Somatotropic hormone (growth hormone, somatotropin) in blood

Somatotropic hormone (growth hormone, somatotropin) is a peptide secreted by the anterior pituitary gland and consisting of 191 amino acids. The daily production of growth hormone is approximately 500 mcg. Somatotropic hormone stimulates protein synthesis, cell mitosis processes and enhances lipolysis. The half-life of somatotropic hormone in adults is 25 minutes. The hormone is inactivated in the blood by hydrolysis. Compared to other hormones, growth hormone is present in the pituitary gland in the largest amount (5-15 mg/g of tissue). The main function of somatotropic hormone is to stimulate body growth. Growth hormone promotes protein synthesis and, interacting with insulin, stimulates the entry of amino acids into cells. It also affects the absorption and oxidation of glucose by adipose tissue, muscles and liver. Growth hormone increases the sensitivity of adipocytes to the lipolytic effect of catecholamines and reduces their sensitivity to the lipogenic effect of insulin. These effects lead to the release of fatty acids and glycerol from adipose tissue into the blood, with their subsequent metabolism in the liver. Somatotropic hormone reduces the esterification of fatty acids, thereby reducing the synthesis of TG. Current data suggest that growth hormone can reduce glucose consumption by adipose tissue and muscles by postreceptor inhibition of insulin action. Growth hormone increases the transport of amino acids into muscle, creating reserves of substrate for protein synthesis. Through a separate mechanism, growth hormone increases the synthesis of DNA and RNA.

Growth hormone stimulates cell growth both directly and indirectly, through IGF-I and II. The main biological effects of somatotropic hormone are provided by IGF-I.

Secretion of growth hormone normally occurs unevenly. For most of the day, its concentration in the blood of healthy people is very low. There are 5-9 discrete releases of the hormone per day. The low initial level of secretion and the pulsating nature of the releases significantly complicate the assessment of the results of determining the concentration of growth hormone in the blood. In such cases, special provocative tests are used.

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Normal serum growth hormone concentration

Age	STH, ng/ml
Blood from the umbilical cord	8-40
Newborns	10-40
Children	1-10
Adults:
Men	0-4.0
Women	0-18.0
Over 60 years old:
Men	1-9.0
Women	1-16

Regulation of growth hormone secretion

The regulation of growth hormone secretion is mainly carried out by two hypothalamic peptides: STRG, which stimulates the formation of growth hormone, and somatostatin, which has the opposite effect. IGF-I also participates in the regulation of growth hormone secretion. An increase in the concentration of IGF-I in the blood suppresses the transcription of growth hormone genes in the pituitary somatotrophs according to the principle of negative feedback.

The main disorders of the somatotropic function of the pituitary gland are represented by excessive or insufficient production of growth hormone. Gigantism and acromegaly are neuroendocrine diseases caused by chronic hyperproduction of growth hormone by somatotrophs of the anterior pituitary gland. Excessive production of growth hormone during the period of osteogenesis before the closure of the epiphyses leads to gigantism. After the closure of the epiphyses, hypersecretion of growth hormone causes acromegaly. Pituitary gigantism is rare, it occurs at a young age. Acromegaly occurs mainly at the age of 30-50 years (average frequency is 40-70 cases per 1 million population).

The development of pituitary dwarfism (dwarfism) in the vast majority of cases is associated with insufficiency of the somatotropic function of the anterior pituitary gland, up to its complete loss. Disruption of the production of growth hormone by the pituitary gland is most often (approximately 70% of cases) caused by primary damage to the hypothalamus. Congenital aplasia and hypoplasia of the pituitary gland are detected very rarely. Any destructive changes in the hypothalamic-pituitary region can lead to growth cessation. Most often, they are caused by craniopharyngiomas, CNS germinomas and other tumors of the hypothalamic region, tuberculosis, sarcoidosis, toxoplasmosis and cerebral aneurysms.

There are forms of dwarfism, mainly hereditary, in which the formation and secretion of growth hormone are not impaired. In particular, children with Laron syndrome have all the signs of hypopituitarism, but the concentration of somatotropic hormone in the blood is increased against the background of reduced levels of IGF-I. The main defect is due to the inability of growth hormone to stimulate the production of IGF-I.

In many patients with hypopituitarism, no obvious structural damage to the hypothalamus or pituitary gland can be detected; in such cases, the pathology is often caused by functional defects of the hypothalamus. STH deficiency can be either isolated or combined with deficiency of other pituitary hormones.

The daily rhythm of growth hormone secretion with peaks of its concentration above 6 ng/ml 1-3 hours after falling asleep regardless of the time of day is formed by 3 months after birth. The average daily concentration of growth hormone increases during puberty, decreases after 60 years; by this time, the daily rhythms disappear. Gender differences in growth hormone secretion have not been identified.