Growth Hormones

Several hormones play important roles in human growth. The major human growth hormone (hGH), or somatotropin, is a protein made up of 191 amino acids secreted by the anterior pituitary and coordinates normal growth and development. Human growth is characterized by two spurts, one at birth and the other at puberty. HGH plays an important role at both of these times. Normal individuals have measurable levels of hGH throughout life. Yet, levels of hGH fluctuate during the day and are affected by eating and exercise. Receptors that respond to hGH exist on cells and tissues throughout the body. The most obvious effect of hGH is on linear skeletal development. But, the metabolic effects of hGH on muscle, liver, and fat cells are critical to its function. Humans have two forms of hGH, and the functional difference between the two is unclear. They are both formed from the same gene, but one lacks the amino acids in positions 32–46.

Additional hormones that affect growth are the somatomedins, thyroid hormones, androgens, estrogens, glucocorticoids, and insulin. Somatomedins are small proteins produced in the liver in response to stimulation by hGH. The two major somatomedins are insulin-like growth factor I and II (IGF-I and IGF-II). IGF-I causes increased cartilage growth and collagen formation, and its plasma levels peak between the ages of 13 and 17. IGF-II is important during fetal development and is present at constant levels in adult brains; however, its neuronal role is unclear. IGH-II increases protein synthesis as well as RNA and DNA synthesis. Levels of all these hormones are measured in the plasma, which is the liquid, cell-free, portion of blood.