The body requires certain hormones in order to promote cell growth. These include Human Growth Hormone and Insulin Growth Factor. HGH and IGF-1 work together starting around the time children reach the age of nine. After this age and on through the teen years, people experience large growths in muscle, bone, organs, skin, and nerves.

Around the time of puberty the pituitary gland will signal the secretion of HGH. HGH will quickly move to the liver where IGF-1 is produced. IGF-1 will then prompt the cells of the body to grow and reproduce. HGH and IGF-1 work together to allow the systems of the body to grow at the same rate. IGF-1 is able to signal several systems at one time, including skeletal muscle, the liver, the lungs, the nerves, the skin, cartilage, the hematopoietic cell, the kidneys, and bones.

Following the completion of puberty, around the age of sixteen or seventeen, HGH production slows, resulting in the slow of IGF-1 promotion. Since the body systems are no longer being signaled to grow the body goes into a state of rest, called the resting period. During the resting period the body continues to produce IGF-1 for repairs and maintenance of cells.

Sometimes the body is unable to produce human growth hormone, so synthetic hormones are administered. Children with deficiencies are normally prescribed the human growth hormone itself. Children often lack the ability grow at young ages because HGH is no being produced.

Adults with deficiencies, in turn, are often prescribed IGF-1. HGH is difficult to measure in the blood since it is absorbed by the liver so quickly, thus IGF-1 is normally administered. Prescriptions of IGF-1 are often given to individuals with certain types of dwarfism. IGF-2 is a protein that is closely related to IGF-1, though rather than being secreted during puberty and adulthood, IGF-2 is secreted during the period of gestation only.