GIS CH cohort with a first bsTSH between 6 and 19.9 mIU/L, unlike previous studies that have studied cases recruited on the basis of higher bsTSH levels or known organification defect (10,12). Here, DUOXA2 is linked to cyclic hematopoiesis.