The consistency of the FT3/FT4 ratio during childhood is useful for the screening of several genetic disorders related to peripheral thyroid hormone metabolism, including Allan-Herndon-Dudley syndrome (MCT8 deficiency) [4], selenocysteine insertion sequence binding protein 2 deficiency [5], thyroid hormone resistance [6, 7], and others. The gene discussed is SECISBP2; the disease is Allan-Herndon-Dudley syndrome.