Growth hormone, also known as somatotrophin, is one of the hormones produced by the anterior pituitary gland. It has numerous functions that are essential for normal growth and development in humans.
This article will discuss the function of growth hormone, the regulation of its axis and relevant clinical conditions to its dysfunction.
Growth Hormone Function
GH has direct metabolic effects on tissues by binding to cells, and has indirect effects by stimulating cells in the liver to produce insulin-like growth factors (IGFs or somatomedins). The main IGF is IGF-1.
Growth hormone, either directly or indirectly, affects almost every tissue in the body, especially skeletal muscle and cartilage cells (chondrocytes).
The overall effects, arising from an interplay between GH and IGF-1, are important for the following:
- Skeletal growth
- Muscle strength
- Bone density
- Cardiac function
- Increased lipolysis – cells use the resultant fatty acids to generate ATP, sparing glucose
- Glycogenolysis in the liver (breakdown of glycogen to glucose), which is a diabetogenic effect, due to the significant increase in blood glucose levels
- Stimulation of stem cell division and differentiation of daughter cells in epithelia and connective tissues
The effects of GH via IGF-1 can be thought of as “anabolic” (compound building) like insulin and include:
- Increase protein synthesis and cell growth
- Increase carbohydrate oxidation
IGF-1 is important especially after a meal when glucose and amino acids are available in the blood. Glucose is taken up into cells through the action of insulin for ATP synthesis. At the same time, IGF-1 binds to plasma membrane receptors to increase their uptake of amino acids for protein synthesis, which uses up energy.
Regulation of Growth Hormone
Hypothalamic Control of Growth Hormone
The hypothalamus secretes growth hormone releasing-hormone (GHRH). GHRH stimulates somatotroph cells of the anterior pituitary to release growth hormone (GH), also known as somatotropin.
Several factors including stress, exercise, nutrition, hormones such as ghrelin (synthesised by the stomach) and sleep modulate the production of growth hormone.
The growth hormone axis is different to the typical endocrine axis. Whilst GHRH promotes GH release, the hypothalamus also produces growth hormone inhibiting hormone, also known as somatostatin, which inhibits GH.
IGFs directly supress GH secretion by somatotrophs. IGFs also indirectly suppress GH secretion by inhibiting GHRH release and stimulating GHIH release.
Clinical Relevance – Growth Hormone Deficiency
In children, a deficiency of GH could result in short stature due to slow bone and muscle maturation and delayed puberty. In adults, changes are more subtle and include:
- Reduced muscle mass and strength
- Reduced bone mass
- Reduced energy
- Possible cardiac dysfunction
One famous example of Growth Hormone deficiency is that of footballer Lionel Messi. The Ballon d’Or winner moved to FC Barcelona at the age of 13, as the club promised to fund his treatment of the condition.
Clinical Relevance – Growth Hormone Excess
In adults, excess GH is called acromegaly and is mostly caused by a pituitary tumour secreting GH.
Clinical features include:
- Large extremities with growth of hands, feet and jaw
- Paraesthesia in the extremities
- Coarse facial features, wide nose and rounded face
- Cardiomegaly (enlarged heart)
In children, hypersecretion of GH before the bony epiphyses have fused results in gigantism, whereby the child grows very tall.
Treatment involves surgery to remove the pituitary tumour, via the sphenoid bone (“transsphenoidal”) if the tumour is large enough, or medically by using somatostatin analogues.