In fact, this model of IGHD, in which most adults have never received GH replacement therapy, makes it possible to analyse the effects of the somatotrophic axis (pituitary GH and circulating IGF1) and extrapituitary circuits (IGF2 and local production of IGF1 and IGF2) on body size and body functions. This evidence concerns the gene IGF1 and isolated congenital growth hormone deficiency.