The liver is the primary site for the production of both circulating insulin-like growth factor 1 (IGF-1) (Kita et al., 1996) and
plasma triiodothyronine (T3) through thyroxin (T4) deiodination, both processes being under the control of GH via hepatic GH receptor (Burnside and Cogburn, 1992). Two iodothyronine deiodinases are expressed in the liver.