Chapter 16
The Endocrine System
605
16
bodybuilding properties, and some parents seek to give their
children the hormone in an attempt to make them taller. How-
ever, while muscle mass increases, there is no objective evidence
for an increase in muscle strength in either athletes or the el-
derly, and only minimal increases in stature occur in normal
children. Moreover, taking GH can lead to fluid retention, joint
and muscle pain, diabetes, and may promote cancer.
Thyroid-Stimulating Hormone (TSH)
Tyroid-stimulating hormone (±SH)
, or
thyrotropin
, is a tropic
hormone that stimulates normal development and secretory activity
of the thyroid gland. Its release follows the hypothalamic–pituitary–
target endocrine organ feedback loop described earlier and shown
specifically for TSH in
Figure 16.8
.
±e hypothalamic peptide
thyrotropin-releasing hormone
(±RH)
triggers the release of TSH from
thyrotropic cells
of
the anterior pituitary. Rising blood levels of thyroid hormones
act on both the pituitary and the hypothalamus to inhibit TSH
secretion. GHIH also inhibits TSH secretion.
Adrenocorticotropic Hormone (ACTH)
Adrenocorticotropic hormone (AC±H)
(ah-dre
0
no-kor
0
tĭ-ko-
trōp
9
ik), or
corticotropin
, is secreted by the
corticotropic cells
of
the anterior pituitary. It is split from a
prohormone
(a large precursor
molecule) with the tongue-twisting name
pro-opiomelanocortin
(POMC)
(pro
0
o
0
pe-o-mah-lan
0
o-kor
9
tin). ACTH stimulates the
adrenal cortex to release corticosteroid hormones, most impor-
tantly glucocorticoids that help the body resist stressors.
ACTH release, elicited by hypothalamic
corticotropin-
releasing hormone (CRH)
, has a daily rhythm, with levels
peaking in the morning, shortly before awakening. Rising levels
of glucocorticoids feed back and block secretion of CRH and
ACTH release. Internal and external factors that alter the nor-
mal ACTH rhythm by triggering CRH release include fever, hy-
poglycemia (low blood glucose levels), and stressors of all types.
Gonadotropins (FSH and LH)
Follicle-stimulating hormone (FSH)
and
luteinizing hormone
(LH)
(lu
9
te-in-īz
0
ing) are referred to collectively as
gonadotro-
pins
. ±ey regulate the function of the gonads (ovaries and tes-
tes). In both sexes, FSH stimulates production of gametes (sperm
or eggs) and LH promotes production of gonadal hormones. In
females, LH works with FSH to cause an egg-containing ovarian
follicle to mature. LH then triggers ovulation and promotes syn-
thesis and release of ovarian hormones. In males, LH stimulates
the interstitial cells of the testes to produce the male hormone
testosterone.
Gonadotropins are virtually absent from the blood of prepu-
bertal boys and girls. During puberty, the
gonadotropic cells
of
the anterior pituitary are activated and gonadotropin levels rise,
causing the gonads to mature. In both sexes,
gonadotropin-
releasing hormone (GnRH)
produced by the hypothalamus
prompts gonadotropin release. Gonadal hormones, produced
in response to the gonadotropins, feed back to suppress FSH
and LH release.
Prolactin (PRL)
Prolactin (PRL)
is a protein hormone structurally similar to
GH. Produced by
prolactin cells
, PRL’s only well-documented
effect in humans is to stimulate milk production by the breasts
(
pro
5
for;
lact
5
milk). ±e role of prolactin in males is not
well understood.
Figure 16.7
Disorders of pituitary growth hormone.
An
individual exhibiting gigantism (center) is flanked by a pituitary
dwarf (left) and a woman of normal height (right).
Hypothalamus
Anterior pituitary
Thyroid gland
Thyroid
hormones
TSH
TRH
Target cells
Stimulates
Inhibits
Figure 16.8
Regulation of thyroid hormone secretion.
TRH
5
thyrotropin-releasing hormone, TSH
5
thyroid-stimulating hormone.
previous page 639 Human Anatomy and Physiology (9th ed ) 2012 read online next page 641 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off