Resistance to thyrotropin (TSH) caused by monoallelic or biallelic TSHR variants has a special place in the etiology of primary CH, with phenotypes ranging from mild persistent hyperthyrotropinemia (PHT) with normal gland size to severe permanent CH (PCH) with orthotopic hypoplasia (61, 63, 64, 65). This evidence concerns the gene TSHR and cyclic hematopoiesis.