GH and IGF-1 Production

The release of GH in the pituitary gland, a pea-sized gland located below the brain, is governed by two hormones:
growth hormone-releasing hormone and growth hormone-inhibiting hormone. [3] Different external stimulatory and inhibitory factors may affect the release of GH including: [4]

• Fasting, low blood sugar and vigorous exercise
• Ghrelin (an enzyme that stimulates appetite)
• Growth hormone-releasing hormone (somatocrinin)
• Sex hormones such as androgens and estrogen

Some inhibitors of growth hormone secretion include: [5]

• Circulating concentrations of GH and IGF-1 (Insulin-like growth factor 1)
• Dihydrotestosterone (active form of testosterone)
• Growth hormone-inhibiting hormone (somatostatin)
• Glucocorticoids (powerful anti-inflammatory compounds)
• Hyperglycemia (high blood sugar)

Young adolescents especially those in the puberty period, secrete GH at the rate of about 700 μg/day, while healthy adults secrete GH at a lower rate of about 400 μg/day – these surges of GH secretion occur during the day every 3 to 5 hours. [6] When secreted into the bloodstream, GH remains active for only a few minutes, but this is enough time for the liver to convert it into growth factors, which are necessary for the stimulation of growth in living cells. [7]
The most essential of all growth factors is the insulin-like growth factor 1 (IGF-1), also called somatomedin C, which boasts a host of anabolic properties. IGF-1 is a hormone that resembles the molecular structure of insulin. It supports cellular division, growth of muscles and organs, helps repair nerve damage in different vital organs, reduces body fat by using fat as a source of energy instead of glucose, and it helps increase the number and size of cells in the body. [8] IGF-1 is produced primarily by the liver as an endocrine hormone. Its production is stimulated by GH and can be retarded by a number of factors such as malnutrition, growth hormone insensitivity and lack of growth hormone receptors. [9]
Measurements of IGF-1 are adjusted for age because its levels tend to decline over time. Normal ranges of IGF-1 by age are: [10]

• 16 to 24 years old: 182 to 780 ng/mL
• 25 to 39 years old: 114 to 492 ng/mL
• 40 to 54 years old: 90 to 360 ng/mL
• 55 years old and above: 71 to 290 ng/mL

Relationship between HGH and IGF-1

GH stimulates the production of IGF-1. GH is broken down in the liver and is converted to IGF-1. Additional IGF-1 is also generated within target tissues. IGF-1 by itself and in combination with other growth factors play an important role in healing, muscle and bone growth, repair processes, and other essential functions in the body. Most of the effects of GH are mediated through IGF-1. [11] When the levels of GH rise or fall below the normal, the same thing will happen to IGF-1. [12] The GH/IGF1 system is dynamic and its activity is greatly influenced by age, sexual maturation, body composition and other factors.