Many hormones of the anterior pituitary stimulate the body indirectly by exerting the effect principally on other major endocrine glands, causing these glands to release hormones that carry the intended message to a target tissue. Growth hormone is unique because it is a hormone of the anterior pituitary that stimulates almost all of the body’s tissues directly.
Also called somatotropin, human growth hormone (hGH) belongs to a class of hormones called somatotropes. hGH is comprised of 191 amino acids compiled into a single chain. The hypothalamic release of growth-hormone-releasing factor prompts growth hormone secretion. hGH stimulates body growth; increases secretion of IGF-1; stimulates breakdown of lipids; and opposes insulin action on carbohydrate and lipid metabolism. Generally, hGH causes growth in almost every tissue in the body capable of growing.
In an experiment to elucidate the effect of growth hormone, ten rats of the same litter were selected. Five rats received daily injections of growth hormone, while the other five rats were made a control. Accelerated growth, evidenced by increased body weight, was seen throughout the lifespan of the experimental population (Figure 1). Measurements of body mass index (BMI) revealed that in early stages tissue growth accelerated uniformly. However, after reaching the age of sexual maturity, growth in soft tissue continued, but marked decline in growth of dense bone was evident. This phenomenon can be explained by the closure of the epiphyseal plates of long bones.
Figure 1. Average body weight of rats with (purple) or without (green) daily injections of growth hormone as a function of time in days.
Gigantism, a disease of accelerated growth, can be caused by primary tumors of the anterior pituitary gland. In patients with tumor-induced gigantism, levels of growth-hormone-releasing factor would expected to be:
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