The observation that CGL was required for lowering circulating levels of T4 and IGF-1 in two different models of hypothyroidism, combined with the ability of TH to activate GHR/IGF-1 gene expression (Koenig et al., 1987, Tsukada et al., 1998), suggests that the compromised ability to reduce circulating T4 may be partially responsible. This evidence concerns the gene IGF1 and hypothyroidism.