496
UNIT THREE
Thyroid-stimulating hormone
, also called
thyrotropin,
is a glycoprotein. It controls secretion of certain hormones
from the thyroid gland. TSH can also stimulate growth of
the gland, and abnormally high TSH levels may lead to an
enlarged thyroid gland, or
goiter.
The hypothalamus partially regulates TSH secretion by
producing thyrotropin-releasing hormone (TRH). Circulating
thyroid hormones help regulate TSH secretion by inhibiting
release of TRH and TSH; therefore, as the blood concentra-
tion of thyroid hormones increases, secretion of TRH and
TSH decline
(f g. 13.16)
.
External factors infl
uence release of TRH and TSH. These
include exposure to extreme cold, accompanied by increased
hormonal secretion and emotional stress, which can either
increase or decrease hormonal secretion, depending upon
circumstances.
PRACTICE
15
How does growth hormone af
ect the cellular metabolism oF
carbohydrates, Fats, and proteins?
16
What are the Functions oF prolactin?
17
How is TSH secretion regulated?
Adrenocorticotropic
(ah-dre
no-ka
te-ko-tro
¯p
ik)
hor-
mone
is a peptide that controls the manufacture and secre-
tion of certain hormones from the outer layer (cortex) of the
adrenal gland. The secretion of ACTH is regulated in part
by
corticotropin-releasing hormone
(CRH), which the hypo-
thalamus releases in response to decreased concentrations of
TRH
Thyrotropin-
releasing
hormone
TSH
Thyroid-
stimulating
hormone
Thyroid
Mammary gland
Adrenal
cortex
Ovary
Testis
Bone
Muscle Adipose
tissue
PRL
Prolactin
Hormones from Anterior Pituitary
Hormones from Hypothalamus
ACTH
Adrenocorticotropic
hormone
LH
Luteinizing
hormone
FSH
Follicle-
stimulating
hormone
GH
Growth
hormone
PRF
Prolactin-
releasing
factor
CRH
Corticotropin-
releasing
hormone
SS
Somatostatin
GnRH
Gonadotropin-
releasing
hormone
GHRH
Growth
hormone-
releasing
hormone
PIH
Prolactin-
release
inhibiting
hormone
Bloodstream
Hypothalamus
Anterior
pituitary
gland
TRH
TSH
Thyroid
hormones
inhibit TRH
and TSH
Target
cells
Thyroid
hormones
stimulate
target cells
Thyroid
hormones
Thyroid
gland
TSH
stimulates
the secretion
of certain
thyroid
hormones
+
+
Release into
bloodstream
Stimulation
Inhibition
FIGURE 13.16
Thyrotropin-releasing hormone (TRH) From the
hypothalamus stimulates the anterior pituitary gland to release
thyroid-stimulating hormone (TSH), which stimulates the thyroid gland
to release hormones. These thyroid hormones reduce the secretion oF
TSH and TRH by negative Feedback. (
+
= stimulation;
= inhibition)
FIGURE 13.15
Hormones released From the hypothalamus, the corresponding hormones released From the anterior lobe oF the pituitary gland,
and their target organs.
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