HHEX and cyclic hematopoiesis: Of the 21 candidate genes, TPO, SLC5A5, TG, TSHR, DUOX2, DUOXA2, SLC26A4, FOXE1, PAX8, NKX2-1, NKX2-5 and IYD were previously reported possible causative genes for CH, while the other nine genes: DIO1, DIO2, THRA, THRB, DUOX1, DUOXA1, GNAS, SLC16A2 and HHEX were not well-established disease genes and were included based on their function associated with the production and transportation of thyroid hormone.