Source: Secreted by the anterior pituitary.
Function:
- Growth hormone is a major anabolic growth-promoting and stress hormone. All anabolic hormones such as growth hormone, insulin, thyroid hormones, and androgens are required for normal growth.
- It has a direct catabolic effect as a stress hormone, decreasing glucose uptake in adipose tissue and muscles which raises blood glucose, and mobilizing fats by increasing the activity of hormone sensitive lipase, leading to increased free fatty acids.
- Its direct anabolic effect as a growth hormone includes increased uptake of amino acids into cells.
- It also has indirect anabolic effects through growth factors.
- Most of the anabolic actions of growth hormone result from increased production of growth factors, also known as somatomedins or insulin-like growth factors (IGFs). A major growth factor is somatomedin C, or IGF-I. IGF-I's major known anabolic effect is increased synthesis of cartilage (chondrogenesis) in the epiphyseal plates of long bones, thereby increasing bone length. It is hypothesized that circulating IGFs increase lean body mass. The decreased lean body mass of aging may be due, in part, to the concomitant decrease in IGFs. IGFs also decrease in catabolic states, especially in protein-calorie malnutrition.
- It increases insulin resistance, leading to a diabetogenic effect.
Regulation:
- Growth hormone is released in pulses in response to growth hormone releasing hormone (GHRH). Its secretion increases during exercise and sleep. Secretion is inhibited by glucose and somatostatin release via negative feedback by somatomedin.
- During the sixth decade of life and later, growth hormone secretion diminishes considerably in both men and women. The cause of this decrease is unknown.
Disturbance of GH function:
- These disturbances may be either hypofunction or hyperfunction. According to the site of disturbance. It may be: -
Primary: if it is pituitary in origin.
Secondary: if it is hypothalamic in origin.
Effects of hyperfunction:
- Excessive secretion of GH (Growth Hormone), often due to a pituitary adenoma, can cause acromegaly in adults or gigantism in children.
- If the excess growth hormone is produced before the epiphysis closes, it leads to a condition called gigantism. This is characterized by tall stature and long limbs.
- After the closure of the epiphysis, the same excess hormone results in a condition known as acromegaly. Despite being caused by the same hormone surplus, the symptoms of gigantism and acromegaly are not alike.
Acromegaly.