Chapter 16
The Endocrine System
607
16
(tri
0
i-o
0
do-thi
9
ro-nēn), or
T
3
. T
4
is the major hormone secreted by
the thyroid follicles. Most T
3
is formed at the target tissues by con-
version of T
4
to T
3
. Both T
4
and T
3
are constructed from two linked
tyrosine amino acids. ±e principal difference between them is that
T
4
has four bound iodine atoms, and T
3
has three (thus, T
4
and T
3
).
TH affects virtually every cell in the body
(Table 16.2)
.
Like steroids, TH enters a target cell, binds to intracellular recep-
tors within the cell’s nucleus, and initiates transcription of mRNA
for protein synthesis. Effects of thyroid hormone include:
Increasing basal metabolic rate and body heat production,
by turning on transcription of genes concerned with glucose
oxidation. ±is is the hormone’s
calorigenic effect
(
calori-
genic
5
heat producing).
Regulating tissue growth and development. TH is critical for
normal skeletal and nervous system development and matu-
ration and for reproductive capabilities.
Maintaining blood pressure by increasing the number of
adrenergic receptors in blood vessels.
Internally, the gland is composed of hollow, spherical
folli-
cles
(Figure 16.9b). ±e walls of each follicle are formed largely
by cuboidal or squamous epithelial cells called
follicular cells
,
which produce the glycoprotein
thyroglobulin
(thi
0
ro-glob
9
u-
lin). ±e central cavity, or lumen, of the follicle stores
colloid
, an
amber-colored, sticky material consisting of thyroglobulin mol-
ecules with attached iodine atoms.
Tyroid hormone
is derived
from this iodinated thyroglobulin.
±e
parafollicular cells
, another population of endocrine cells
in the thyroid gland, produce
calcitonin
, an entirely different
hormone. ±e parafollicular cells lie in the follicular epithelium
but protrude into the so² connective tissue that separates and
surrounds the thyroid follicles.
Thyroid Hormone (TH)
O²en referred to as the body’s major metabolic hormone,
thy-
roid hormone (TH)
is actually two iodine-containing amine
hormones,
thyroxine
(thi-rok
9
sin), or
T
4
, and
triiodothyronine
Table 16.2
Major Effects of Thyroid Hormone (T
4
and T
3
) in the Body
PROCESS OR SYSTEM
AFFECTED
NORMAL PHYSIOLOGICAL
EFFECTS
EFFECTS OF HYPOSECRETION
EFFECTS OF HYPERSECRETION
Basal metabolic rate (BMR)/
temperature regulation
Promotes normal oxygen use and
BMR; calorigenesis; enhances
effects of sympathetic nervous
system
BMR below normal; decreased
body temperature, cold
intolerance; decreased appetite;
weight gain; reduced sensitivity
to catecholamines
BMR above normal; increased body
temperature, heat intolerance;
increased appetite; weight loss
Carbohydrate/lipid/protein
metabolism
Promotes glucose catabolism;
mobilizes fats; essential for
protein synthesis; enhances liver's
synthesis of cholesterol
Decreased glucose metabolism;
elevated cholesterol/triglyceride
levels in blood; decreased
protein synthesis; edema
Enhanced catabolism of glucose,
proteins, and fats; weight loss; loss
of muscle mass
Nervous system
Promotes normal development
of nervous system in fetus and
infant; promotes normal adult
nervous system function
In infant, slowed/deficient brain
development, retardation;
in adult, mental dulling,
depression, paresthesias,
memory impairment, hypoactive
reflexes
Irritability, restlessness, insomnia,
personality changes, exophthalmos
(in Graves' disease)
Cardiovascular system
Promotes normal functioning of
the heart
Decreased efficiency of heart's
pumping action; low heart rate
and blood pressure
Increased sensitivity to
catecholamines leads to
rapid heart rate and possible
palpitations; high blood pressure;
if prolonged, heart failure
Muscular system
Promotes normal muscular
development and function
Sluggish muscle action; muscle
cramps; myalgia
Muscle atrophy and weakness
Skeletal system
Promotes normal growth and
maturation of the skeleton
In child, growth retardation,
skeletal stunting and retention
of child's body proportions; in
adult, joint pain
In child, excessive skeletal growth
initially, followed by early
epiphyseal closure and short
stature; in adult, demineralization
of skeleton
Gastrointestinal (GI) system
Promotes normal GI motility
and tone; increases secretion of
digestive juices
Depressed GI motility, tone, and
secretory activity; constipation
Excessive GI motility; diarrhea; loss
of appetite
Reproductive system
Promotes normal female
reproductive ability and lactation
Depressed ovarian function;
sterility; depressed lactation
In females, depressed ovarian
function; in males, impotence
Integumentary system
Promotes normal hydration and
secretory activity of skin
Skin pale, thick, and dry; facial
edema; hair coarse and thick
Skin flushed, thin, and moist; hair
fine and soft; nails soft and thin
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