Growth Hormone is also called somatotropin hormone.
It is synthesized by the acidophils of anterior lobe and
stored in very large amounts in the pituitary gland. Daily
GH output(adults/children) is 0.2 to 1 mg/day, with
biological half life 6-20 min.
Human GH is a single unbranched polypeptide chain
containing 191 amino acids with molecular weight of
It varies considerably in structure from species to species.
Growth Hormone does not function through a target gland
but exerts its effects directly on all tissues of the body .
MECHANISM OF HORMONAL ACTION
1.ACTION THROUGH MEMBRANE PERMEABILITY:
bind with the receptor that causes confirmational
change in protein receptor that results opening or
closing of channels.
2.ACTION THROUGH THE EFFECT OF GENE EXPRESSION
these hormones are lipophilic they easily pass through the
3.ACTION THROUGH SECONDARY MESSENGER:
It include AMP & GMP SYSTEM.
4.ACTION THROUGH TYROSINE KINASE ACTIVATION:
It include -hormone receptor that possess intrinsic tyrosine
-hormone receptor that NOT possess intrinsic
ACTIONS OF GROWTH HORMONE
1. STIMULATION OF GROWTH OF BONE ,
CARTILAGE AND CONNECTIVE TISSUE
The effects of growth hormone on skeletal growth
mediated by family of polypeptide called
GROWTH HORMONE increase the number of cells.
e.g. in muscles and bones .
THYROID HORMONE AND INSULIN
They are also necessary for normal osteogenesis
insulin increase cytoplasmic growth.
Thyroid hormone required for full effect of GH on
DNA replication .
• GROWTH FACTORS
a. Somatomedin A and B;
b. Insulin like growth factor I and II ;
c. Nerve growth factor (NGF);
d. Ovarian growth factor (OGF);
e. Epidermal growth factor (EGF);
f. Fibroblast growth factor (FGF);
h. Multiplication simulating activity(MSA); and
i. Platelet derived growth factor(PDGF).
INSULIN LIKE GROWTH FACTOR I AND II
(or somatomedin C)
(or multiplication stimulating
1. Secretion : Independent of
GH before birth but is
stimulated by GH after birth
;peak secretion at the time of
puberty and decreases in old
2. Plasma level :10-700 ng/ml
3. Receptor: Similar to insulin
4. Major action:
i.Growth stimulating activity;
ii.Control of skeletal and
1. Independent of GH. Its
secretions are constant
2. 300-800 ng/ml
receptor involved targeting
proteins to intracellular
4. Growth during foetal
‘Receptors’ for somatomedins exist in
chondrocytes,hepatocytes,adipocytes and muscle cells.
SOMATOMEDIN has insulin like effect on
tissues,including lipolysis,increased glucose oxidation
fat and increased glucose and amino acid transport by
Somatomedin activity rises peak 16-20 hours after
injection of GH.
SOMATOMEDIN ACTIVITY REDUCES BY
Glucocorticoids and protein deficiency.
Before Epiphysial Closure-
GH through somatomedin , stimulate proliferation of
chondrocytes , appearance of osteoblast.
Stimulation of DNA & RNA synthesis and collagen
Increase in the thickness of epiphysial end plate.
• After epiphysial closure-
bone length can no longer increase by GH but bone
thickening can occur through Periosteal growth .
Seen in hypersecretion of GH (ACROMEGALY)
2.Effects on protein and Mineral metabolism
• On protein metabolism : GH is protein anabolic
Mechanism of action:
a. It effects ribosomal attachment
b. It increase transport of neural and basic amino acid
into cells from E.C.F. Therefore , plasma amino acid
level decreases. This effect is unaffected by protein
synthesis blocking drugs.
c. It increases excretion of amino acids
i.e. 4-hydroxyproline,which comes from collagen.
Thus , hydroxyproline excretion is increased in:
- diseases associated with increased collagen
- when synthesis of soluble collagen is increased.
d. It stimulates erythropoesis.
• On mineral metabolism:
a. Increases Ca2+ absorbtion from GIT.
b. Decreases Ca2+ ,K+ , Na+ and phosphorus excretion
from kidneys, because these minerals are diverted
from kidneys to the growing tissues.
3. Effect on carbohydrate and fat
On carbohydrate- GH is DIABETOGENIC
a. Increasing hepatic glucose output ; and
b. Directly antagonizing the insulin effect on adipose
tissue and skeletal muscle .
• On fat metabolism :
a. GH has catabolic effect. i.e. it increases
mobilization of fat from adipose tissues , increase
circulating ‘FFA’ Level. This provides ready source of
energy for tissues during hypoglycemia,fasting and
other stressful stimuli.
b.GH has ketogenic effect. i.e increases hepatic
oxidation of fatty acid to keto bodies.
Increases ability of pancreas to respond to
4. On kidneys
Following removal of anterior pituitary,
i. Kidney size decreases
ii. GFR decreases
iii. Renal blood flow decreases and
iv. Tubular secretion of PAH decreases
5.On Thymus: GH increases growth of thymus (which is
often enlarged in ACROMEGALY).
6.Increases milk production : GH can increase lactation
Can occur when GH levels
Are increased and somatomedin levels are depressed
e.g. in kwashiorkor
Their plasma IGF-I conc.
Fails to increase in puberty.
• Laron Dwarfism: there is
Co genital abnormility of
GH receptors therefore
Plasma conc. of GH binding
decreases and IGF-I
not secreted in sufficient amount.
• Gigantism: due to over production
of GH in adolesence
i.e. before epiphysial closure.
• Acromegaly :It is associated with
hypersecretion of prolactin .
It causes growth of those areas
where cartilage persist.
GUYTON AND HALL