**3.1 Insulin-like growth factor-1(IGF-1) production and circulation**

Insulin-like growth factor-1 is produced primarily by the liver. Then it is carried in blood bound to a carrier protein, which prolongs its half-life. Its level is therefore more constant than that of growth hormone [68]. Human growth hormone stays in the blood stream only for a short time, (few minutes). Every organ and system of the body is affected by growth hormone, either directly or indirectly, through the mediator IGF-1 [69]. Similar to that, it encourages young children's bone growth. The majority of organs and tissues experience growth, including the brain, which is also impacted. Most of the advantages and effects of human growth hormone are caused directly by IGF-1. IGF-1 has a ten-fold greater potency than HGH [70]. With a half-life of 21 to 40 hours. In addition, it is tightly bound to its binding protein in plasma. GH stimulates IGF-1 not only secretion but also the production of IGF binding protein [71]. IGF-1 mediates the anabolic growth promoting effects of GH in the muscle [72].
