Growth Hormone (Somatotropin)

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Growth hormone (somatotropin) is very special because it affects almost every tissue in the body. Although it has several effects in the organism, we can distinguish three main ones: 1) growth, 2) utilization of fat as a source of energy, 3) diabetogenic effect.

Its first effect is obviously growth. Growth hormone increases the absorption of amino acids, and it also stimulates the synthesis of RNA, DNA, and proteins. As a result, there's a greater number of cells that are also bigger. This increases the size of organs and simply said stimulates growth of almost every tissue. It also affects the development of bones because it stimulates chondrocytic and osteogenic cells to reproduce and it overall, stimulates cartilage metabolism. All of this then enables the person to grow. This is the reason why we call the hormone growth hormone.

Its second effect is that the organism uses more fat as a source of energy. That's because growth hormone releases fatty acids from adipose tissue and they are then available to be used for energy, which as a result increases lean body mass.

Growth hormone's third effect is often called diabetogenic. Growth hormone decreases absorption of glucose in skeletal muscle and adipose tissue and it also increases glucose production in the liver. As you can imagine, if the organism doesn’t absorb that much glucose and it actually produces even more, there's higher glucose concentration in blood. And for that reason, more insulin needs to be secreted. Now, if someone has an actual diabetes, they are, in a way, in a similar situation: they have a high concentration of glucose, which requires more insulin, which they don’t have, or the tissue is resistant to it. Growth hormone creates something similar: the high glucose concentration and the need for more insulin. That's why its effect is called diabetogenic.

Growth hormone can function either directly or indirectly. Direct action happens, for example, in skeletal muscle or adipose tissue; however, growth hormone can also work indirectly through special substances called somatomedins, which are produced in the liver. The hormone stimulates their production and they in turn stimulate bone growth. These somatomedins fuction through the so-called IGF receptors. They are called IGF receptors because somatomedin stimulates growth in a similar way that insulin does. For that reason, somatomedins are also called insulin-like growth factors or IGFs and their receptors IGF receptors.

Secretion

Growth hormone is secreted from the anterior pituitary in a pulsatile pattern. That simply means that not the same amount of the hormone is secreted all the time. There are certain factors that simulate its secretion like hypoglycemia (low glucose concentration) and low concentration of fatty acids. That's because, as we said, growth hormone decreases absorption of glucose and causes its further production and it releases fatty acids from adipose tissue to the blood. Other factors are excitement, exercise, but also trauma. What’s also important is that growth hormone is secreted in different amounts during life. First, it increases from the moment we're born. Then its secretion remains quite stable during childhood, but then at puberty, it raises enormously. This burst is induced by estrogen in females and testosterone in males. This is the reason why people grow so much during puberty. Then after puberty, its secretion rates start to decline and when we're old they reach their lowest.

Regulation

Growth hormone is regulated by hypothalamic hormones – growth hormone-releasing hormone and somatostatin. Growth hormone-releasing hormone activates somatotrophs, the cells in the anterior pituitary that produce growth hormone. Somatostatin, on the other hand, inhibits somatotrophs so that they don’t release the hormone. Negative feedback also plays a role in growth hormone's regulation: growth hormone and somatomedins stimulate the secretion of somatostatin, which then inhibits growth hormone's further secretion.

Pathophysiology

Deficiency of the growth hormone (= too little of the hormone) causes failure to grow, delayed puberty and also mild obesity, because, as we said, growth hormone also helps to utilize fat as a source of energy. This condition is called dwarfism and can be treated with human growth hormone replacement.

Excess of the growth hormone (= too much of the hormone) can have two different outcomes depending on whether there's too much of the hormone before or after puberty. If there’s too much of the hormone before puberty, it results in gigantism – the person simply grows too much. If it’s after puberty, the person cannot get any taller, but their organs can increase in size, as well as their hands, feet, etc. They can also suffer from insulin resistance because of growth hormone's diabetogenic effect. This is referred to as acromegaly. The patient is treated with somatostatin analogues, which inhibit somatotrophs and the release of the hormone.

References:
Costanzo, L. S. (2018). Physiology. Elsevier.
Hall, J. E., Hall, M. E., & Guyton, A. C. (2021). Guyton and Hall Textbook of Medical Physiology. Elsevier.