IGSF1 and congenital hypothyroidism: Isolated congenital central hypothyroidism is rare (incidence 1 in 65 000),1 with genetic causes comprising mutations in the TSH beta subunit (TSHB), the immunoglobulin superfamily member 1 (IGSF1) or, less frequently, the TRH receptor (TRHR) genes.2 Central hypothyroidism is associated with either subnormal or inappropriately normal TSH levels and therefore evades diagnosis in the UK and Irish TSH‐based newborn screening programmes for congenital hypothyroidism (CH).