IGSF1 and Central hypothyroidism: Loss-of-function of the immunoglobulin super family member 1 gene (IGSF1, OMIM#300888) causes an X-linked syndrome, characterized in males by congenital central hypothyroidism, delayed testosterone rise in puberty despite normal timing of testicular enlargement, adult macro-orchidism, and in some cases deficiencies of prolactin (PRL) and/or growth hormone (GH) (1,2,3).