TPO and cyclic hematopoiesis: Emerging evidence suggests that genetic defects in iodotyrosine deiodinase (IYD) (3), sodium-iodide symporter (NIS) (4), pendrin (SLC26A4) (5), thyroperoxidase (TPO) (6), thyroglobulin (TG) (7), dual oxidase 2 (DUOX2) (8), and dual oxidase maturation factor 2 (DUOXA2) (9) can cause goitrous CH, including transient or permanent types.