IGSF1 and cyclic hematopoiesis: Detection of CeCH by neonatal screening requires the inclusion of thyroxine (T4) in the CH screening programme, as in the Netherlands, where an algorithm including TBG enables detection of permanent neonatal central CH.1 In keeping with current recommendations, our data support early initiation of levothyroxine replacement in IGSF1 deficiency, as sequelae attributable to hypothyroidism were evident in 70% affected males.9 Additionally, associated pubertal delay and GH deficiency may be amenable to treatment following endocrine diagnosis.