350
Chapter 11
Table 11–6
Major Hormones Infl uencing
Growth
Hormone
Principal Actions
Growth hormone
Major stimulus of postnatal growth:
Induces precursor cells to differentiate
and secrete insulin-like growth factor I
(IGF-1), which stimulates cell division
Stimulates liver to secrete IGF-1
Stimulates protein synthesis
Insulin
Stimulates fetal growth
Stimulates postnatal growth by
stimulating secretion of IGF-1
Stimulates protein synthesis
Thyroid hormones
Permissive for growth hormone’s
secretion and actions
Permissive for development of the
central nervous system
Testosterone
Stimulates growth at puberty, in large
part by stimulating the secretion of
growth hormone
Causes eventual epiphyseal closure
Stimulates protein synthesis in male
Estrogen
Stimulates the secretion of growth
hormone at puberty
Causes eventual epiphyseal closure
Cortisol
Inhibits growth
Stimulates protein catabolism
testosterone, synthetic androgens, and the hormones dehy-
droepiandrosterone (DHEA) and androstenedione. However,
these steroids have multiple potential toxic side effects, such as
liver damage, increased risk of prostate cancer, and infertility.
Moreover, in females, they can produce masculinization.
Cortisol
Cortisol, the major hormone the adrenal cortex secretes in
response to stress, can have potent
antigrowth
effects under
certain conditions. When present in high concentration, it
inhibits DNA synthesis and stimulates protein catabolism in
many organs, and it inhibits bone growth. Moreover, it breaks
down bone and inhibits the secretion of growth hormone. For
all these reasons, in children, the elevation in plasma corti-
sol that accompanies infections and other stresses is, at least
in part, responsible for the retarded growth that occurs with
chronic illness. Furthermore, the administration of pharma-
cological glucocorticoid therapy for asthma or other disorders
may temporarily decrease linear growth in children.
This completes our survey of the major hormones that
affect growth.
Table 11–6
summarizes their actions.
Acromegaly and Gigantism
Acromegaly
and
gigantism
arise when chronic, excess
amounts of growth hormone are secreted into the blood.
In almost all cases, acromegaly and gigantism are caused
by benign (noncancerous) tumors of the anterior pituitary
gland that secrete growth hormone at very high rates. These
tumors are typically very slow growing, and, if they occur
after puberty, it may be decades before a person realizes there
is something seriously wrong.
If the tumor arises before puberty, when the
epiphyseal growth plates are still open, the individual
will develop gigantism (“pituitary giant”) and grow to
extraordinary heights (
Figure 11–29
). Some pituitary
giants have reached heights over 8 feet! If the tumor arises
after puberty, when linear growth is no longer possible, the
ADDITIONAL CLINICAL EXAMPLES
condition is known as acromegaly. Such people will be of
normal height but will manifest many other symptoms that
also occur in pituitary giants.
Even when linear growth is no longer possible (after
puberty), very high plasma levels of GH and IGF-1 result in
the thickening of many bones in the body, most noticeably in
the hands, feet, and head. The jaw, particularly, enlarges to
give the characteristic facial appearance called
prognathism
that is associated with acromegaly. In addition, many internal
organs such as the heart also become enlarged, and this can
interfere with their ability to function normally.
All adults continue to make and secrete GH even after
growth ceases. That is because GH has metabolic actions in
addition to its effects on growth. The major actions of GH
in metabolism are to increase blood sugar levels, increase
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