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Growth hormone
• Human Growth Hormone (hGH) is a polypeptide chain, composed of
191 amino acids and with a molecular mass of 21,500 Da. It is
released by the anterior pituitary of both men and women. The
secretion is stimulated 3 –4 hours after a meal, about 1 hour after
the beginning of sleep, and after physical exercise.
• Hyposecretion of hGH becomes apparent in infants a few months
after birth and may result in dwarfism. In the opposite case,
hypersecretion of hGH results in gigantism and may be due to
hypophysic tumors.
• In adults, when epiphyses are closed, hypersecretion of hGH
provokes an increase in volume of soft tissues (hands, feet, lips), a
proliferation of bones (acromegalysyndrome), and a limited
tolerance of glucose Plays an important role in growth control.
•
Its major role in stimulating body growth is to stimulate the liver and other
tissues to secrete IGF-1.
• It stimulates both the differentiation and proliferation of myoblasts.
• It also stimulates amino acid uptake and protein synthesis in muscle and
other tissues.
• Growth hormone secretion is regulated by three feedback loops, including
both long, short and ultrashort loops.
• 1) GHRH inhibits its own secretion from the hypothalamus via an ultrashortloop feedback.
(2) Somatomedins, which are by-products of the growth hormone action on
target tissues, inhibit secretion of growth hormone by the anterior pituitary.
(3) Both growth hormone and somatomedins stimulate the secretion of
somatostatin by the hypothalamus.
• Growth hormone is secreted in a pulsatile pattern, with
bursts of secretion occurring approximately every 2
hours.
• The largest secretory burst occurs within 1 hour of falling
asleep (during sleep stages III and IV). The bursting
pattern, in terms of both frequency and magnitude, is
affected by several agents that alter the overall level of
growth hormone secretion.
• The normal values often increase to as high as 50 ng/ml
after depletion of the body stores of proteins or
carbohydrates during prolonged starvation.
• In brief, it has been found that growth hormone causes the liver
(and, to a much less extent, other tissues) to form several small
proteins called somatomedins that have the potent effect
of increasing all aspects of bone growth.
• Many of the somatomedin effects on growth are similar to the effects
of insulin on growth. Therefore, the somatomedins are also called
insulin-like growth factors (IGFs). At least four somatomedins have
been isolated, but by far the most important of these is
somatomedin C (also called IGF-1).
• The molecular weight of somatomedin C is
about 7500, and its concentration in the
plasma closely follows the rate of growth
hormone secretion.
• Some aspects of the somatomedin
hypothesis are still questionable. One
possibility is that growth hormone can
cause the formation of enough
somatomedin C in the local tissue to
cause local growth.
• It is also possible that growth hormone itself is directly
responsible for increased growth in some tissues and
that the somatomedin mechanism is an alternative
means of increasing growth but not always a
necessary one Short Duration of Action of Growth
Hormone but Prolonged Action of Somatomedin C.
• Growth hormone attaches only weakly to the plasma
proteins in the blood. Therefore, it is released from the
blood into the tissues rapidly, having a half-time in the
blood of less than 20 minutes.
• By contrast, somatomedin C attaches
strongly to a carrier protein in the blood
that, like somatomedin C, is produced in
response to growth hormone. As a result,
somatomedin C is released only slowly
from the blood to the tissues, with a halftime of about 20 hours. This greatly
prolongs the growth-promoting effects
of the bursts of growth hormone secretion
shown in Figure
• The serum level of insulin-like growth factor-1 (IGF-1)
and of its major circulating binding protein IGFBP-3 are
types of these of biomarkers. They are used to diagnose
and assess the degree of the human growth hormone
(hGH)-deficient state, and to measure responsiveness
and predict the outcome of hGH therapy of a patient.
They are also used in the differential diagnosis of growth
disorders in order to elucidate the etiology of the
disease. In addition, IGF-1 and IGFBP-3, i.e. their ratio,
can serve as safety biomarkers of hGH therapy.
• IGF-1 measurement before and after growth hormone
therapy.The Human Growth Hormone (hGH) ELISA is a
solid phase sandwich ELISA method. The samples and
anti-hGH-HRP conjugate are added to the wells coated
with hGH MAb. hGH in the serum binds to anti-hGH MAb
on the well and the anti -HGH second antibody then
binds to hGH. Unbound protein and HRP conjugate are
washed off by wash buffer. Upon the addition of the
substrate, the intensity of color is proportional to the
concentration of hGH in the samples. A standard curve
is prepared relating color intensity to the concentration of
the hGH.
• The diagnosis of growth hormone defiance
cant be made in a single random growth
hormone level because growth hormone is
secreted in pulses.
• Some pediatric endocrinologists diagnosis
growth hormone defeciency based on an
extremely low level of insuline like growth
hormone which varies much less in the
course of the day than growth hormone.
• IGF1 levels are dependent on the amount
of growth hormones in the blood but can
also be low in normal, young children, so
the test must be interpreted carefully.
• A more accurate but still imperfect way to
diagnosis growth hormone deficiency is a
growth hormone stimulation test. In this
test, your child has a blood drawn for
about 2 to 3 hours after being given
medications to increase growth hormone
• If the child does not produce enough
growth hormone after this stimulation, then
the child is diagnosed with growth
hormone deficiency. However, growth
hormone stimulation tests can over
diagnose growth hormone deficiency.
Growth hormone stimulation tests vary
and are complicated, so they are usually
performed under the guidance of a
pediatric endocrinologist.
• Usually, other tests to check the pituitary
or to evaluate the brain MRI are performed
when treatment is considered.
Clinical significant
• In children, ascertaining linear bone
growth along the epiphyseal plate.
Abnormally elevated levels lead to
gigantism while complete absence slows
the rate of growth one third to one half of
normal. In adults, the epiphyseal growth
plates ha fuse so hGH excess gradually
produces acromegaly, a coarse thickening
of the bones of the skull, hands and feet.
Growth hormone test
• The specimens shall be blood, serum in type
and the usual precautions in the collection of
venipuncture samples should be observed.
• For accurate comparison to established normal
values a fasting morning serum sample should
be obtained.
• The blood should be collected in a plain redtop
venipuncture tube without anticoagulant or
additive. Allow the blood to clot. Centrifuge the
specimen to separate the serum from the cells.
Sample storage
• Samples may be refrigerated at 2-8 C for
maximum period of five days.
• If the specimen can not assayed within
this time, the samples may be stored at
temperatures of -20C for up to 30 days.
• Avoid use of contamination devices. Avoid
repetitive freezing and thawing. When
assayed duplicate, 0.100 ml of the
specimen is required.
Principle
• In this method, GH calibrator, patient specimen or control is first added to
streptavidin coated well.
• Biotinylated monoclonal and enzyme labeled antibodies(directed against
district and different epitopes of GH) are added then the reactants mixed.
• Reaction between the various GH antibodies and native Gh forms a sandwich
complex that binds with streptavidine coated to well.
• After completion of the required incubation period, the
enzyme-growth hormone antibody bound conjugate by
aspiration or decantation.
• The activity of the enzyme present on the surface of the well is
quantized by reaction with a suitable substrate to produce
color.
• Hyperglycemia inhibits growth hormone
secretion. Age is an important factor in
growth hormone concentrations. At birth,
GH is high and generally declines with the
exception of a burst during the growth
phase of adolestronce. Women typically
have a 50% higher level than their agemached males.